Despite their pretty and perfumed faces, gardenias have sour and wet blanket dispositions. In other words, they prefer acidic soil and a blanket of humidity! Because these Southern belles will only survive outdoors in U.S. Department of Agriculture plant hardiness zones 7 to 11, they must remain hothouse flowers in a large percentage of the country. Whether you grow them indoors or out, a little water and vinegar can make a big difference.
Gardenias need their soil to be kept moderately moist, but not soggy, at all times. Because tap water tends to be alkaline, it's best to use rainwater, or you can acidify your tap water with vinegar. For an outdoor gardenia, add 1 tablespoon of vinegar to 1 gallon of water every time you irrigate the plant, or give it 1 cup of vinegar per 1 gallon of water once a month. For indoor plants, you can just keep a 1-gallon jug of room-temperature water ready at all times with 1 tablespoon of vinegar added to it. Since gardenias can get root rot from too much moisture, an outdoor plant’s soil should be amended with compost or peat to improve its drainage. If you keep an indoor gardenia in a clay pot filled with 2 parts potting soil to 1 part peat, it shouldn’t get too mucky.
To feed gardenias, look for a water-soluble fertilizer formulated for acid-loving plants that also contains micronutrients such as iron. For an indoor gardenia, apply it every two weeks from spring until fall at the amount indicated for houseplants — usually about 1 teaspoon of crystals dissolved in 1 gallon of water. For an outdoor plant, you can use the same food at the rate of 1 tablespoon of crystals per 1 gallon of water. If you prefer an organic fertilizer, spray your plant every couple weeks with a mix recommended by the Dirt Doctor website. It consists of 1 cup compost tea, 1 ounce molasses, 1 ounce vinegar and 1 ounce liquid seaweed combined with 1 gallon of water. You won't want to use this smelly concoction on a potted gardenia unless it is outdoors at the time.
You can provide more humidity for an indoor gardenia by bathing it at least once a month with lukewarm water and moving it outdoors to a partially shaded position during the summer. To avoid shocking the plant, bring it back inside before you begin running your furnace on a regular basis, while the indoor air is still relatively moist. Set the gardenia on a tray containing gravel with a little water at the bottom, and position it on an east-facing windowsill where it will get at least four hours of sunlight per day. For the best results, place it in a room where the thermostat can be lowered at night. Nighttime temperatures around 60 degrees Fahrenheit will raise humidity levels somewhat, as well as cuing gardenias to set buds.
Pests and Diseases
Although gardenias like humidity, it's best to avoid spraying water directly on their leaves, as that can cause fungus problems. If your gardenia gets leaf spot or powdery mildew anyway, vinegar also makes an effective fungicide when 3 tablespoons of it are mixed into 1 gallon of water. Spray the plant in early morning or early enough in the evening to allow it time to dry off before dark. For insect pests such as scale or thrips, add about 2 1/2 tablespoons of horticultural oil to the gallon of water instead. Spray the gardenia thoroughly in the early evening, and rinse it off the following morning with plain water.