Download Signs of a tree under stress

Contents
Introduction
Learning outcomes
2
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Tree disorders
3
Climatic and atmospheric factors
3
Effects of other organisms
7
Signs of a tree under stress
11
Causes of stress in trees
12
Selection of planting material
13
Planting techniques
15
Damage to the roots of established trees
17
Damage caused by roots
27
Damage to kerbs, footpaths and paving
30
Summary
33
Check your progress answers
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Introduction
Upon completion of this section the learner will be able to:

identify, prevent and/or treat common disorders.
You will know you have achieved this when you are able to:
2

identify abiotic and biotic factors which result in tree disorders

identify four common symptoms of stress

identify common disorders affecting a given list of tree species.
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Tree disorders
Tree disorders are a range of problems caused by a variety of factors and
requiring analysis and relevant treatment to alleviate or remove the
condition. Apart from sudden physical damage such as that caused by storm,
accident or vandals, most of these problems are a result of the tree becoming
‘stressed’. Conditions causing stress to trees result in reduced vigour and
reduced ability to respond to those conditions. Shigo defines stress as: ‘a
condition where a system or its parts begins to operate near the limits for
which it was designed.’
Reductions in plant vigour usually arise from exposure to stress: drought,
poor soil aeration, freezing or extreme temperature fluctuation, defoliation,
nutrient deficiency, chemical injury, mechanical damage, or transplant
shock. This stress may manifest as visible symptoms or it may have a more
subtle effect of weakening a plant and increasing its susceptibility to
pathogens or attack by boring insects. The greatest threat to survival faced
by urban trees is mistreatment by humans, giving rise to stress.
Climatic and atmospheric factors
Temperature
This is possibly the most fundamental plant selection criterion. Plants have
evolved under certain temperature regimes so a tropical plant placed in a
cool temperate landscape (or vice versa) is going to suffer. The extremes of
temperature affect growth and survival. Some plants are frost tolerant,
others are not. Consider whether or not you are in a frost-prone area. Take
into account the fact that heat may be reflected off buildings or hard
surfaces.
Light
Has the plant evolved in sun or shade? Light conditions may change during
the life of a plant; for example, other trees may grow or be removed,
buildings may come and go. Photosynthetic efficiency may be reduced if
plants have to compete for light.
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If a plant is suddenly exposed to more light and heat it may suffer from
sunburn. Over-pruning smooth-barked trees such as Ficus spp and
Fraxinus spp can have the same effect.
Photoperiod (the relative amount of light and dark) may determine a tree’s
seasonal behaviour. Some plants, when grown at lower latitudes than those
to which they are native, may experience a reduced growing season and will
therefore be smaller than normal. In almost all photoperiod-sensitive woody
species, long days and short nights promote vegetative growth.
Night lighting such as street lights may affect plants sensitive to day length.
One general observation is that vegetative growth may be promoted and
dormancy delayed. Continuous lighting suppresses chlorophyll formation
and promotes lengthening of shoot internodes and leaf expansion. Thus trees
in this situation may be more susceptible to air pollution and water stress.
Next time you are out at night have a close look at trees growing close to
lights and see if you can see any of these effects.
Rainfall
The amount of rain that falls and its distribution influence plant selection
and requirements for irrigation.
Drought is a common stress factor. Some plants have adaptations for
drought hardiness (eg reduced leaves, sunken stomates) but others must
simply be watered more often or not planted in situations where drought is
common. Borer infestations tend to be more common during dry periods.
Excess rainfall can also cause problems through reducing soil aeration,
while high humidity may encourage diseases.
Hail damage may predispose plants to certain foliar diseases; for example,
shoot blight (Sphaeropsis sapineae, previously Diplodia pinea) of Pinus
radiata (see Hadlington and Johnston, p 96). Bark can also be killed (as well
as the cambium) on trees with thin bark. These damaged areas may also be
involved by saprophytic or parasitic organisms.
The weight of snow on branches may be enough to break them.
Wind
Knowing the direction of prevailing winds will tell you about the
temperature, moisture and salt conditions that may exist in an area during
particular seasons.
Windbreaks can be grown using hardy species to redirect or lessen the effect
of such winds.
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Strong winds and storms are less predictable and may cause extensive
damage to trees, especially when the soil is moist. In areas where strong
winds are common, early thinning of trees may lessen any damage by
allowing the wind to move through the trees more easily. Trees in windy
areas tend to be shorter, with thicker trunk bases and a more pronounced
trunk taper than trees grown in a more protected environment or close
together.
What are the prevailing wind directions in your area, and from which
directions do storms come?
City landscapes can increase wind velocity by creating wind tunnels.
Turbulence may be created by the many solid barriers in street scapes.
Lightning
The nature and extent of lightning damage to a tree can vary greatly.
Branches and barkk may be blown completely apart; the crown may be
killed or large limbs broken out; trunks may be split open; in rough-barked
trees, a continuous groove of bark and/or wood may be stripped out or
injured without any external evidence; part or all of the root system may be
killed. These variations appear to be related to the intensity of the lightning,
the species, the amount of water on or in the bark, and the character of the
branch and trunk tissues (see Figures 8.8 and 8.9, Hadlington and Johnston).
Lightning is most likely to strike a lone tree, the tallest in the group, a tall
tree at the end of a row or on the edge of a grove, a tree growing in moist
soil or adjacent to bodies of water or a tree closest to a building. Plants
under or near lightning-struck trees are often killed or injured.
Protection
Trees can be protected against lightning if means are provided by which a
lightning discharge can enter or leave the ground without causing damage or
loss. Trees close to buildings (within 3m), specimen trees, historic trees, and
those under which people may shelter could be equipped with a lightning
protection system. In a group of trees, only tall trees need to be protected.
Lightning protection systems should be inspected regularly and air terminals
may need to be raised every two to three years. A tree protected with a
lightning rod will actually be more likely to be struck by lightning, but a
properly installed system should protect the tree and people underneath from
harm.
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Treatment
Depending on the extent of damage, trees may need to be removed, others
treated. Since the extent of internal damage cannot be assessed immediately,
repairs should be limited to safety pruning and treatment of bark wounds. A
period of six months is usually enough to assess whether or not the tree will
survive. Root growth should be stimulated by watering and a light
fertilising. Struck trees may decline over several years.
Pollution
Leaves are the main plant parts to show symptoms of air pollution injury.
On broad-leaved plants, leaves may develop interveinal necrotic or chlorotic
areas, marginal or tip necrosis, stippling of the upper surface or silvering of
the lower surface. Conifer needles may show patterns of chlorosis and
necrosis. These are symptoms of acute toxicity and usually result from a
short exposure to high concentrations of a gaseous pollutant.
Pollution injury symptoms can be very variable depending on such factors
as:

