How to Grow Gardenias
The gardenia shrub requires lots of sun, regular water and acidic well-drained soil in order to produce those stunning, fragrant blossoms gardeners love.
It's hard not to think of gardenia bushes (Gardenia jasminoides) as the southern belles of the backyard garden. Gardenia blossoms are pure-white and waxy, with an alluringly sweet, come-hither fragrance. They contrast elegantly with the thick, glossy dark leaves of this semi-tropical beauty.
Gardenias thrive in mild, humid climates, which are generally found in the American South. And like southern belles, these picky shrubs are not happy unless their growing conditions are optimal. Many warm-climate gardens are graced by gardenias, so if you are willing to go the extra mile -- and live in U.S. Department of Agriculture plant hardiness zone 8 through 10 -- you can grow gardenias too.
Things You'll Need
Soil test kit for pH
Organic compost or composted manure
Drip irrigation system
Granular fertilizer for "acid-loving plants"
Grow gardenias in full sun in cool or warm locations. In very hot locations, grow gardenias in partial shade. You can also grow these shrubs in full shade, but you may not get as many flowers. Gardenias have little tolerance for sea salt, so do not expect beautiful, fragrant flowers if you live on the coast.
Because they require acidic soil to thrive, test the pH of your soil before you plant gardenias using a home kit or the laboratory at your local university. The ideal pH for gardenias is between 5 and 6 or 6.5. A higher pH -- indicating more alkaline soil -- increases the possibility of mineral deficiencies and can lead to a yellowing of the leaves termed chlorosis. Don't plant these shrubs near the house foundation or other areas of cement since the cement can leach into the soil making it alkaline.
Amend an average loam with organic compost or composted manure before you install gardenias. Although gardenias grow in clay and sandy soil, they prefer well-drained loam high in organic content.
Space gardenia shrubs between 36 and 60 inches apart depending on the mature size of the species or cultivar. These shrubs can grow from 2 to 8 feet tall, with a similar width. If you are growing a few specimen plants, leave even more room between them to allow air currents to waft the heavenly fragrance around the garden. If you grow gardenias in a hedge, space them a little closer to one another.
Water your gardenias regularly throughout the year absent sufficient rainfall. Keep the soil moist at all times, but not wet. Soggy soil deprives the plant of oxygen, damaging its roots. Do not allow the soil to dry out, since this also damages the shrub's roots. Apply several inches of organic mulch to the surface of the soil around the shrub to hold in water and help maintain pH, but don't mulch up against the trunk of the shrub. The ideal water system for gardenias is a drip system, since it delivers regular water to the plants on an ongoing basis without getting water on the foliage.
Feed your gardenia shrub with all-purpose, organic plant food. Sprinkle a granular fertilizer formulated for gardenias around the drip line of the shrubs in early spring, then repeat eight weeks later. Work the fertilizer gently into the top few inches of the soil, then water it in well. Use 1 1/2 tablespoons of granular fertilizer per foot of canopy spread.
Pinch back newly planted young gardenias in early summer and summer's end to encourage branching of the stems. Prune established gardenia immediately after blooming so that they retain the shape and size you desire. Trim off scraggly branches and remove dead flowers and foliage. You should prune early enough that, by early October, new shoots have grown to some 6 inches long. Always sterilize pruners before using them to prevent spreading disease. Soak them in a mix of one part water to one part denatured alcohol.
Keep your eye out for pests on your gardenia plants. Aphids are small insects that cluster on foliage and flowers. Mealybugs look like bits of cotton and are usually found on the underside of leaves. Scales are brown or green and hard, generally attaching to the foliage. Spider mites also attach to the underside of foliage. Remove these insects with a spray of hose water. Alternatively, remove them with a brush and soapy water. Whiteflies also look like cotton underneath leaves, and you can recognize them by a resulting accumulation of sooty black mold. Treat these by picking off infected leaves by hand or removing adults with a small, hand-held vacuum when it is cool outside. You can also purchase yellow sticky traps from your garden supply store and place them in the area.
Test the pH of your soil again if your plant develops chlorosis, since this is usually due to soil that is too alkaline. To deal with the problem organically, add a 2-inch layer of organic compost to the soil, or a mulch of 2 inches of pine needles. Otherwise you'll have to amend the soil with elemental sulfur. It is much better to do this before you plant the gardenias, however, since they do not like their roots disturbed. You'll need to work in 1.5 pounds of sulfur to a depth of 6 inches to change the pH of 100 square feet from 7.5 to 6.5. To alter the pH from 8 to 6.5, you'll need 3.5 pounds of sulfur per 100 square feet.
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