Download Gardenias

Circular 1098
June 1994
Gardenias1
Kathleen C. Ruppert and Joan Bradshaw2
DESCRIPTION AND USE
A favorite landscape shrub in Florida, the
gardenia has very fragrant creamy-white flowers and
glossy, dark-green leaves. The genus Gardenia is
believed to have been named after Alexander Garden,
a physician in Charleston, South Carolina, during
colonial days.
Gardenias are a member of the family Rubiaceae
and belong to the genus Gardenia. There are over
200 species of Gardenias. In Florida, two species are
of primary importance:
Gardenia jasminoides
containing many cultivars, and Gardenia thunbergia,
grown primarily as a rootstock. Gardenia jasminoides
is native to China although most named cultivars have
arisen in cultivation. Gardenia thunbergia, named for
C. P. Thunberg, an 18th century Swedish botanist, is
native to South Africa. This latter species is valuable
due to its nematode resistance and the vigor it
imparts to species grafted on its root.
Gardenias can be used as screens, hedges, borders
or ground covers. They also may be used as freestanding specimens or in mass plantings.
These shrubs are excellent choices for fragrant
flowers and handsome foliage. If you want to enjoy
the flowers’ fragrance, plant in areas with good air
circulation near patios or windows where the
fragrance will be noticed. Many cultivars bloom in
the spring, while others bloom throughout most of the
growing season.
Plant gardenias in full sun, partial shade, or
shifting shade for best flower production. Prolonged
shade may reduce flowering.
CULTIVARS
In Florida, gardenias are available on their own
root system ("own root") or grafted on Gardenia
thunbergia (Plate 1) rootstock. Grafted plants are
usually more vigorous and produce more and larger
flowers than "own root" plants. Those grafted on
Gardenia thunbergia are not as cold hardy north of
Tampa, Orlando and Cocoa. Many grafted gardenias,
however, are grown as interior plants in homes,
offices, and shopping malls in northern states.
Many cultivars of Gardenia jasminoides grow in
Florida. Most are not from breeding but through
mutation, and therefore, can be increased only by
vegetative propagation.
There is considerable
variation in flower size and form, blooming time and
duration, and plant growth among cultivars, which
include:
’Aimee Yashioka’ (Plate 2) which has brilliant
dark green foliage with large flowers, 4-5 inches
in diameter; produces an abundance of flowers in
late spring;
1.
This document is Circular 1098, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First
published: June 1993. Reviewed: June 1994.
2.
Kathleen C. Ruppert, assistant professor, Environmental Horticulture Department, University of Florida, Gainesville; Joan Bradshaw, Extension
Agent II, Urban Horticulture, Pinellas County Cooperative Extension Service; Cooperative Extension Service, Institute of Food and Agricultural
Sciences, University of Florida, Gainesville FL 32611.
The term "plates," where used in this document, refers to color photographs that can be displayed on screen from CD-ROM. These photographs
are not included in the printed document.
The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational
information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national
origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office.
Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / John T. Woeste, Dean
Gardenias
’August Beauty’ (Plate 3) which has dense foliage
with large double white flowers, flowers heavily,
is 4-6 feet high and blooms spring to fall;
’Belmont’ (Plate 4) which has dark green foliage
with large flowers, 4-5 inches in diameter; blooms
throughout most of the growing season;
’Coral Gables’ (Plate 5) which has dark green
foliage with large flowers on compact plants;
blooms throughout summer months;
’Fortuneiana’ (Plate 6) which has double,
carnation-like flowers up to 4 inches in diameter;
’Glazerii’ (Plate 7) which has medium green
foliage with heavy peak bloom in April in south
Florida;
’Golden Magic’ which has almost double, pure
white flowers that age to deep golden yellow;
plants grow 3 feet tall by 2 feet wide in three
years;
’Miami Supreme’ (Plate 8) which has medium to
dark green foliage with large flowers, 4-6 inches
in diameter;
’Mystery’ (Plate 9) which has 4-5 inch diameter,
double white flowers on a 4-8 foot rather upright
growing shrub; needs pruning to keep it neat;
’Radicans’ (Plate 10) (’Prostrata’) which is a
small-leaved, almost creeping version of the
species; the small, lustrous leaves are especially
handsome and coupled with the 1-inch diameter
fragrant flowers make this a good choice for many
landscapes; grows 1-2 feet high with a 4-foot
spread, forms a graceful, flowering evergreen
shrub; good ground cover, or mass or facing
plant;
’Radicans Variegata’ (Plate 11) which is a
variegated version of ’Radicans’ with creamywhite leaf margins and the same flowers as
’Radicans’; it may produce branch reversions that
need to be removed;
’Veitchii’ (Plate 12) which grows 2-4 feet high and
produces 1-1 1/2 inch diameter white flowers;
blooms profusely from spring to fall;
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’Veitchii Improved’ (Plate 13) which grows taller
than ’Veitchii’ to 5 feet and produces slightly
larger (2 1/2 - 3 inch) flowers in greater numbers.