plant species and stage of growth

the type of concentration of pollutants

the length of exposure

moisture content of the leaves

humidity

light

temperature

wind

nutritional level.
Long periods of exposure may result in chronic injury which may be
difficult to distinguish from other forms of poor growth. Affected plants will
be low in vigour, leaves will be pale green tree, leaves and fruit will colour
early and more brightly and will drop earlier. Affected plants are usually
more susceptible to other disorders. Air pollution may be from particulates
and/or from airborne chemicals. Particulates include various dusts (chemical
or natural), soot, oil and lead. Stomates and lenticels may be blocked and
photosynthesis may be reduced.
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There are four major chemical air pollutants:
1
sulphur dioxide: a point-source pollutant from coalburning industries,
oil and gas refineries
2
fluorides: a point-source pollutant from industries such as steel and
aluminium refineries and the production of ceramics, glass and
phosphate fertihsers
3
ozone
4
peroxyacetyl nitrates (PAN): oxidants formed in the atmosphere by
photochemical reactions between hydrocarbons and nitrogen oxides in
the presence of sunlight. Photochemical oxidants can cause plant injury
as far as 120-200 km from the origin of the primary pollutants. The
hydrocarbons come from the combustion of coal and petroleum-mostly
from vehicular exhausts.
Some pollutant-tolerant trees and shrubs
(From Bernatzky 1978)
Buxus sempervirens (sulphur dioxide)
Platanus x hybrids (sulphur dioxide)
Acer platanoides (fluorine, oxone)
Euonymus europaeus (fluorine)
Quercus robur (fluorine, oxone, nitrogen
trioxide)
Chamaecyparis pisifera (fluorine, nitrogen trioxide)
Fagus sylvatica (nitrogen dioxide)
Ginkgo biloba (nitrogen dioxide)
Robina pseudoacacia (nitrogen dioxide, nitrogen
trioxide)
Acer negundo (nitrogen trioxide)
Effects of other organisms
Other organisms tend not to be important as selection factors but are
commonly involved in tree decline. They may either be the primary cause of
damage or they may be secondary organisms taking advantage of a tree
weakened by other factors.
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People
People can do the right or wrong things by trees. The right things are careful
selection and appropriate maintenance. The wrong things include poor
selection, lack of maintenance, careless use of machines (particularly lawn
mowers and ‘whippersnippers’), inappropriate use of herbicides, and
disruptive construction around trees. Unfortunately, deliberate damage to
trees is an increasing problem. Usually this is by acts of indiscriminate
vandalism. However, sometimes a tree is poisoned or otherwise damaged by
someone angered by the loss of a water view or engaged in a dispute with
neighbours. Some people don’t like trees because they drop leaves, shade
their gardens or may shelter perverts!
Animals
Large animals such as horses, cattle, sheep and goats can damage trees by
grazing and debarking and by compacting the soil underneath. The excreta
of dogs and cats may enrich the soil around young trees causing nutritional
problems.
Soil organisms
These may be macro or micro, plant or animal, beneficial or detrimental.
Earthworms, mycorrhizal fungi, nitrogen-fixing bacteria and the multitudes
of micro-organisms involved in nutrient recycling have beneficial effects on
the soil ecosystem and thus on tree health. A change to less favourable
conditions especially anaerobic conditions may favour organisms which can
have detrimental effects on plant growth. Some tree species are more
susceptible than others to certain root rots. For example, Eucalyptus
marginata (jarrah) is sensitive to Phytophthora cinnamomi. However, in this
case, extra stress is placed on the trees by the effect of burning practices on
the nature of the understorey.
Pathogens
Like all organisms, trees are subject to certain diseases. Susceptibility to
disease depends on a number of factors including genetic predisposition,
changes in the tree’s environment, and wounding. Most pathogens appear to
be secondary problems arising as a result of a primary cause of stress. Wood
decay is an example of this process and will be discussed in Section 4.
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Insects
Like soil organisms, these can be useful (predators, parasites and
pollinators) or detrimental (leaf-chewers, sap-suckers, borers, and rootchewers). Native plants will always be eaten by native insects as everything
has its place in the food chain. A borer infestation is a symptom of a tree
already under stress. Repeated defoliations of a tree may require control of
the insect or may be a factor in future species selection. A knowledge of the
type of insect and the extent of damage may influence tree-care practices.
Borers, leaf-chewers and sap-suckers will be covered in other Unit 4?.
Birds
Most birds cause no problems to trees and, in fact, control many insect
pests. In some areas the overclearing of low, dense understorey plants has
destroyed the habitats of many small, insectivorous birds, which has allowed
the balance to tip in favour of the insects.
In some instances, however, birds can be harmful to trees. For example,
white cockatoos have been known to strip trees and snap branches, possibly
in the search for borers or possibly for mischief! Also, the mistletoe bird is
closely associated with the mistletoe plant which is a destructive parasite on
other plants (see next section on ‘Other plants’).
Other plants
Other trees and shrubs can compete for water, nutrients and light. In natural
systems this leads to the survival of the fittest.
Turf and ground covers
These present few problems for mature trees but turf in particular can reduce
the growth of young trees. Grass roots not only compete for water and
nutrients but may also have allelopathic effects (ie growth inhibition as a
result of toxic plant exudates).
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Climbing plants
These may affect trees in several ways:

constriction of young trees, causing deformity

competition for light-trees may be smothered

constriction of vascular tissues

competition for water and nutrients

increase in crown weight and wind resistance

concealment of structural weaknesses, decay or insect damage.
If you want a specimen tree, don’t grow a climber up it. If you do choose to
have a climber on a tree, then understand the consequences. Once climbers
such as Ivy, Wisteria and Bougainvillea are established they are very
difficult to remove. Just one more point: the adventitious roots of Ivy are not
parasitic; that is, they do not penetrate and feed off the host tree.
Mistletoe
There are over 70 species of mistletoe in Australia. Mstletoe is a native to
Australia and is adapted to drought and low-nutrient soils by being a parasite
(ie it feeds off a living host—it may eventually kill the host). Some
mistletoes are parasitic on a wide range of hosts, and others are very host
specific. It may be possible to find a mistletoe growing on a mistletoe
growing on another mistletoe! Mistletoes are found in all parts of Australia
except Tasmania and alpine areas. They are found in all plant communities
including rainforests, mangroves, and areas, woodlands and coastal areas
Mistletoe flowers twice a year and the fleshy green pseudoberries it
produces are eaten by a range of birds and animals. One of the main agents
of spread is the mistletoe bird Dicaeum hirundinaceum. This small bird
feeds on insects, fruits and particularly the pseudoberries of mistletoe. (By
the way, the bird is not found in Tasmania or alpine areas.) It has a modified
digestive system which allows the seed to pass undigested through the gut.
The sticky seed is then excreted onto (rather than off) a branch as the bird
has to wipe it on the branch to get rid of it. A sticky coating of viscin glues
the seed to the host’s branch. When conditions are right, the seed germinates
and produces an haustorium (the point of attachment between mistletoe and
host). Some mistletoes produce one point of attachment whilst others
produce a long creeping limb that repeatedly penetrates the host. Mistletoes
are usually well camouflaged in the host.
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Control
This should be considered for specimen trees and those of high value for
shelter or amenity. The best method is to remove the mistletoe from the
branch if the mistletoe is small enough or, if the infestation is heavy, to
remove the entire branch. This is labour intensive but is the most effective
method. A weak solution of herbicide can also be used but this is hazardous.
There is some biological control by fungi, insects, possums and birds (see
Hadlington and Johnston, pp 129).
Dodder
This is a leafless, rootless, twining plant that is parasitic on other plants. It is
a native to Australia but is also found in all other temperate and tropical
areas of the world. The vine, sometimes known as devil’s twine, develops
an extensive branching habit and contacts the host at many points,
producing haustoria. Dodder is propagated by seeds and by broken-off
segments that accidentally come in contact with a fresh host. Manual
removal is recommended as a control but this can be difficult and timeconsuming.
Signs of a tree under stress
The signs of stress in a tree vary from being quite obvious to very subtle. It
is an advantage, in recognising stress, to know what a healthy specimen of
the species in question looks like.
The signs of trees under stress can include:

wilting

leaf fall and a thin crown

‘dieback’- dead twigs and branches

epicormic shoots

basal suckers

small leaves and buds

pale or discoloured leaves

reduced or compressed extension growth

borer damage

early autumn colouring

slow callus growth

spring growth starting late

fruit and bud drop
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
heavy flowering or fruiting.
Once the symptoms are observed it is important to diagnose the cause of the
stress and correct or treat it if possible.
Causes of stress in trees
Symptoms of stress in trees may be caused by any of the following factors:
Abiotic factors—those relating to the physical and chemical properties of
the soil or environment. These include the suitability of:

soil type

soil pH levels

soil aeration and compaction

soil water availability and drainage

climatic factors such as light, temperature, wind exposure

cultural techniques employed (eg mowing, mulching, pruning)

