The gardenia (Gardenia jasminoides) is a beloved plant for the very simple reason that few natural scents are as remarkable, evocative, and memorable. If not for this spectacular appeal, few gardeners would try to grow high-maintenance gardenias, which are vulnerable to many insects and diseases. They are most commonly found in conservatories and greenhouses. Gardenias are a popular gift plant and almost irresistible when in bloom, but they are notoriously difficult to grow in normal indoor conditions. Nevertheless, even a few months with a blooming gardenia in the house makes them a worthwhile addition to your collection.
They grow outside only in U.S. Department of Agriculture plant hardiness zones 8 to 11, which range across the South and the Pacific Coast. If you live in cooler climates, you can take your houseplant gardenia outside during the spring and summer after the temperature stays above 60 degrees Fahrenheit. But be careful to bring it in any night when the temperature falls below that minimum and as soon as fall makes its appearance.
As with all houseplants, your gardenia has the best chance to thrive if you pay close attention to the conditions it likes best.
Light: Bright light, but avoid direct sunlight, especially during summer.
Water: Keep soil continuously moist but reduce watering in the winter.
Temperature: Above 60 degrees is required. Avoid cold drafts if the temperature drops lower.
Soil: Use a non-alkaline soil mix, such as a rhododendron mix. Gardenias prefer slightly acidic soil.
Fertilizer: Feed biweekly with non-alkaline fertilizer. A teaspoon of agricultural sulfur in the soil may help lower soil pH.
It's a good idea to repot your gardenia in the spring or every other spring as needed. If it seems to be pot-bound or not as healthy as it had been but you find no insects or diseases, it usually is a good signal that it needs to be repotted. Use a low pH soil formulated for rhododendron or gardenias.
The basic species features glossy green leaves and waxy, highly fragrant white flowers. Native to China, this plant has been widely cultivated, so there are many cultivars available. In warmer climates, where gardenia is grown outdoors, many plants are sold grafted on a Gardenia thunbergia rootstock. The grafted plants tend to be more vigorous, with better, larger blooms, but they are even less cold tolerant than the basic species.
Gardenias are acid-loving plants, so they prefer soil with a slightly lower pH. With normal potting mixes, this shouldn't be a problem, but you should be aware of it when you chose your potting soil.
Aside from cold temperatures and inconsistent watering, which will cause bud and leaf drop, the most common problems are insects, especially scale, aphids, spider mites, mealybugs, and whiteflies. Gardenias also are vulnerable to powdery mildew, leaf spot, dieback, anthracnose, and sooty mold.
A well-tended gardenia will be compact, with deep leaves, and bloom in early spring to summer, depending on location, when the nighttime temperatures are above 60 degrees and daytime temps are between 75 and 82 degrees.