CULTURAL REQUIREMENTS
Soil Characteristics and Fertilization
Gardenias grow in a variety of soil conditions in
Florida but they do best in well-drained soil high in
organic matter. Soil pH is important because it
affects availability of mineral elements and should be
maintained between 5.0 and 6.5 for most Florida soils.
Where soil pH is above 7.0 because of naturallyoccurring lime (like limestone, marl, or sea shells), a
constant effort will be needed to avoid micronutrient
deficiencies, notably iron. Since there is no practical
way to permanently lower the pH of such soils,
growing a more tolerant species than gardenia may be
wise.
If you suspect a soil pH problem, have the soil
tested before applying any material. Your County
Extension Office has information on how to take a
soil sample and have it analyzed. It is very important
to take the soil sample properly so your results will be
correct.
Proper fertilization is important for gardenia
growth and flower production. Most established
gardenias grow well with two or three applications per
year. One application is normally scheduled around
February (south Florida) or March (north Florida)
and another in September (north) or October (south).
A third application may be made during the summer.
A complete fertilizer with a ratio of approximately
3:1:2 or 3:1:3 (e.g. 15-5-10 or 15-5-15) of nitrogen
(N), phosphorus (P2O5) and potassium (K2O) is
generally recommended unless the soil test reveals
that phosphorus and potassium are adequate. For
each application, apply a maximum of one pound of
nitrogen per 1000 square feet. This rate is easy to
calculate from the information given on the fertilizer
bag. Simply divide the nitrogen percentage (the first
number of the analysis) into 100. For example, if you
purchased 15-5-10 then you would divide 15 into 100
which would equal 6.6 pounds.
Therefore, 6.6 pounds of 15-5-10 will supply one
pound of nitrogen to be distributed over 1000 square
Gardenias
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feet of landscape area. This would be approximately
1/2 pound per 100 square feet. Ideally, 30-50 percent
of the nitrogen should be water insoluble or slowrelease. In south Florida or where soil potassium is
frequently inadequate, a fertilizer containing 30-50%
slow-release potassium should be used.
Frequently plants will become yellow (chlorotic)
due to a deficiency of one or more micronutrients,
usually iron. The deficiency can often be corrected by
acidifying the soil or by foliar application of the
deficient nutrient. Elemental sulfur added to soil will
result in a lower soil pH but the decrease will only be
temporary if the soil contains natural lime. One
technique is to dig a small hole about a foot deep and
8 to 10 inches in diameter near the dripline of the
plant. Mix 2 to 3 tablespoons of agricultural grade
sulfur into the soil taken from the hole, and return
the amended soil to the hole. Repeated every year,
that volume of acidified soil usually prevents
micronutrient deficiencies commonly associated with
high soil pH. Foliar applications of iron are also
effective. Follow the directions on the product label.
The leaves’ loss of normal dark green color may
be due to any of several causes, not just nutritional
deficiencies.
These potential causes include
insufficient light, overwatering or poor drainage, too
low soil temperature, nematode damage or diseases.
For instance, several investigations have indicated that
a soil temperature below 70°F induces yellowing. Tip
burn, which occurs particularly at vein terminals,
causes the leaves to lose their color and die. This
may be caused by inconsistent watering. Some leaf
yellowing on older leaves is normal. This may occur
during the winter months, before new growth appears,
and is typical of many broadleaf evergreens.
Pruning
Pruning keeps plants shapely and in scale with the
landscape. Pruning should be done just after the
plant finishes blooming. Pruning after October 1st
decreases next year’s blooms.
Research in Florida suggests that a combination
of long nights, low temperatures, and wood of the
proper age, aid in bud initiation and development.
Pruning should be early enough to allow new growth
to be at least 4 to 6 inches long by approximately
October 1. Young plants, growing vigorously during
their first year, may be pinched once in June and
again in August to encourage heavy branching.
Irrigation
Watering during dry periods is necessary for
healthy gardenias.