sudden or gradual changes to any of the above

storm, accident or vandalisation
Biotic factors—those relating to other living organisms.
Competition for space, light, water, nutrients
Pathogens (pests and diseases) and parasites
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Selection of planting material
With regard to the climatic factors, it is important to choose plants which are
suitable to the local region. Plants struggling to survive outside their
‘comfort range’ will always be stressed and susceptible to problems.
Similarly plants (particularly large, long lasting trees) need to have adequate
space to grow, and not be subjected to ‘cutting back’ unnecessarily.
When people go to a nursery to buy plants, most spend some time looking at
the size and condition of the stem and leaves. Few stop to consider the root
system concealed in the container. Root characteristics are important when
selecting stock for planting. Root and shoot quality can determine not only
performance but survival. A well developed, healthy root system is essential
to a vigorous plant.
If you intend to buy a large number of trees from a nursery, it may be wise to
randomly select several plants for a close inspection of the roots. Containergrown stock should have fibrous roots sufficiently developed so that the root
mass will retain its shape and hold together when removed from the
container or when handled during planting. Plants should not be ‘potbound’: that is, have too many overdeveloped roots which may circle the pot
or which may be kinked.
If you can’t lift the container from the ground because the roots have grown
out into the ground, leave it and go elsewhere. Circling of roots within the
container can take place several times if plants are not potted-on at the right
stage. Even if peripheral roots are satisfactory (ie fibrous, brown roots with
white tips), roots within the centre of the root ball may be girdled from
previous potting. Be suspicious of this if, when removing the plant from the
container, loose potting mix falls away from the outer edges of the root ball.
Check also that the tree has grown into its container before you buy it,
particularly as in NSW trees and shrubs are priced and sold according to
container size. The tree may appear to be the right size for the container but
may not have grown into it.
If you are unsure of the condition of the inside of a root ball, then select a
random sample from a large order and wash the potting mix from the roots
and check for disorders. Some problem roots may be corrected by pruning
and ‘teasing’ at planting. However, if the roots are very woody and circled it
may be cheaper in the long run to throw it away and plant a better specimen.
Specifications for the purchase/supply of plants could include aspects of
root condition and/or inspection for root defects as a condition of acceptance
on delivery.
It is not uncommon to find that trees in the landscape are not thriving
because they were planted in a pot-bound condition. Signs of this can
include poor stability in the ground (hold the trunk and push the tree back
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and forth-if the tree moves in the ground, suspect a problem with the roots).
Other symptoms include undersized and paler leaves and susceptibility to
borers.
Figure 1: Pot bound roots
Container size, colour and shape may influence root development. Taller,
deeper pots provide good drainage characteristics; tapered and ribbed pots
tend to produce less root circling than flat-based pots; light-coloured pots
are cooler than black pots but are more expensive, are often translucent and
are short-lived.
A recent development in the in-ground production of super-advanced trees
has been the use of special fabric root-control bags that stimulate secondary
root branching within the bag. Trees are planted into the bags, which have
been ‘planted’ into the soil. They are filled with the same soil as surrounds
the bag. The roots grow outwards and when they contact the bag, they
penetrate it. However, as the roots that penetrate the bag increase in
diameter, the fabric restricts root expansion and causes a swelling or nodule
to develop just inside the bag. It is from this nodule that fibrous, secondary
roots develop. Following removal of the bag at planting time in its new site,
many root tips develop rapidly into the surrounding soil. This method has
advantages over traditional in-ground or open-ground production of trees in
that a maximum number of roots are retained after digging. Likewise, it
compares favourably to above-ground containerising of plants in that it
avoids high temperatures within the container and the problem of containers
being blown over. The system was developed by Carl Whitcomb from the
United States and is being used successfully in several large nurseries and
tree farms in Australia.
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Planting techniques
Many people feel that they have an innate ability to plant a tree. However,
like most things horticultural, planting is a skill! Planting has been covered
in module 5862MD Planting Skills but it is important to re-consider some
aspects of planting now, as the roots may be damaged at this stage.
The planting hole needs to be wider and deeper than the root ball.
It is most important in fine-textured soils (ie clays) that the sides and bottom
of the hole are opened up and are not smooth-sided—otherwise it is similar
to putting the plant in a larger container.
The excavated material should be broken up to a fine tilth.
Do not dig into the subsoil—especially in clays—as it may create a well and
therefore create anaerobic conditions.
With large plants or transplants, the soil may need to be mounded.
It is not recommended that organic matter be placed at the bottom of
planting holes as anerobic breakdown may either produce toxic gases or
displace soil oxygen.
Similarly, do not place a layer of gravel or coarse material in the bottom of
the planting hole. (Contrary to popular belief, this does not improve
drainage but actually impedes drainage as it usually creates a perched water
table. The soil and ensuing maintenance procedures should supply the roots
with adequate water, oxygen, nutrients and support.)
When planting, carefully remove the container. Matted and circling roots
should be cut or teased. It may be enough to just roughen the outside of the
root ball. This will break small roots and encourage branching of the root
system.
When placing the root ball in the hole, it is most important that the top of
the root ball is level with the soil surface, not below it. (One common cause
of poor establishment is planting trees too deeply. It is particularly important
to avoid this in fine-textured soils. Due to the difference in texture between
the soil and the potting mix (usually very coarse), any water applied tends to
stay in the soil and will not move into the root ball until saturation of the
soil occurs. Apart from aeration problems and possible trunk damage, trees
planted too deeply tend to suffer water stress. Another certain way to
damage tree roots is to dig too small a hole and then push the root ball in
with a size 10 boot!)
When backfilling, ensure good contact between soil and root ball. The plants
should be watered in and mulched.
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If staking is required, do not drive the stake or stakes through the root ball—
this also damages roots. (Staking will be covered in Unit 6.)
When planting bare-rooted stock, remove dead, diseased and twisted roots.
It is most important that bare-rooted trees are planted at the correct depth (it
is usually possible to see a soil level mark on the plant) and that the soil is in
good tilth for backfilling. There must be good contact between the roots and
soil, with no large air pockets otherwise the roots will dry out.
Figure 2: Correct techniques for planting
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Damage to the roots of established
trees
Stress factors can affect the roots of existing trees directly by killing them or
by preventing or reducing branching, elongation, radical growth and
mycorrhizal formation. Indirectly, stress can affect roots by reducing
photosynthesis and other physiological processes, which may lead to less
sugar for use by, or storage within, the roots, or disrupt transport of
materials between roots and leaves.
Stress factors which damage roots can include:

excavation and construction work

soil compaction

changes in the water table

soil buildup or surface sealing

contamination and gas leaks

pH and salinity changes

competition with turf and other trees

insects and diseases

drought

nutrient problems

girdling roots

damage to the crown or trunk.
Each of these causes will be discussed and some possible solutions
suggested.
Excavation and construction damage
Visit a construction site where trees have been retained and observe the
amount of room required for building work to proceed.
Apart from direct loss to roots by trenching, excavations and foundations,
construction around trees often leads to chemical and structural degradation
of the soil and changes in the water table. Direct root loss will make a tree
more susceptible to drought, and the reduction in water uptake will lead to a
reduction in the crown and hence photosynthesis. Trees may also be
destabilised and be more subject to windthrow.
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Recommendations
If trees are to be retained on building sites where excavation is to occur, as
much of the root system as possible (at minimum, to the drip zone) should
be protected by a root curtain or by sheet-piling.
Sheet-piling involves contiguous sheet-piles being driven vertically down
through the roots before excavation commences. (See Figure 3.)
Figure 3: Sheet-piling
This will cut the roots cleanly, retain the root ball intact and allow
excavation to occur up to a solid barrier. The sheet-piling should be in short
lengths to minimise crown damage. Once excavation and construction work
is completed, the piling is removed. A root curtain involves manually
digging a trench at least 500 millimetres wide and 1.5 metres deep between
the tree and the future excavation site. All roots are cleanly cut. (This
technique is used also when root pruning for transplanting.) On the
development side of the trench, a ‘fence’ is constructed of stakes (steel or
timber), wire mesh and hessian. (See Figure 4.)
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Figure 4: Root curtain
The trench is backfilled taking care to replace soil horizons in the correct
sequence. New roots can grow from the cut ends. Application of a rooting
hormone may assist in the initiation of new roots. Excavation of the site can
occur up to the root curtain.
If root treatment can be done only during the excavation, the roots should be
protected from drying out. An alternative could be to spray concrete onto the
exposed root ball. With all techniques it is important to maintain adequate
moisture and drainage within the retained root ball.
When trenching is necessary for laying of pipes or cables, it should ideally
be carried out beyond the drip zone even though roots will still be destroyed.
If it is impossible to trench so far away, then it is best to take the trench
below the centre of the tree where there are fewer roots. As far as possible,
hand digging should occur in sensitive areas and large roots tunnelled under
rather than severed. If water pipes are to be installed they should be of PVC
with solvent-weld joints and with enough space between root and pipe to
allow root expansion (see Figures 2.6 to 2.9, Hadlington and Johnston 1988
or later).
If developers are serious about retaining trees on construction sites, and
governments are serious about imposing tree preservation orders, then good
specifications must be written and followed in order to provide the tree with
means of survival and growth. It is inevitable that some damage will occur
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but carefully prepared specifications and liaison between engineers and
horticulturists and arborists may keep it to a minimum.
Soil compaction
Take a screwdriver and, using the palm of your hand, try pushing it into an
area of lawn or garden which receives little traffic. Try again in an area of
heavy traffic. Feel the difference?
Compaction is common in parks, streets and public areas and is a greater
problem in fine-textured soils than in coarse soils. Soil aeration is reduced
and eventually root functions are impaired. This leads to a deterioration of
the crown and a reduction in overall growth, although the effects on growth
may not be noticeable for several years. Various researchers have found that
when compaction reduced air space to less then 15%, root growth was
inhibited; when dropped to 2%, root growth was negligible. Air space
should be at least 20% for adequate gas exchange between soil and
atmosphere, and oxygen concentration should not drop below 10%.
Recommendations
All problems have a cause. What is the cause of the compaction? Can the
cause be treated?
The most common cause is foot and vehicle traffic. In some cases traffic can
be redirected, but in many cases (eg parks, golf courses) it is unavoidable
and hence symptoms must be treated. Soil can be aerated by coring, slicing
or fracturing with compressed air.
Changes in the water table and soil water levels
Whether the water rises from below or excess water is applied to the
surface, flooding, like compaction, leads to poor soil aeration. However, in
the case of flooding, the oxygen deficiency is immediate, whereas the effects
of compaction occur gradually. Flooding immediately affects most aspects
of tree growth—height, leaf growth, cambial activity, reproduction, leaf
initiation and it may also induce leaf senescence and abscission. High soil
moisture levels may also encourage pathogens such as Phytophthora fungi,
and predispose otherwise resistant species to attack. Waterlogged soils
encourage anaerobic breakdown of organic material, which can lead to the
production of phytotoxic compounds such as sulphides. The roots of some
trees under waterlogged conditions manufacture products which can also be
toxic to the tree; for example, cherries produce cyanide. Plants can be
indirectly affected by flooding through the effect on micro-oganisms
involved in nutrient recycling. For example, the bacteria involved in the
nitrogen cycle stop functioning in saturated conditions.
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Rising water tables can have even more devastating effects on tree growth if
the water is saline. Lowering water tables by drainage can also damage trees
which have adapted to wet conditions. These trees will suffer from a water
shortage.
Some species of trees (eg Nyssa aquatica, Taxodium distichum) found in
swamps and floodplains, have evolved special anatomical features that
allow conduction of oxygen to at least 300 millimetres below the surface of
the water and special metabolism that eliminates toxic byproducts of
anaerobic respiration. Other species which grow in wet sites (eg Willows Salix spp) have shallow and multi branched root systems.
Recommendations
Sometimes flooding occurs because of accidents such as burst water mains
or sometimes as a result of excessive rainfall. In these situations it is
important to recognise that the trees will be stressed and that some dieback
is likely. Removal of excess water as quickly as possible is the only
remedial action that can be taken at the time.
In areas prone to flooding, species adapted to such conditions should be
planted.
Water tables may be changed as a result of construction and site
development. This is best anticipated at the planning stage and decisions
made to either provide drainage or to supply water to trees retained on the
site.
Soil build-up and surface sealing
In your travels around streets, car parks and other built-up areas, assess the
health and vigour of trees that have been retained whilst the area around
them has changed.
When an area is sealed or the soil level is built up, soil aeration and water
penetration are reduced. This not only directly affects root functions and
thus overall plant growth but it also affects soil organisms. The most useful
soil organisms are aerobic. Under anaerobic conditions, aerobic organisms
are replaced by anaerobic species which may produce undesirable products
such as toxic gases. Mycorrhizal fungi will also be affected, thus further
reducing root functioning. A reduction in water and nutrient uptake
eventually affects the leaves and their ability to photosynthesise. Hence
sugar production and, the products of metabolism are reduced and the tree
may become susceptible to secondary problems such as borers.
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Figure 5 shows the effect that build-up or surface sealing may have on a
tree.
Figure 5: The effect of soil buildup
One of the signs of soil level build-up is an absence of flaring at the base of
trees. The tree comes out of the ground like a pole.
Recommendations
In some cases, increasing the soil level around trees can be avoided. For
example, on construction sites materials can be stored away from trees. If
level changes are unavoidable, maintain as large an area of the drip zone at
original level and then backfill with very coarse material (eg gravel).
However, this may not guarantee survival. Care should be taken to avoid
excess run-off into these situations as this will cause drainage problems.
Drainage systems may have to be installed as well as the gravel. If surface
sealing around existing trees is necessary, use a material or materials which
will allow some aeration, such as, in decreasing order of permeability,
gravel, unit pavers on coarse sand (no mortar), asphalt and concrete. Other
materials include decomposed granite, no-fines concrete, ‘B.G.’ blocks or
grass-crete blocks, Ritter-rings® and similar products.
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Figure 6: Alternative to impervious paving.
In many cases of build-up or surface sealing, trees may take up to ten years
to die. Trees planted either in the new soil levels or after paving survive
better than those older trees retained on the site. Note that brick or dry-stone
collars are not effective in preserving trees after soil buildup (see Figures 2.4
and 2.5, Hadlington and Johnston 1988 or later). This is because they collect
