Download Topic 1 Plant Growth

Topic 1
Plant Growth
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1.1 Plant Physiology
Plant physiology is the study of the way in which plants function and the
processes they perform.
The basic requirements for plants to be able to function and grow are:

light

water

air – carbon dioxide and oxygen

nutrients – macronutrients and micronutrients temperature.

space
We could add to this list, support, in the form of soil or media to anchor the
plant in, and sufficient space to develop properly.
All these environmental factors must be at optimal levels for the plant being
grown in order to achieve the best growth results. Also, all the processes
discussed below will be affected by any change in light levels, amount of
available water, the oxygen and carbon dioxide levels, the amount of
available nutrients and the temperature levels.
In plants the basic processes leading to growth and reproduction are:

photosynthesis

respiration

transpiration.
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Photosynthesis
Photosynthesis is the process in green plants that synthesises (builds up)
sugar and starch from water and carbon dioxide, using energy absorbed by
chlorophyll (a pigment in leaf cells) from sunlight. It is an energy - storing
process.
Location
Photosynthesis occurs in the chloroplasts, which are microscopic organelles
in the cells of green parts of plants. So it occurs largely in leaves, but also in
green stems and young fruit, and the sepals of some flowers, etc.
Chloroplasts contain the pigment chlorophyll.
Basic requirements
The basic requirements of photosynthesis are light from the sun, water
absorbed by the roots from the soil, and carbon dioxide from the air, which
enters the leaves through microscopic pores in the leaf epidermis (or skin).
Basic process
Energy in the form of sunlight is absorbed by the pigment chlorophyll in the
chloroplasts and is used by the plant to combine carbon dioxide with water to
produce a sugar. Oxygen is a by-product of this reaction and diffuses out of
the leaf into the atmosphere. The energy is now fixed as chemical energy in
the bonds of the sugar molecule. This process can be summarised as
follows. Note that in reality it is extremely complex and consists of many
steps. We need only concern ourselves with the ingredients and the
products.
Sunlight
Carbon dioxide + water
sugar + oxygen
Chlorophyll
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Result
The sugar produced is the primary form of food required by the plant for its
energy and carbohydrate needs.
The sugar is:

built up into more complex organic materials such as complex
carbohydrates, proteins, fats;

broken down by the process of respiration to release energy for other
reactions in the plant; or

stored as starch to be mobilised later as required.
Significance
Photosynthesis is an enormously important process. The dependence of
most other organisms on the food produced by plants in this way makes it
the first step in the food webs of whole ecosystems. The oxygen produced
by this process is also vital to the existence of most living things. Therefore,
photosynthesis is the foundation of nearly all life on earth.
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Respiration
Respiration is usually thought of as ‘breathing’; but the true reactions of
respiration take place at a cellular level in all living things. This is the process
or series of chemical reactions by which an organism breaks down food
substances to release energy for all its other activities.
Location
Respiration proceeds continuously in every living cell. This is the reason that
mangrove roots inundated by salt water, and deciduous trees in winter and
stored seeds (and hibernating bears) still need an air supply.
Basic requirements
The basic requirements of respiration in plants are sugars (which have been
produced by photosynthesis) and oxygen from air.
Basic process
Sugars and oxygen are converted into carbon dioxide and water and the
energy stored in the sugar molecules is released for other reactions.
sugar + oxygen
carbon dioxide + water + energy
Result
The energy stored in the sugar molecules is now made available for other
physiological processes in the plant.
You will notice that the two processes of photosynthesis and respiration are
opposite in action. If a plant is only photosynthesising sufficiently to provide
for its respiration requirements the plant will not thrive. This sometimes
occurs at low light levels and the plant cannot continue in this state for very
long. Under normal conditions, photosynthesis will produce far more sugar
than is needed for respiration, so a healthy plant will have plenty of sugar for
both growth and storage.
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Transpiration
Transpiration is the loss of water vapour from (mainly) the leaves of plants. It
differs from simple evaporation in that it takes place from living tissue and is
therefore influenced by the physiology of the plant.
Because plants need a constant supply of carbon dioxide for photosynthesis
(in sunlight) and produce oxygen at the same time, the leaves have special
pores in the epidermis called stomates. These allow the carbon dioxide to
diffuse in and the oxygen produced to diffuse out of the leaf. This is known
as gas exchange.
Because the inside of the leaf is saturated by water and water vapour it is
often much more humid inside the leaf than in the outer air. A diffusion
gradient is set up and water vapour diffuses out of the leaf. Under certain
conditions, such as very hot sun, dry wind, low soil water, the plant may lose
so much water by transpiration that it suffers water stress. The stomates will
then close to prevent more water loss. Photosynthesis may cease due to
lack of carbon dioxide in the leaf, but this is less damaging than continuing
water stress would be.
When transpiration is excessive it is often harmful to the plant, causing
wilting, and even, in extreme cases, permanent wilt. However moderate
transpiration is beneficial as it facilitates the upward movement of water and
nutrients through the stem to the leaves. It may also be beneficial in
preventing overheating of leaves in direct sunlight as the evaporation causes
cooling.
Many Australian native plants have evolved modifications to reduce and
control water stress through excessive transpiration. Plants growing in arid
regions, or on free draining soils in areas of sporadic rainfall often have
leathery leaves, of reduced size, coverings of white reflective hairs etc to
reduce transpiration losses.
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1.2 Environmental effects on plant
growth
We know that environmental factors affect the basic plant processes of
photosynthesis, respiration and transpiration. Let us now consider how they
affect plant growth.
Growth
Growth is the sum total of the various physiological processes that combine
to cause an increase in the dry weight of an organism and an irreversible
increase in size.
In plants, growth involves:

photosynthesis - fixation of light energy and inorganic carbon into
sugars

uptake of mineral nutrients and water

assimilation and metabolism of these basic components into all the
complex substances required for life to proceed.
Plant growth is usually confined to meristems, where cell division occurs,
and adjacent regions where cell elongation and enlargement is still
continuing.
All plants depend for optimal growth on the following:

Light

Water

Nutrients

Air (carbon dioxide and oxygen)

Temperature.
Plants differ as to the amount of these essentials that they require for
healthy growth; for example, cacti like very dry conditions and plenty of heat
and bright light, whereas most ferns need shaded, cool and moist conditions.
While each species has an optimal level of light, moisture, nutrients and
temperature at which maximum growth is assured, they will tolerate a range
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of conditions above and below the optimal, although growth and general
vigour may decrease. Some plants tolerate great variations from their
optimal conditions while others have a very narrow tolerance range.
For example, Melaleuca quinquinervia will tolerate great variations in soil
moisture from dry to waterlogged conditions. It will also tolerate great
temperature variations, from frosty cold nights to hot days. Cyclamen
persicum is a plant whose tolerance is very poor; it needs well drained soils,
otherwise it develops bulb and stem rot, and it will go limp and wilt under
warm to hot temperatures.
Light
Light influences various plant processes (not just photosynthesis) in a
number of ways. The nature of the light that plants receive is not constant;
any of its three main properties, which may affect plants, can alter. These
properties are intensity, quality and duration.
Light intensity
Light intensity at the earth’s surface varies with location and season. The
main factors are the angle at which the sun’s rays strike the surface and the
amount of the earth’s atmosphere that the rays have to penetrate. This
depends upon latitude, topography, season and time of day, and factors
such as dust, cloud and pollution.
Figure 1 - Light intensity and angle of incidence vary with latitude.
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Figure 2 - Seasonal effect on light intensity.
Other influences on the intensity of light experienced by plants will include:

overshadowing by other plants, buildings, walls, etc.

