Gardenias are grown for their large, sweetly fragrant flowers and glossy evergreen foliage. They are meant for warm climates and sustain substantial damage when exposed to temperatures below 15 degrees F. (-9 C.). Most cultivars are hardy only in USDA plant hardiness zones 8 and warmer, but there are some cultivars, labeled as cold-hardy, that can withstand winters in zones 6b and 7.
How to Winterize Gardenia Outside
Be ready for unexpected cold snaps by keeping supplies on hand to protect your plant. On the fringes of the recommended climate zones, you can protect gardenias in winter by covering them with a blanket or cardboard box during brief cold snaps.
A cardboard box large enough to cover the shrub without bending the branches is a must when temperatures drop. Gardenia winter care in areas that experience snow includes protecting the branches from the weight of heavy snow accumulation. Cover the plant with a cardboard box to prevent the weight of the snow from breaking the branches. Have old blankets or straw available to insulate the shrub under the box for an extra layer of protection.
Outdoor container grown plants can be overwintered in a sheltered location and insulated with bubble wrap in areas just outside their growing zone, or one zone lower. For colder areas, however, these should be brought inside (see care below).
Despite your best efforts, the tips of the branches may die and turn black from frost or cold damage. When this happens, prune the branches a couple of inches (5 cm.) below the damage with sharp pruning shears. If possible, wait until after it blooms.
Indoor Winter Care for Gardenias
In colder areas, plant gardenias in containers and provide winter care for gardenias indoors. Clean the plant with a strong spray from a water hose and thoroughly examine the foliage for insect pests before bringing it indoors. When wintering over gardenia plants indoors, keep in mind that these are evergreen shrubs that don’t go dormant in winter, so you will need to continue to provide optimal growing conditions.
A gardenia kept indoors over winter needs a location near a sunny window where it can receive at least four hours of direct sunlight each day.
Indoor air is dry in winter, so you will have to provide extra humidity for the plant during the winter months. Place the plant on top of a tray of pebbles and water or run a small humidifier nearby. Although you should mist the plant occasionally, misting alone doesn’t provide enough humidity for good health.
Gardenias overwintered indoors need cool night temperatures of around 60 degrees F. (16 C.). The shrub will survive warmer night temperatures but it may not flower well when you take it back outdoors.
Keep the soil lightly moist and use a slow-release azalea fertilizer according to the package instructions.