water and litter, can be a danger to small animals and children, and lead to
poor aeration, drainage and water infiltration.
Drought and nutrient problems
Drought can affect roots both directly and indirectly. Root tips can die from
lack of water; drying of soil prevents root penetration; photosynthesis is
reduced and hence food for root growth and storage is reduced. Therefore,
drought can reduce root growth during the current season and also at the
beginning of the next season through its effect on food reserves. The
deficiency or excess of certain nutrients may directly damage roots or may
indirectly affect plant growth and storage of reserves. Plants require 16
essential elements in the correct proportions, for normal growth and
reproduction, so fertiliser application must be carefully considered.
Excessive fertiliser causes problems with salinity and toxicities.
Some plants have adapted to drought stress by morphological modifications
(eg sunken stomates, pubescent new growth, waxy leaves). Likewise, many
plants (particularly some Australian natives) have ways of coping with low
nutrients: nodulation in legumes and Casuarina spp for nitrogen fixation;
for example: nitrogen-fixing associations between blue-green algae and
cycads: mycorrhizal associations: proteoid roots; lignotubers and epicormic
buds. These are explained more in Section 4.
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Recommendations
When selecting plants for sites, consider available moisture and nutrients
and either select accordingly or provide adequate moisture and nutrients.
Avoid applying excessive fertilisers and avoid changing the pH, to avoid
upsetting nutrient availability. If nutritional problems are suspected, check
the pH and, if necessary, have the soil analysed. If plants have been
overfertilised, leaching may be required. If pH is a problem, adjust it
accordingly.
Contamination, pH changes and salinity
A significant proportion of the root system of trees is concentrated in the top
few millimetres of soil and extends well beyond the drip zone. The roots
intermingle with leaf litter, grass roots and are even found in the cracks and
crevices in concrete and asphalt paving. Due to this proximity to the surface
they are vulnerable to damage by chemical contamination caused by
herbicides used to kill broad-leaved weeds.
Dicamba is an example of a herbicide which is known to damage trees.
Some organisations have found problems in trees where Roundup(has been
used repeatedly around their roots. This is a widely used herbicide but more
research on its action in soil and tree roots is required. Applications of
Velpar , bromacil and other sterilants and residual herbicides can kill trees
over 20 metres distant from where they have been applied. Other chemicals
that can damage roots include fuels and oils (including diesel), solvents,
detergents and chemicals used for root control in pipes. Gas from gas leaks
or from anaerobic respiration (eg old tip sites) can also damage roots
directly and indirectly by displacing soil oxygen.
Rapid changes in pH can occur around building sites due to contamination
by lime and cement. This can upset nutrient availability, leading to
deficiencies and/or toxicities of various elements.
Salinity from rising salt or from over-fertilising damages roots and interferes
with water uptake.
Recommendations
If residual herbicide damage is suspected, several diagnostic tests can be
used.
A bio-assay using annual plants can be carried out to see if the soil is still
toxic enough to damage new plantings. Toxic levels of herbicide will twist,
stunt or kill the test plants.
The damaged tree can be analysed for herbicide residues.
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The surrounding soil can be analysed for residues.
Chemical analysis can be quite expensive. If you suspect a particular
herbicide, check with the manufacturer about symptoms of phytotoxicity
and necessary treatment. Soil containing a toxic level of herbicide can be
removed, mixed with organic matter, leached or planted with tolerant plants,
depending on the herbicide, soil texture, drainage, area involved, desired
planting and the available budget. Activated charcoal, at 40 grams for each
150 millimetres depth of soil in a hole one metre in diameter, should be
thoroughly mixed into the soil. This approximates the recommended rate of
150 parts (weight) of activated charcoal to one part of active herbicide in the
soil. Other researchers have reported that nitrogen-enriched sawdust could
be as effective as charcoal.
Gas leaks can be detected by smell and by gas detection meters. The source
of the leak should be found and repaired, the soil aerated; and the plant
replaced. Plants tolerant of poorly aerated soils (such as swamps) may be
more tolerant of gas-contaminated soils.
pH and salinity problems should be treated at the cause. In some cases the
pH and salinity are site constraints and therefore tolerant plants should be
selected.
Competition
Turf and ground covers are not usually a great problem for established trees.
However turf can greatly reduce the growth of young trees. The roots of
grass intermingle with tree roots and directly compete with them for water
and nutrients. Turf may also increase run-off. These factors tend to
exacerbate the effects of other stresses such as drought and nutrient
deficiences. Some turf species have been found to have allelopathic effects
on young trees. Activities associated with lawn care, such as applications of
lime, herbicides and fertilisers, may also damage tree roots.
Competition can also come from other trees and shrubs in the landscape.
Recommendations
Adequate irrigation and appropriate fertilising may help to overcome some
of these competition problems. If turf can be kept at least 300 millimetres
from the trunks of young trees and shrubs, most retarding effects will be
eliminated. The area should be mulched. Trials using applications of organic
mulch versus grass and bare soil as surface treatments have shown that
mulching produces several beneficial results. Under organic mulch, rooting
density and root surface area were increased and soil moisture content was
significantly higher.
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The mulched circle also reduces the exposure to damage from whippersnipper type trimmers.
Insects and diseases
When you’ve been digging in the garden, have you ever noticed curled
white larvae with quite prominent legs? (See Figure 3.6, Hadlington and
Johnston 1988 or later.) They are probably Christmas beetle or scarab beetle
larvae. These larvae live underground and feed on plant roots. In favourable
seasons, these larvae can seriously debilitate and even kill young trees. They
can also be a serious pest of containerised plants.
There are several species of root rot fungi that can cause death or instability
including Armillaria spp and Phytophthora cinnamomi. Information on
these organisms is provided in Hadlington and Johnston pp 93-94.
In the production of trees in nurseries, seedlings may be susceptible to
damping-off diseases. The organisms concerned are varied and include
Pythium spp, Phytophthora spp, Rhizoctonia spp, and Fusarium spp.
Damping-off is favoured by low soil temperatures, surface compaction, soils
of poor structure and excessive watering.
Recommendations
If Christmas beetle larvae are numerous and causing major problems, an
insecticide such as ‘Lawn Beetle Killer’ & can be applied as directed on the
label.
For Amillaria and Phytophthora refer to Hadlington and Johnston or your
Pest and Diseases modules.
Damping-off diseases are best controlled by avoiding the conditions that
cause them, by pasturising potting mixes at 60’C for 30 minutes or by using
specific fungicides.
These problems and others will be dealt with in more detail in Section 5.
Girdling roots
Trees can be weakened or killed by roots that girdle the trunk or main roots.
Circling roots usually start in the nursery as a result of inadequate pottingon. Sometimes surface roots are deflected by poor soil or rocks and may
grow back towards the plant.
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Recommendations
Girdling roots are best avoided by potting-on at the right stage, by checking
roots before planting and by providing adequate room for root growth.
Sometimes surface girdling roots can be removed with a chisel and mallet. If
one root is girdled, others lower down may also be girdled. This may require
careful excavation with a hand trowel close to the trunk to determine the
extent of girdling.
If girdling is severe, the plant will not only be inefficient but may be
unstable. In severe cases the tree should be removed.
Figure 7: Girdling roots
Damage caused by roots
Most people tend to ignore the root systems of trees—that is, until the roots
cause problems. A healthy, vigorous root system is essential for a healthy
and vigorous tree but it is true that the root systems of trees can themselves
cause problems. Fore-knowledge about which trees are likely to have too
vigorous root systems for a particular site can lead to more careful plant
selection and avoid serious problems later.
Roots can crack and plug sewerage and drainage lines, lift and break kerbs,
footpaths, paving and foundations, and may exacerbate the problems of
swelling and shrinking clays which may cause further structural damage.
Tree-caused damage makes up a large component of the insurance and legal
costs of local councils. This includes damage caused by street trees to
private property and to pavements, in the latter case making them uneven
and causing people to have accidents. This section will look briefly at the
main types of problems caused by roots and at some possible solutions.
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Damage to sewerage and drainage lines
Roots can damage pipes in two ways: by entering through cracks in the
pipes and then growing and expanding, or by cracking them by external
pressure as a result of root expansion.
Roots are not the only cause of damage to pipes. Other factors to consider
are:

soil movement caused by site disturbances or natural causes

movements due to shrinking clays

the age and type of pipes

the quality of the installation work.
Damage to sewers can be prevented by laying PVC plastic pipes with
solvent-weld joints, rather than earthenware pipes. Other preventive or
treatment measures include the following:
Avoid planting large trees with vigorous root systems within two metres of
sewers and drains. (Several local councils, the Water Board and other
authorities provide lists of plants considered unsuitable for planting near
drains and sewers).
Have a plumber remove root blockages—this is temporary and may have to
be carried out on an annual basis.
Root-prune near pipes on a regular basis.
Excavate a trench along the line of the sewer or drain and encase the
pipeline with about 150 millimetres of concrete all around the pipe.
Excavate a trench between the tree and the pipe and fill the trench with
concrete.
Treat drains and sewers with commercially available chemicals (but
carefully investigate the effects of these chemicals on trees, sewers and drain
outfalls before you use them).
Trees commonly associated with drain problems include Ficus spp, Populus
spp, Salix spp, Cinnamomum camphora, Erythrina spp, Robinia
pseudoacacia and Ulmus spp. (Refer to the authorities mentioned above for
more complete lists.)
Structural damage to foundations
Roots can cause damage to foundations of buildings by expanding and
physically disrupting foundations, and by removing water from soils near
foundations causing the soil to shrink and thus the foundations to settle. The
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latter problem is particularly serious if the soil is a shrinking and swelling
clay and is made worse by dry seasons and droughts.
As a tree’s demand for water levels off after it reaches maturity, it is less
likely that the roots of mature trees will cause damage to new foundations
nearby, unless the roots are cut during excavation work. Trees planted after
construction will cause most damage. Older houses with shallow footings
are most susceptible.
Tree removal is one solution to the problem, but if the tree is to be retained,
the following actions could be considered:
Soak the soil near the foundations to alleviate shrinkage damage.
Underpin the footings to a depth which avoids the drying effects of the tree
(see Figure 8).
Figure 8: Underpinning of foundations
In shrinking clays, position foundations carefully by carrying them through
the clay layer to firm sand or bedrock.
Construct a root barrier between the tree(s) and footings by trenching and
then filling with lightly reinforced concrete (see Figure 9).
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Figure 9: Root barrier between tree and footings
Trees near houses should be selected for shallow and compact root systems
if foundations problems are likely. Not only can the growing of trees near
foundations be a problems, but so too can the removal of trees, especially on
the shrinking clays. Due to the fact that a tree uses water in the ground, the
removal of a tree may mean extra water in the soil, expansion of clay
particles and thus heaving of the ground under wet conditions.
Damage to kerbs, footpaths and paving
Footpath and kerb damage is a common occurrence and major expense for
local government. Roots cause problems to these structures by simply
attempting to grow and survive in a fairly hostile environment. Due to soil
compaction around street plantings and the aerobic nature of roots, many
problems are caused by roots forced to grow near the soil surface, and
subsequently expanding. The root/trunk interface is prone to maximum
growth and commonly causes buckling and cracking of paving when too
small a hole has been provided for the plant. Most footpath problems are
associated with the roots of young and vigorous trees.
To avoid future problems with pavement distortion and kerb damage, the
following techniques could be considered:
At planting, provide planting holes that are as large as possible with deep
soil and good drainage. Adequate soil depth discourages an excess of
surface rooting. The soil should be as uniform as possible to avoid interface
problems.
Select trees that have deep roots; are small to medium at maturity, and are
suited to harsh environments. In some situations it may be suitable to plant
shrubs such as Photinia fraseri, Nerium oleander or other species which can
be trained as trees.
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As roots follow the path of least resistance, create aerated and friable
conditions away from problem areas. This may be done by ‘water-jetting’
(forcing water deeper into the soil to create spaces) or by deep ripping
before pavement construction.
Avoid heavy pruning and lopping of trees as this can upset the root/crown
ratio and lead to increased root growth.
Prune the roots periodically. As with pruning the above-ground parts of
trees, root pruning should start early in a tree’s life to avoid major damage to
the plant. Modified concrete cutters can be used for root pruning near
paving.
The use of root-control boxes or deep-root planters has been tried in some
areas. These planters are designed to force root growth downwards.
However, because of the tree’s need for aeration, the roots tend to grow
back up towards the surface after being temporarily deflected. Therefore
these products generally delay root problems rather than solve them.
In Canberra, polythene sheeting and the root-retarding herbicide dichlobenil
have been trialled in an attempt to limit root invasion. A trench was dug and
dichlobenil (as Gasoron ) was applied at the rate of one kilogram per cubic
metre of backfill. This herbicide kills treated root tissue without affecting
untreated parts. For the trees used in the trial (Eucalyptus bicostata—
Tasmanian Blue Gum), the barriers were still working after eight years (see
Figure 10).
Figure 10: Root control Barrier
The digging of trenches has been indicated as a possible means of
controlling problem roots, in combination with root pruning. However,
trenching has some problems of its own:
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
it may destabilise trees;