the lower leaves or branches of plants being shaded by leaves or
branches above

water vapour (present as cloud and fog) in the atmosphere

solid particles (dust, smoke and pollution) in the atmosphere.
The effect of light intensity on plants
Some plants have adaptations which suit them to growing in situations of
high light intensity. These plants often have a reduction in leaf size, reflective
surfaces and leaves arranged for maximum shading. Other plants are
adapted to areas of low light intensity. They often have thin stems, large thin
leaves and the rate of production of chlorophyll may be greater than for sun growing plants. Rainforest plants and plants suited to indoors usually have
very dark green leaves.
Growing plants in higher light intensities than they are adapted to can result
in the plant producing more sugars but having less water available than
normally. Plants in such conditions will generally produce smaller and thicker
leaves with heavier cuticle (waxy leaf covering). The plants are often shorter,
with shorter internodes and a greater amount of supporting and conducting
tissue. Higher light intensity can also cause the production of more flowers
and fruit.
Growing plants under lower light intensities than they require will lower the
rate of photosynthesis and production of sugars, and so growth can be
retarded. Shaded leaves are generally thinner.
When plants are moved from high to low light conditions, it takes six weeks
for them to become acclimatised to the lower light conditions.
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Plants growing in the absence of light will generally show etiolation. Etiolated
plants produce long, thin stems with long internodes. Leaves fail to expand
and the plants are white, due to lack of chlorophyll development.
Rate of growth
In general, increasing light intensity will cause an increase in growth,
because photosynthesis will be increased by the increase in temperature of
leaf surfaces and by the direct influence of light causing the stomates to
open. As well, increased transpiration will bring about an increase in
absorption of water and nutrients from the soil, and also an increase in the
rate of translocation of dissolved substances through the plant.
Once the increase in transpiration becomes high enough, the plant will lose
too much water, leading to an internal water deficit which will restrict growth.
So, increasing the light intensity will cause an increase in growth—but only
up to a certain point.
Light quality
Light quality will vary according to what wavelengths of light are contained in
the light. The chlorophyll pigment works with light of the violet - blue and
orange - red wavelengths. In open sunlight there is no problem, but placing
plants under coloured awnings (green particularly) results in very little
growth. Shadecloth is not a problem, because white light passes through the
holes in it, but barriers like solid green fibreglass will seriously restrict plant
growth.
Plants growing under glass or artificial light will also experience poorer light
quality than in natural conditions. Glass screens out shorter wavelengths
(violet end of the spectrum) and artificial lights do not provide the same
range of wavelengths as natural sunlight.
Duration of light
Day length depends on latitude, time of the year (season) and the local
influence of topography of the land (for example, compare a plant on a
mountain top and one in a deep ravine).
Apart from the direct effect that duration of light has on photosynthesis and
growth, there are also some other special responses. Day length, or more
accurately night length, triggers production of flower buds in many species.
This period can be manipulated in glasshouses to provide out of season
flowering.
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Temperature
Limited on the lower side by the freezing of water and on the upper side by
protein denaturation, life exists only in a narrow range of temperature.
For each plant, there is a range of temperatures over which it will be able to
survive. Within this range lies the temperature at which growth will be
maximised. On either side of the range for growth there are temperatures
which are not extreme enough to cause death, but at which growth will
cease and the plant will show heat or cold rigour. At these temperatures
plants will survive for a time on reserved food, but will not be able to exist
permanently.
Figure 3 - Effect of temperature on plant growth.
The ranges of temperatures a plant can survive depends on the type of
conditions it has evolved in. Cool temperate plants have a different range
from sub - tropical plants.
In general, chemical reactions increase with increasing temperature—usually
doubling for every +10C. However the reactions in plants are controlled by
enzymes, and these are denatured by excessive heat. (Compare the
changes occurring when we fry an egg, which is largely protein.) So the
chemical rule only holds true for biological reactions up to a certain
temperature (about 50C); above that, the processes gradually slow down
until growth ceases and eventually death occurs.
A combination of morphological and physiological features allow many
species to withstand extreme temperatures in their environment. Reduced
leaf size, white reflective hairs, glossy leaf surfaces and the angle at which
leaves are held by the stem can all reduce leaf heating. For cold resistance,
deciduous leaves, high sugar content of leaf sap (which acts like anti freeze to prevent frost damage), dormancy and bud scales all permit low
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temperature survival.
Within an acceptable range of temperatures, an increase in temperature will
cause:

an increase in transpiration

an increase in the absorption of water and nutrients due to increased
transpiration

an increase in respiration

an increase in photosynthesis

an increase in transport within the plant.
Heat injury
Temperatures above 40C will usually cause inactivation of enzymes and so
many growth processes stop. Most plant tissue dies if exposed to
temperatures between 50(and 60C.
Heat injury may be evident as:

sunscald, which damages woody stems, leaves, flowers and fruits—
this damage is due more to a sudden fluctuation in temperature rather
than degree of heat;

stem girdle, caused by high temperatures at the soil surface
damaging stems of herbaceous plants.
Cold injury

chilling injury - tropical plants show chilling injury if they are exposed
to temperatures below about 10(C.

freezing injury - this occurs when water inside the plant cells or in
intercellular spaces freezes and forms ice. The cells may be ruptured
by expansion of the ice crystals, or the tissues become dehydrated as
water is no longer available.

