Gardenias are fragrant bushes with heavily scented star-shaped flowers. The glossy green leaves and semi-climbing habit of many of these bushes add year-round color and texture to the home garden. Gardenias are sensitive to freezing temperatures and can acquire a heavy amount of damage even in short freezes. The most hardy gardenia is safe to grow in United States Department of Agriculture zones 7 to 10, although they may survive in zone 6 if grown in protection and are well mulched.
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Gardenias that have run into an unexpected period of freezing will have foliar and twig damage. The flowers will turn brown and "melt" if they are still on the bush. Buds will fall off. The leaves can have mild damage in the form of black tips or they may be entirely dark and fall off after a couple of days. Twig die-back and blackening will occur in temperatures below freezing. If the plant is grown in areas where the winter temperatures are consistently below freezing the entire root system will die and the plant will fail.
Cold-damaged wood should not be cut off until the danger of frost has passed. If you cut off the damage, tender tissue is exposed to potential cold. It could also spur new growth, which is more susceptible to cold temperatures. You can manually defoliate the tree at any point in the season. Wait until spring to prune when new leaves have formed. This will help you identify what parts are dead. Prune back to healthy growth, taking care to cut just below active buds.
Food and Water
Damaged gardenia plants need tender care after experiencing cold stress. The plants will benefit from a regular watering schedule, which keeps them moderately moist but not soggy. They should also have a foliar feed in the form of liquid nitrogen sprayed on the foliage as soon as the leaves re-grow. Spray them monthly with this organic mix diluted in water to help feed the foliage and stems. Damaged shrubs should not have a synthetic fertilizer applied the first year after damage because it creates weak cellular growth. Pinch the tips of new growth in June to force healthy branching.
Most gardenias can survive a frost with light foliar damage. If you grow gardenias in areas where freezing temperatures are possible, you should take a few winter preparations. Water the plants thoroughly the day before the freeze. Cover them with plastic or even a blanket from top to root. A solid form of some sort or even stakes are useful to keep the cover from crushing the foliage. Leave the cover in place as long as temperatures are freezing at night and no more than 50 degrees Fahrenheit in the day. If you use a plastic cover you should remove it during periods of sun to prevent the light from burning the plant.