Gardenias are not the easiest shrub to grow, but their beautiful white flowers make up for the extra attention they require. Gardenias have been a popular shrub in South Carolina since the 18th century and have been grown for thousands of years by the Chinese. Gardenias are named after the Scottish naturalist Alexander Garden.
Gardenia (Gardenia jasminoides) is also known as cape jasmine. It is an evergreen shrub and depending on the cultivar grows to a height and width of 3 to 8 feet. Gardenias have glossy, dark green foliage that is 2-to 4-inches long and about half as wide. The flowers are waxy, white, very fragrant, can be either single or double depending on the cultivar, and can be 2- to 4-inches in diameter.
Since gardenias are grown primarily for their fragrant flowers, they should be planted where people will notice their fragrance. The flowers open over a long period of time, from May through June, and sporadically thorough summer. Gardenias are considered deer resistant shrubs.
Fall is the best time to plant gardenias. Plant in light to partial shade, preferably with morning sun and afternoon shade. Gardenias dislike root disturbances. Smaller cultivars will grow well in containers placed where their fragrance can be enjoyed.
Gardenias prefer acidic (a pH less than 6.0), moist, well-drained soils. Add organic matter such as compost or ground pine bark to the planting bed and till into the soil before planting. Mulch plants with 2- to 3-inch dep layer of pine straw, compost, or ground bark.
Fertilize gardenias lightly in the spring once the frost has passed with a well-balanced, extended-release, acid-forming, azalea fertilizer. Fertilize the gardenias again 6 weeks later to encourage extra flowers or faster growth of young shrubs. A well-balanced fertilizer has a ratio of 2-1-1. Examples include:
• Pennington Evergreen & Rhododendron Plant Food (12-6-6),
• Lilly Miller Ultra Green Azalea, Camellia & Rhododendron Food (10-5-4),
• Sta-Green Azalea, Camelia & Rhododendron Food (10-5-4),
• Pennington Azalea Food (10-6-8),
• Scotts Evergreen, Flowering Tree & Shrub Fertilizer (11-7-7),
• Vigoro Azalea, Camellia & Rhododendron Food (10-8-8).
Complete, acid-forming organic fertilizers are great choices for gardenias for spring and early summer fertilization. These fertilizers are typically not as nutrient rich, and because of the low nitrogen content and their inability to burn roots, they can be mixed lightly into the soil in the fall at planting to enhance root growth. Organic fertilizer examples include:
• Espoma Holly-Tone (3-4-3),
• Fertrell Holly Care (4-6-4),
• Jobe’s Organics Azalea Camellia & Rhododendron Fertilizer (4-4-4),
• Natural Guard Evergreen & Holly Food (5-4-5).
Do not fertilize gardenias in the fall. Doing so will stimulate tender growth, which can be killed if the winter temperature drops below 15 degrees. Gardenias are cold sensitive.
Products containing iron may be applied during the growing season to correct yellowed new foliage caused by an iron deficiency. This may occur if gardenias are limed or planted near a new concrete foundation. Southern Ag Essential Minor Elements is an example of an iron-supplement that contains iron and numerous other trace elements.
Gardenias should be pruned after they have finished flowering in the summer to remove straggly branches and faded flowers. Gardenias should be watered weekly during periods of drought in the summer. Drip irrigation is an excellent way to keep water off the foliage and blossoms and prevent leaf spots.
Most of the older gardenia cultivars are cold hardy to USDA zone 8, but many of the newer and smaller cultivars are hardy to at least USDA zone 7a. Dwarf cultivars are more cold sensitive.
Whiteflies are the most common problem on gardenias. Whiteflies have piercing-sucking mouthparts, that penetrate the cells of a leaf, then suck out the leaf sap. The top sides of the leaf may become pale or spotted. As the whitefly removes the plant sap, it excretes a large amount of clear, colorless, sugary waste, which drips onto the leaves below. This waste is called honeydew, and it quickly colonizes a black mold (sooty mold), which coats the leaves in summer.
Whiteflies can be controlled with an insecticidal soap solution or a horticultural oil. Both kill by suffocation. Sprays will need to be repeated every few days until the whitefly population is under control. Follow label directions for mixing insecticidal soap sprays. Use horticultural oils as sprays between 45- and 90-degree temperatures. Spray in the early morning or late evening to slow the drying time of spray. Use a 2% solution (5 tablespoons of horticultural oil per gallon of water.
Synthetic pyrethroid insecticides (such as bifenthrin, Cyfluthrin, and lambda cyhalothrin) will also control whiteflies if sprayed on the flower surfaces of leaves. Acephate is a foliar systemic insecticide that will move through the leaf tissue, therefore you should spray the upper leaf surface. All foliar sprays may need to be repeated one or twice at 10-day intervals because they typically do not kill the eggs. Do not spray plants in bloom to prevent injury to pollinating insects.
A single drench of imidacloprid can be applied at the base of the shrub in the spring as new growth appears to give season long control. This will prevent numerous insecticidal sprays.
Some cultivars to be on the lookout for include the following:
Larger, upright, double-flowered cultivars:
• ‘August Beauty’
• First Love® (‘Aimee’): May be listed as ‘Aimee Yoshida’
• ‘Frost Proof’
• Summer Snow® (PP22797)
Smaller cultivars with repeat blooms:
• ‘Chuck Hayes’
• Crown Jewell® (PP19896)
• ‘Double Mint’ (PPAF)
• Heaven Scent® (‘Madga I’, PP19988)
• Jubilation™ (‘Lee One’, PP21983)
• ‘Kleim’s Hardy’
• Pinwheel® (‘PIIGA-1’, PP22510)
• Scent Amazing™ (‘LeeTwo’, PPIP)
• ‘Fragrant Pathways’
• ‘White Gem’
For more information on gardenias, please visit the Home and Garden Information Center website at hgic.clemson.edu. Tune in on Tuesday nights to watch “Making it Grow” at 7pm on SCETV or mig.org. Email Outen at [email protected]
Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, gender, religion, national origin, disability, political beliefs, sexual orientation, gender identity, marital or family status and is an equal opportunity employer.