Moist soil is essential for
successful gardenias. Watering is important because
it largely controls the number of flower buds that
remain on a plant to maturity. If water stress occurs
in a heavily budded plant, many buds will fall before
opening. Therefore, while the plant is in bud, large
variations in soil moisture should be avoided. Use
mulch and avoid cultivation around the base of the
plant to help maintain adequate moisture.
PROPAGATION
Cultivars of Gardenia jasminoides grown in
Florida can be propagated by cuttings or grafting.
Plant production for north Florida should be
restricted to "own root" because plants grafted on
Gardenia thunbergia rootstock are not hardy in
outdoor planting areas at temperatures below 28°F.
Gardenias produced for south Florida should be
grafted, because plants grafted onto Gardenia
thunbergia are superior to "own root" ones.
Cuttings can be taken any time during the year,
but are most successful in June, July, and August.
Gardenia thunbergia can be propagated from seeds or
cuttings.
Tip or midsection cuttings with wood 6 to 8 weeks
old should be cut 4 to 5 inches long with at least 2 or
3 sets of leaves. Cuttings can be taken at or between
nodes as they root from the cut end. Leaf removal is
unnecessary and undesirable because it results in a
longer rooting period.
Rooting of cuttings is best under continuous or
intermittent mist, or in a closed-case propagating
device. Rooting media should be a 50:50 combination
of clean, sharp builders’ sand and peat moss; or a
50:50 combination of peat moss and perlite.
In south Florida, propagation should be by
grafting scion from a desired cultivar to a seedling
rootstock of Gardenia thunbergia.
Rootstock
seedlings, however, are difficult to obtain.
If
collecting seed yourself (seed pod, Plate 14), simply
sow the seeds from the berry in flats or pots
containing a 50:50 combination of peat moss and
perlite or 50:50 combination of peat moss and sand.
Seeds germinate slowly and erratically. Seedlings
should be removed when they form their second true
leaf. By waiting until they produce their second true
Gardenias
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Figure 1. Making the splice-graft: a) single sloping cuts are
made on the scion and rootstock; b) join so that cambial
layers meet; and c) bind them together.
leaf you may increase their survival rate by 20
percent.
When seedling rootstocks are approximately 6
inches or taller, and approximately a pencil thickness
in stem diameter, they are ready to be grafted. Don’t
graft too low on the plant because the mature plant’s
branches may droop to the ground. As a result, the
branches may root and may become infested with root
knot nematodes. Pruning or grafting high prevents
the problem.
The most successful grafts are the splice-graft and
inverted saddle graft. For the splice-graft (Figure 1)
remove the entire rootstock top with a long, sloping
cut. Also, remove any drooping side branches
beneath the graft. Select a scion from a desired
cultivar that has a diameter similar to the rootstock
and cut with a similar sloping cut. Join scion and
rootstock so cambial layers meet, or align on both or
at least one side. The cambium is a thin, green,
actively growing layer of cells located between the
bark and wood of a plant. Bind them with a rubber
budding strip and wax or wrap with a plastic tie strip.
In either case, the joined area should be completely
covered to prevent drying and water entry which
might prevent proper callusing of the cambium.
The inverted saddle graft (Figure 2) is easier to
make and stronger than the splice-graft. Remove the
top from the rootstock with a horizontal cut at a point
Figure 2. Inverted saddle graft: a) split top 1/2 inch of
rootstock; b) two slanting cuts on opposite sides of scion; and
c) align cambial layers and bind together.
where the stem diameter is equal to or slightly less
than that of a pencil. Split the top 1/2 inch of the
rootstock. Select a scion from the desired cultivar
with a similar diameter. Cut the scion base in a
wedge shape and insert it into the split top of the
rootstock with cambial layers aligned. Treat the
joined area the same as that of the splice-graft.
After grafting, place the plant in a shaded spot
and maintain the humidity as close to 100 percent as
possible to prevent the scion’s wilting. Mist plants
throughout the day to prevent wilting or place plants
inside a plastic enclosure in a shaded area to maintain
necessary humidity. Grafts should begin to callus
within 2 weeks and be self-supporting within a month.
