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Univerisity of Florida IFAS Extension Reprint

Gardenias

Bradshaw, Joan

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 free-standing 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.

Plate 1.
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;

  • '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 creamy-white 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;

  • '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.

Plate 2.

Plate 3.

Plate 4.

Plate 5.

Plate 6.

Plate 7.

Plate 8.

Plate 9.

Plate 10.

Plate 11.

Plate 12.

Plate 13.

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 naturally-occurring 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 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 slow-release. 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 leaf you may increase their survival rate by 20 percent.

Plate 14.
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.

Figure 2.
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 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.

Figure 4.
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.

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.

"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.

Insects

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.

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.

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 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 root-knot 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.

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.

Bud Drop

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.

Root injury may occur because of nematode infestations, poor watering practices, poorly drained soils, excessive fertilization or mechanical injury.

  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.

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.

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.


Footnotes

1. This document is Circular 1098, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date January 1998. Reviewed October 2003. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.

2. Joan Bradshaw, Multi-County Extension Agent/Natural Resources Citrus County Extension Office, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.


Copyright Information

This document is copyrighted by the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all conventions, but permits free reproduction by all agents and offices of the Cooperative Extension Service and the people of the State of Florida. Permission is granted to others to use these materials in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the publication, its source, and date of publication.

 

 

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