Institute of Food and Agricultural Sciences
Hendry County Horticulture News
Gardenia - Fragrant Landscape Favorite
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. 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.
Gardenias are available on their own root system ("own root") or grafted on Gardenia thunbergia 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 but this is of little concern in our area.
Many cultivars of gardenia 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.
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.
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 applied around February and another in October. 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(P) and potassium(K) is generally recommended. 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 ½ pound per 100 square feet.
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, over watering or poor drainage, too low soil temperature, nematode damage or diseases. 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.
The most serious gardenia disease is stem canker. 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. Good luck and good gardening.
Gene McAvoy is the horticulture agent with the Hendry County Extension Service. Direct your horticulture questions to PO Box 68, LaBelle, FL 33975, e-mail - firstname.lastname@example.org or phone 863-674-4092 or 863-983-1598. You are also welcome to visit the Hendry County Extension Office at 225 Pratt Blvd., LaBelle. Office hours are from 8:00 - 5:00.
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