it may lead to crown dieback and susceptibility to other stresses and
secondary problems;

suckering from severed roots may occur in some species, (eg Elms—
Ulmus spp);

it may stimulate new root growth;

it needs to be an ongoing treatment unless permanent barriers are
constructed within the trench.
As with most tree problems, the possible problems caused by roots should
be anticipated before planting and either suitable species selected or
appropriate maintenance procedures initiated early.
In the following Units we will continue to learn about tree disorders and
stress, in relation to tree wounding, pruning techniques and pests and
diseases.
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Summary
‘Out of sight, out of mind’—this attitude is a major reason why tree roots
are so easily damaged. When you look around the typical urban environment
it is easy to see why so many trees are declining. Apart from being damaged,
roots can also cause damage to roads, buildings and other structures. Both
problems of damage to roots and damage caused by roots should be
addressed in the planning and implementation stages of projects so that the
problems are miminised or avoided. The next unit looks at damage to the
trunks and branches of trees.
1 Read the following and circle the letter of the best answer.
(a) When selecting container-grown stock, the root systems of the plants should:
(i) consist of large numbers of woody roots circling the pot
(ii) have no or few roots visible near the outer edges of the root ball
(iii) be sufficiently developed to retain the shape of the root ball but not
be overgrown
(iv) always be reduced by half to produce hardy plants.
(b) When planting container-grown stock:
(i) avoid digging into the subsoil, thus creating a well
(ii) always plant the tree slightly deeper to encourage stability
(iii) leave smooth sides on holes in clay to encourage the roots to grow
down
(iv) place a layer of gravel at the base of the hole to provide drainage.
2
Label the following statement True (T) or False (F).
(a) Direct loss of roots may predispose a tree to windthrow.
(b) Damage to mycorrhizal fungi will have no effect on the health and vigour of a tree
(c) Sheet-piling is a technique for protecting trees on construction sites.
(d) If trenching is to take place close to trees, it is best to take the trench below the
centre of the tree.
(e) Compaction occurs readily in sandy soils.
(f) The acceptable minimum for soil oxygen concentration is 5%.
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(g) The nitrogen cycle is unaffected by soil moisture levels
(h) If the soil level around a tree must be increased then it is best done gradually
with coarse material
(i) A bio-assay using annual plants is a useful tool for detecting the presence of
residual herbicides
(j) Turf growing to the base of young trees has no effect on their growth rate.
3
Why do oxygen concentrations decrease and carbon dioxide concentrations increase in
the soil in the growing seasons?
4
Why may trees be damaged by broad-leaf herbicides applied to lawns?
5
You have been called to inspect two trees, both ten years old Corymbia maculata
(Spotted Gum). Tree number one is reported to be dying (and has borer damage) and
your clients are worried about the second tree. On questioning the owners, the
following sequence of events emerges:

Two years ago, an in-ground pool was installed and the area of soil between the
house and the pool was built up by 200mm, concreted and paved. The trunk of
tree number one was protected by sacking. Tree number two was unaffected by
pool construction as there was no access for large machines.

One month ago a new drain was constructed to carry overflow from the pool and
stormwater from the rool. A -small trenching machine dug a trench 800 mm
deep, 250 mm wide and 1 m inside the side fence. The stormwater pipe was
installed but as yet the trench has not been refilled.
(a) In point form explain why tree number one is exhibiting the symptoms of die-back
and borer damage.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(b) List two steps that could be taken to compensate for the trenching injuries to tree
number two.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
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6
Answer the following questions, imagining that you have been called to a site where
trees are suspected of damaging sewer pipes.
(a) How do roots damage pipes?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(b) Apart from roots, what other factors may be involved in damage to pipes?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(c) How would you confirm the cause of the problem?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(d) If tree roots were the cause, how might you treat the problem:
(i) in the short term?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(ii) in the long term?
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
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7
You are in charge of planning a new street planting in a shopping centre that is soon to
be paved. Your employer is keen to minimise the risk of litigation as a result of root
problems. List five possible methods of avoiding pavement distortion and kerb
damage.
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
_____________________________________________________________________
Check your answers with those given at the end of this section.
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Check your progress answers
Check your progress 1
1
(a) When selecting container-grown stock, the root systems of the
plants should:
(iii) be sufficiently developed to retain the shape of the root ball
but not be overgrown
(b)When planting container-grown stock:
(i) avoid digging into the subsoil, thus creating a well
2
a, c, d, h, i all True
b, e, f, g, j all False
Respiration by roots and soil organisms and decomposition of organic
matter all use oxygen and produce carbon dioxide
4 Surface roots in the top levels of the soil can absorb them.
5 (a)
possibly:
physical root damage
compaction and/or soil level build up cause poor aeration and
restricted water infiltration
(b)
(i)
(ii)
Cut roots cleanly, possibly add a rooting hormone
refill, keep roots moist to reduce stress during recovery
6
(a) Enter through cracks and expand
crush externally by growth
(b) Soil movement
Clay shrinkage
Age and type of pipe
Quality of installation
(c) Excavate or use an electric eel
(d) (i) annual use of the electric eel, or
root-prune near the pipe regularly, or
use chemicals in the drains to kill the roots
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(ii) excavate pipe and enclose in concrete and/or fill trench with
concrete
7
38
Use large planting holes
Choose species with deep root systems
Provide friable and well aerated conditions away from the paths
Avoid heavy pruning and lopping as this increases root growth
Install root control barriers or deep root planters to force growth
downwards.
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