Experts and BHG readers answer.
What do gardenias need to look their best?
With the rich aroma of jasmine tinged with spicy notes of nutmeg and vanilla, graceful white gardenia blooms smell as good as they look. Whether you grow them indoors in Alaska or outdoors in Miami, you can find a glorious gardenia (Gardenia jasminoides) to suit any setting. Native to China, gardenias are semitropical evergreen shrubs. They'll thrive outdoors to Zone 7b, a range that includes much of the lower South, California, and the maritime Northwest.
In addition to mild winters, gardenias need acidic soil. If you're missing one or both of these requirements, don't give up. Gardenias can be happy when grown in containers that are kept indoors during the winter and moved outside after the weather warms in spring. In Texas and other mild-winter climates where the soil is alkaline, grow gardenias in tubs filled with an acidic potting mix, such as potting soil amended with sphagnum peat moss, and keep them outdoors year-round. However you grow gardenias, it's important to know about some common problems that are easier to prevent than to cure.
Yellow leaves. Cool soil temperatures (below 70 F) during the growing season or a pH above 6.5 can cause gardenia leaves to turn yellow, because the plant can't absorb certain nutrients even when those nutrients are present. Gardenias grown outdoors often shed a few yellow leaves in winter, which is normal, but when you see yellowing leaves during the summer or on plants grown indoors, take corrective measures immediately. The best remedy for yellow leaves is to feed plants with a good azalea fertilizer, which contains the three major plant nutrients (nitrogen, phosphorus, and potassium) as well as sulfur, which helps acidify the soil. Even if you never see yellow leaves, feed gardenias monthly during the summer and at least once during the winter. Yellow leaves may also be a symptom of overwatering or an infestation of root-knot nematodes.
If you live in central or southern Florida, grow gardenias that have been grafted onto nematode-resistant rootstocks. Unfortunately, these rootstocks are not hardy below 28 F, so they cannot help gardenia lovers in Zones 8 and 7b. Blossom drop A gardenia that has lovely dark green leaves can still have problems producing flowers, particularly if the plant is grown indoors. It is heartbreaking to watch flower buds slowly form, then drop off just before they are ready to open. The causes could be too little light, over- or under watering, high temperatures, or low humidity.
Give your gardenia extra attention during late spring and autumn, when gardenias are most likely to bloom. Place the plant where it will get bright morning light and cool nighttime temperatures (about 55 F). A spare bedroom where the heating vents can be closed is often a good place. Check the soil every day or two and keep it lightly moist (but not wet) at all times, and keep the air humid around the plant. When the blooms open, take the plant into your favorite room, where the fragrance can be enjoyed to the fullest. Bloom problems on outdoor gardenias are most often due to improper pruning.
Gardenias should be pruned only to control their size or to shape the plants. Plan before you plant so you don't have to constantly prune a plant to keep it in bounds. Most well-sited gardenias need little if any pruning. If you prune your plant, do so in early summer, after spring's flowers have come and gone. Buds that open in spring form in late summer or autumn. Pests Prevent problems with aphids, spider mites, scale, and other small insects by spraying plants every 4-6 weeks with insecticidal soap. If you grow gardenias in pots outdoors in summer, always inspect plants and spray them with insecticidal soap before you bring them indoors in autumn to keep pests from moving in too.