Desiccation - results when high transpiration rates are not matched
by relatively slow absorption of water from cold soils. Freezing of soil
water will stop absorption completely.
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Cold resistance
Adaptations of plants which may help them to survive very cold conditions
include:

thick, corky bark for insulation

deciduous leaves

highly concentrated sap (which lowers freezing point)

underground organs such as bulbs, rhizomes and corms

small, compact cells which resist freezing injury.
Conditioning
During the growing seasons, some plants may be damaged by a heavy frost
but during winter be able to withstand temperatures below freezing. Some
conifers display this feature. With some fruit trees, the trees themselves are
able to tolerate frosts, but a late frost can damage flower buds and young
fruit, ruining the crop.
The gradual exposure of plants to lower temperatures causes hardening of
the plants. Sometimes withholding water is also used to bring about
hardening. Similar practices may also be used to increase a plant’s
resistance to temperature extremes and drought.
Other temperature effects
The seeds of many plants show a higher germination rate when exposed to
alternating night/day temperatures of about 10ºC difference. Most species
will only show normal vegetative growth, flowering, fruiting and seed
germination if they do receive alternating night/day temperatures.
Reduced night time temperatures mean reduced respiration during this time;
so daytime photosynthetic rates are more easily able to make up the night
time loss of sugar used in respiration. For these last two reasons, night time
temperatures in glasshouses are usually less than day time.
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1.3 Climate
There are many factors that influence climate. Variations in these influences
occur daily, seasonally, and yearly. The effect of each influence also varies
from place to place. We will look at:

precipitation

light

temperature

wind.
Precipitation
Precipitation has a most important influence on plants growing in Australia.
There are many forms of precipitation (dew, rainfall, frost or snow) and the
following factors determine the amount of moisture available:

the total amount received

the frequency of the precipitation - how often does it occur?

the intensity

the variability - is it fairly predictable each season, or are there
occasional long dry or wet spells?
Perhaps you could summarise these roughly as how much rain falls, how
often it falls, how heavily it falls and how reliable it is. The amount of
precipitation available for plant growth is also influenced by the evaporation
rate, and this is affected by the temperature of the area, the local wind speed
and cloud cover.
Effective precipitation is the amount of moisture available to plants. Both
precipitation and evaporation determine the effective precipitation.
The availability of water is considered to be the most important factor for
plant growth. The season in which rainfall is concentrated and the
temperature must also be taken into account when determining rainfall
effectiveness.
In Australia, the rainfall decreases from high on the coastal areas to very low
inland, and plants and plant associations change along with the change in
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rainfall. For example, rainforest is found in isolated patches along the East
Coast, where roughly 1400 - 1500 mm of rain falls each year. In places with
lower rainfall, plant communities become more scleromorphic (the plants
have leathery leaves with a thick cuticle). Eucalypt forests dominate these
communities and, in the dry interior, these communities are replaced by
wattle shrublands.
As well as a change in plant communities there are changes to plant
characteristics. For example, the leaf size of many species reduces as the
effective precipitation drops. Leaves on plants in tropical rainforests are
large, deep green and smooth, and in more arid areas leaves may be
smaller, grey - green and hairy (xerophytic). The leaf size, colour and texture
may be related to effective precipitation. Plants that grow where there is high
effective precipitation do not need to modify their leaves to withstand periods
of drought, whereas plants in arid areas may need to reduce water loss
because of the reduction in effective precipitation in these areas.
Light
Plants need light to function - both the amount and intensity of light influence
photosynthesis as well as the ability to produce flowers.
The amount and intensity of light depends on:

latitude

altitude

aspect.
Latitude
Depending on the latitude, the day - length and the intensity of incoming
solar energy will vary. As a result, plant communities at various latitudes are
different.
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Figure 4 - The sun’s rays strike the earth more directly at the equator than at
the poles. Therefore polar regions receive less energy per square kilometre
from the sun than equatorial regions. As a result, places near the equator are
warmer than those near the poles.
Altitude
Light does not change as a result of altitude, except that at high altitudes
where plants receive more ultraviolet light. Solar radiation, relative humidity
and precipitation increase with altitude, and temperature decreases. For this
reason as the altitude increases, the natural vegetation forms zones similar
to those from the equator to the poles.
With increasing altitude, the air temperature decreases while humidity and
exposure to wind increase. On high mountains you will see clear zones of
vegetation that correspond to changes in height above sea.
Figure 5 - Zoning of vegetation as altitude increases
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The effect of altitude may also be seen in changes in members of a plant
species when they grow at different altitudes. For example, the snow gum
(Eucalyptus pauciflora) grows in an almost continuous belt from 1200 m to
1800 m above sea level. There are significant differences in the morphology
of the plant including the following:
The trees at 1200 m are much taller than the trees at 1800 m.
The lengths of leaves vary from long at low altitudes to shorter at high
altitudes.
The fruits are larger at high altitudes.
The trees growing at high altitudes are more resistant to frosts.
Although the same species is present at all altitudes, the plants have
adapted to the conditions brought about by different altitudes.
Aspect
The aspect of a site is the direction in which the slope of the site faces. In
the southern hemisphere north - facing slopes generally have more light,
higher temperatures and are drier (due to greater evaporation) than south
facing slopes of the same altitude. Aspect also influences effective
precipitation.
Aspect may influence the type of plant associations found in an area, with
plants that require more moisture and less light seen on the southerly
slopes.
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Figure 6 - The physiographic factor of aspect influences plant associations.
Temperature
Temperature influences the amount of moisture available to plants, the
effective precipitation, so it has an enormous influence on the growth rate of
plants. It is important to determine the length of time that area experiences
its maximum and minimum temperatures.
Temperature is significant as it places limits on the growth of certain species,
and it affects many biochemical processes such as plant growth rates and
flowering. For example, plants of the genus Atriplex (saltbush), which are
common in arid regions of Australia, germinate in response to temperature.
Below 10°C the seeds survive but germination is extremely slow. If the
temperature rises above 10°C, the rate of germination increases. The
optimum germination rate takes place at 25°C, when germination takes only
30 hours. Above 25°C it slows, until at 300°C germination stops altogether.
This is an adaptation which ensures that germination takes place at a time of
year when survival is most likely.
Another example may be seen in the flowering response of plants. Many
plants from regions with cold winters need a period of cold before they will
flower. For example, bulbs of tulip and hyacinth will not flower in warm
conditions unless they are kept in a refrigerator at about 50°C before
planting.
Certain temperatures may be damaging to plants. For example, there are
some algae and bacteria that can live in water at temperatures up to 80°C,
but most plant tissues die as a result of dehydration or overheating if
exposed to temperatures between 50°C and 60°C.
Plants also suffer from low temperatures. Many plants such as tomatoes are
severely damaged by frosts, while others such as pines survive severe cold
for part of the year. Many plants from cold regions have adapted to the cold
by being deciduous (eg oaks); they drop their leaves and become dormant in
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winter, avoiding leaf damage due to freezing.
Wind
Wind increases the rate of evaporation and therefore reduces the effective
precipitation. Water evaporates from the leaves of plants, and this process is
called transpiration. The amount of water that plants lose in this way
increases as temperatures rise and is high if the atmosphere is dry. Wind
may increase the rate of transpiration from plants.
However, many plants have structures that control the loss of water, and the
mulga tree (Acacia aneura) is a good example. During a drought the amount
of water available in the soil decreases and the daily loss of water from the
mulga also decreases. The leaves of the mulga have a thick waxy cuticle
that is almost waterproof, and their stomata (small pores through which
water evaporates) tend to close for a longer period each day as the drought
progresses. The stomata may only open for a short period around sunrise,
so very little water is lost.
Where wind is constant from one direction, plants may lean in the direction
of the prevailing wind. This is particularly noticeable in coastal areas where
plants exposed to winds from the ocean are ‘wind - pruned’.
Topography
Topography refers to the lie of the land, and variations in topography
influence the effective precipitation, temperatures and wind. Mountains and
hilltops are usually exposed to hot or cold winds. Valleys are more sheltered
but they may be cold on still winter nights.
Proximity to the ocean also shows variations where you find coastal
temperatures are moderated and precipitation is usually higher than inland
areas. Exposed coastal areas are prone to strong winds and salt spray.
Topographical features such as hills and mountain ranges influence weather
patterns. For example, on the upwind side moisture - laden winds are likely
to precipitate (provide rain) as they rise over the barrier. The other side of
the barrier is in a rain shadow and temperatures are usually higher. Rainfall
may be higher on the upwind side of the barrier, and an example is the
orographic (relating to mountains) rainfall on the eastern side of the Great
Dividing Range.
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Figure 7 - Rainfall patterns on the eastern side of the Great Dividing Range
Climatic zones
Now that you have looked at factors which influence climate it’s time to look
at the climatic zones of Australia. They may be defined in a number of ways.
Ian G Read in The Bush: A guide to vegetated landscapes of Australia
subdivides Australia into two major climatic regions:

Temperate, south of the Tropic of Capricorn

Tropical to the north.
Temperate regions have seasonal weather with mild to hot summers and
cool to mild winters. Tropical regions have warm to hot conditions all year
with a period of summer rainfall and winter drought. Different authors may
classify zones differently, depending on their viewpoint.
You are going to study climatic zones from a horticultural viewpoint. Most
texts will describe Australian climatic zones based on factors that influence
climate and on features that influence the selection of plant material.
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1.4 Microclimate
Many factors influence the growth of plants. The natural factors are climatic,
edaphic (due to soil), geological and topographical influences. Plants change
over time due to evolutionary forces, and these changes affect the future
growth patterns of plants. Of course humans can also influence the growth
of plants.
In this section we will look at microclimates and how they can influence how
you successfully design and plant a site.
Microclimatology is the study of climatic conditions within a limited
geographic area. It is sometimes referred to as the ‘science of small - scale
weather’. Whereas the air, buildings or vegetation modifies the general
climate of a region, small areas within the region may experience markedly
different conditions.
An understanding of some of the reasons for the many different
microclimates in an area will help you to successfully design and plant a site,
whether that site is surrounding a house, fence, an underplanting in a
regeneration area or a crop in the shelterbelt of a windbreak. You can also
influence and change the microclimate to improve the existing environment
for your plants.
All these influences that affect climate and microclimates are interrelated. A
change in one may cause a change in another. For example, by planting a
screen you alter the temperature of the area (especially on the eastern or
southern side of the planting) and change the wind speed. You may also
increase the humidity (especially on the shaded side of the screen) around
the area.
Factors that affect the microclimate
Every property has a variety of microclimates. Microclimates are dependent
on the variation of a number of factors which include:

wind speed and direction

landform or topography

existing vegetation

temperature range

structural materials
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
soil types, depth and moisture content

