Trying to keep a potted gardenia (Gardenia jasminoides) thriving indoors through the winter isn't a task for the faint of heart. If you give a gardenia too much, or too little, water or keep it too warm, or too cold, the buds will drop and deny you their beautiful, fragrant white blooms. Grown in U.S. Department of Agriculture plant hardiness zones 8 through 11, gardenias are picky about their humidity and sunlight as well. Preparing the ideal place for these plants to spend the winter will go a long way to keeping it healthy until spring.
Place the gardenia in bright, direct sunlight for four to six hours a day. Put the plant in a window with southern exposure or use fluorescent grow lights. Use a light fixture with two fluorescent light tubes for 12 to 14 hours a day, placed 6 to 12 inches from the plant, to supplement its light needs.
Keep daytime temperatures between 70 and 75 degrees Fahrenheit. Maintain temperatures of 60 F at night. Gardenias do poorly if temperatures fluctuate beyond these ranges.
Water the gardenia so that the soil never completely dries out. Too much water drowns the roots, though, so check the soil moisture levels consistently to make sure it is moist to the touch, but not waterlogged.
Maintain humidity for the gardenia. Place a layer of pebbles or gravel in a tray, then add water, but not quite enough to cover the pebbles. Set the gardenia on the tray. The pot should not be in standing water. You also can give the gardenia time to sit in a steamy bathroom or use a humidifier.
Apply a 30-10-10 fertilizer designed for acid-loving plants. Mix 1 teaspoon of a water-soluble formula into a gallon of water, and use it to water your gardenia weekly.