Gardenias
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PESTS AND OTHER PROBLEMS
Diseases
Probably the most serious gardenia disease is
stem canker, which occurs on the main stem at the
soil line. Fortunately, this disease is not too common
in Florida. Stem canker is distinguished by rough,
cracked areas that form cankerous growths near the
soil line. The disease organism enters the plant
through wounds, so every precaution should be taken
to prevent damage to stems. Destroy any infected
plants to prevent infection of other gardenias. No
fungicides are available to control the disease.
gardenias, root-knot nematodes (Meloidogyne species)
are the most common. Fortunately, the symptoms
they cause are readily recognized: premature wilting,
low vigor, thin canopy, and leaf and/or bloom loss
under relatively mild stress. Roots infected by rootknot nematodes are swollen and gnarled (the
overgrown tissues are usually called galls or knots).
They often deteriorate prematurely because fungi
readily attack the tender tissues that the plant
produces in response to the infection.
"Sooty mold," an organism that looks like a
disease, often occurs on the foliage turning it black.
This black, smut-like substance does not injure foliage
but prevents sunlight from reaching the leaf, thereby
reducing photosynthesis.
The organism is not
parasitic but lives on honeydew secreted by sucking
insects such as aphids, scales, mealybugs and
whiteflies. Sooty mold can be managed best by
controlling these insects.
In south Florida, gardenias are usually grafted on
Gardenia thunbergia rootstock, which resists root-knot
nematode attacks. This rootstock is considered too
cold-sensitive for landscape use in north Florida, so
flowering gardenias (G. jasminoides) are propagated
on their own roots. No chemical treatments are
available for nematode control in landscape plantings.
The best practices to minimize effects of root-knot
nematodes are to use the resistant rootstock where it
is well adapted and apply organic matter liberally to
the soil. The latter encourages natural enemies of the
nematodes and provides gardenia roots with a better
physical and chemical environment.
Insects
Bud Drop
Many insect pests attack gardenias in Florida and
can be troublesome unless proper control methods
are used. The most injurious insects include scales,
aphids, spider mites, thrips and whiteflies.
One of the most difficult problems in gardenia
culture is bud drop or bloom failure. Causes include
root injury, insect damage, and unfavorable weather
conditions.
Scale insects include cottony cushion, Florida
wax, soft brown scale and others. These insects
attach themselves to host plants after hatching and
give stems or leaves a lumpy appearance. Scale
insects are difficult to control especially as they
mature.
Root injury may occur because of nematode
infestations, poor watering practices, poorly drained
soils, excessive fertilization or mechanical injury.
Spider mites can cause considerable damage
especially during hot, dry periods. These small pests
feed primarily on the underside of the foliage, causing
colorless or whitish spots. Therefore, considerable
injury usually has occurred when the homeowner
notices the damage.
Check with your local
Cooperative Extension Office for the most recent
recommendations on insect control.
Nematodes
Nematodes are among the most serious gardenia
pests in Florida.
Nematodes are microscopic,
parasitic roundworms that live in and feed on
gardenia roots. Although many kinds may affect
1. An excessive number of nematodes in the soil
often damages roots and prevents normal uptake
of water and nutrients.
2. Poorly drained, wet soils, or excessive watering
excludes oxygen, thereby causing root injury.
3. Too much fertilizer adds excessive soluble salts
and can cause root-system dehydration. It is
important, therefore, to follow fertilizer
recommendations.
4. Mechanical injury to root systems occurs most
often during transplanting so be careful. Always
plant gardenias at the same depth as they grew in
the nursery. Don’t transplant while they are in
bud unless flowers are unimportant.
Gardenias
Insects damage unopened buds, causing them to
drop. Thrips and aphids are most troublesome.
Usually pear-shaped aphids are visible but tiny thrips
can go undetected until they cause considerable
damage.
During excessively hot, dry weather bud drop is
prevalent because the plant cannot absorb water
rapidly enough to compensate for water loss through
transpiration. Maintenance of adequate soil moisture
and frequent light syringing aids in reducing water
loss and bud drop under such conditions. Gardenias
may also experience bud drop following a rapid drop
in temperature, even if the temperature does not
reach freezing.
Page 6
REFERENCES
Dirr, M. A. 1990 (4th edition). Manual of woody
landscape plants-their identification, ornamental
characteristics, culture, propagation and uses.
Stipes: Champaign, Illinois.
Kidder, G., R. J. Black, & K. C. Ruppert. 1991. Soil
pH and landscape plants. Factsheet SL-113.
Florida Cooperative Extension Service.
Yeager, T.H. & E.F. Gilman. 1991. Fertilization
recommendations for trees and shrubs in home and
commercial landscapes. Circular 948. Florida
Cooperative Extension Service.
Special thanks are extended to Harmon Carroll,
Richard Carroll, and Robert Carroll.