orientation

light.
Temperature
Screen plantings or
structures such as
fences may change
the climate by
protecting the area
from heat or strong,
dry or cold winds. Screen plantings alter wind speed and can change the
temperature of the area on the lee side (the side that is sheltered) of the
structure.
Temperature varies with the elevation. It
drops as the elevation rises
Surface temperatures are higher if the
slope is at right angles to the sun than if
the slope lies in the direction of the sun’s
rays.
Windward slopes may be cool
and moist due to the orographic
(mountain) rainfall and cooling
winds. The lee side (protected)
may be robbed of rainfall, is hotter
and more arid.
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Areas close to bodies of water often have different temperatures to nearby
land. In coastal areas daytime sun heats the land surface and warm air
rises, and cool, moist air from the water body moves towards the land. At
night the cooler air from the land flows to the water body.
Buildings, structures or building
materials may affect temperatures
by reflecting or shading the area.
Some surfaces radiate heat.
The colour of surfaces also
reflects or attracts and
absorbs heat; dark colours
absorb heat which may be
re - radiated for several
hours at night.
Topography
Cool air flows downhill, and so local
depressions may contain cool pockets
of air. These are useful in warm areas
or can create frost pockets.
In cooler climates a preferred site is
usually the upper slope below the hill
crest, as temperatures are higher.
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The crest of a hill will be exposed to
strong, cold winds. This is the case
whether there is a building on the top of
the hill or plants.
Winds may be deflected by vegetation or ground forms or structures. They
may also be made weaker or strengthened.
Steep slopes may be prone to erosion as water runs down the slope at great
speed. This can be reduced by careful plantings
Light
The glare from reflective surfaces
such as water, sand or building
materials (glass, pale surfaces) is
reflected light and may increase
temperature where it strikes.
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Plants next to driveways,
buildings (especially those with
a lot of glass) or bodies of
water may burn owing to
reflected light.
Plants help reduce radiated
heat and create shade. They
increase the humidity and
reduce temperatures.
Deciduous plants may provide shade in summer, cool the area and create
higher humidity. In winter, deciduous plants allow more light and so create a
warmer area.
Topographical features, buildings, trees and other objects reduce the total
hours of daylight. Temperatures are also cooler. This can affect which plants
you select. Plants needing lots of light may become spindly and weak and
may not flower.
Slopes facing north
receive the longest hours
and greatest intensity of
light and heat each day.
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The intensity of the sun
increases during daylight
hours. The morning
(easterly) sun is mild, the
late afternoon (westerly) sun
is hot and glaring.
The sun’s orbit and angle
vary with the seasons,
and light and temperature
alter accordingly.
Shadows change with the seasons
and the changing angle of the sun.
Humidity
Hot winds are cooled as they pass over bodies of water and increase the
humidity (the amount of water vapour in the air). This occurs on a smaller
scale when winds blow across pools, sprinklers, streams and other
waterways. These situations often suit more delicate plant material such as
ferns.
Hot winds increase the
amount of moisture
evaporated from the
soil. Plants growing in
groups tend to
increase the humidity
in the area.
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Wind
Winds dry out soils and
plants, reduce humidity,
may damage plant
material and structures,
and may carry damaging
salt.
Winds or breezes may be affected by buildings,
walls, hedges or mass plantings. You may be
able to channel or redirect wind.
A wind tunnel may be formed by
dense plantings or structures. The
wind tunnel may capture breezes or
increase the wind speed. This may
be detrimental in cold windy areas.
You may use screens of plant material or other structures to reduce the
wind. It is usually better to use a screen that the wind can pass through,
rather than a solid screen that may cause turbulence and damage plants
instead of protecting them.
Manipulating the microclimate
While we do not have any control over the climate of an area, there are
certain measures that we can use to vary the area’s microclimate. A
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summary of these measures is found in the table below.
Table 1 - Ways to manipulate the microclimate
Eliminate the extremes of heat,
cold, humidity, air movement, and
exposure.
This can be achieved by intelligent
selection of the site, planning the layout,
building orientation and creating climate responsive spaces, eg through careful
selection of plant material.
Provide direct structural protection
against the extremes of sun, rain,
wind, storm, heat and cold.
Structures rather than plants are usually
more effective against the extremes of
weather.
Respond to the seasons.
This usually means using different areas at
different times of the year, or using
deciduous plantings that modify the area
they are in, by changing with the seasons.
Plan the use of spaces, indoors
and out, according to the
movement of the sun.
This can be done by arranging living areas,
indoors and out, so that they receive the
preferred type and amount of light and
warmth at the preferred time.
Consider the wind and the
direction it comes from. Can this
wind be modified and used to
lower temperatures or reduce
humidity?
This can be done with structures and plant
material (trees, screens, hedges) arranged
so that they redirect a breeze (into or away
from a particular area) or partially block the
breeze to reduce its strength.
Use the evaporation of moisture as
a primary method of cooling.
Air moving across a spray of water, or a
pool, or indeed any moist surface, be it
masonry, fabric, or foliage, is thereby made
cooler.
Maximise the beneficial effects of
water bodies that are nearby.
This can be done by opening the
surrounding areas to the full effect of
breezes from the body of water. These
temper the atmosphere of the warmer or
cooler adjacent lands.
Introduce water.
The presence of water in any form, from
film to waterfall, has a cooling effect both
physically and psychologically.
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Preserve the existing vegetative
cover.
It shades the surface.
It retains the cooling moisture of rain.
It protects the soils and environs from the
hot and cold winds.
It cools and refreshes heated air by
evapotranspiration.
It provides a sunscreen, shade and
shadow.
It helps to prevent rapid run - off and to
recharge the water - bearing subsoil layers.
It checks the wind.
Install new plantings where
needed.
The new plants may be utilised for various
types of climate control. Windscreens,
wind directors, shade trees and heat absorptive ground covers are examples.
Consider the effects of altitude.
The higher the altitude and latitude, the
cooler or colder the climate. Select plants
accordingly.
Reduce the humidity.
Generally speaking, a decrease in humidity
creates an increase in bodily comfort. Dry
cold is less chilling than wet cold. Dry heat
is less enervating than wet heat. Humidity
can be decreased by induced air circulation
and the drying effects of the sun. However,
increased humidity may be required if you
wish to grow ‘soft’ plant material (eg ferns).
Be aware of undrained air
catchment areas and frost pockets.
These areas are made when an air dam is
formed by a solid material across a slope,
stopping the cold air on its downward
journey (cold air settles to the lowest point
and collects). In some situations a
collection of cool air will be desirable, but in
others it will create problems.
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