Gardenia (gardenia jasminoides) is a broadleaf evergreen that produces fragrant, white blossoms on glossy, dark green leaves. This shrub thrives in well-drained, acidic soil, and grows up to 6 feet tall. Gardenia is native to tropical climates and hardy in U.S. Department of Agriculture plant hardiness Zones 8 to 11. Gardenia is a sensitive plant that needs special care, but perfect growth conditions in a greenhouse including temperature, humidity, ventilation and soil can produce healthy gardenias year round.
Choose a right-sized pot that is at least 6 inches deeper and 4 inches wider than the size of the gardenia's root ball. The pot should also have drainage holes at the base.
Place the gardenia pots in bright sunny areas of the greenhouse. Maintain a high humidity level with temperatures ranging from 68 to 74 degrees Fahrenheit during daytime and at least 60 degrees F at night. Space the pots so that the leaves do not touch the foliage of adjacent plants. It is best to keep the potted gardenias on a well-draining surface filled with gravel.
Water the gardenias every day or alternate day during their bloom season. Keep the soil continuously moist, but reduce watering during winter. Water sufficiently so that the droplets drip out of the pot’s drainage hole. Touch the soil base to check if the plant needs water. If the base feels too dry, water the plant. If it is moist, wait another day to water the gardenia.
Prune the stem tips lightly to remove unwanted branches and faded flowers. This encourages the plant to grow bushier, and bloom well in the spring. Undertake pruning immediately after the plant finishes blooming. Delay in pruning reduces the next year’s blooms. Pinch vigorous, young plants during their first year to encourage heavy branching.
Fertilize the greenhouse gardenias with 30-10-10 NPK (nitrogen-phosphorus-potassium) fertilizer during their growing season. Dilute 1 teaspoonful of fertilizer in 1 gallon of water, and apply in two to four-week intervals. Feed the gardenias with fish emulsion or blood meal once a month to encourage continuous blooms.
Inspect the greenhouse gardenias for pest infestations including spider mites, scales, whiteflies and aphids. Dilute 3 ounces of horticultural oil and 6 ounces of lime sulfur in 1 gallon of water, and spray over the foliage with a garden spray. Read the label for detailed instructions and application rates
Repot the gardenias once every two years. Prepare sterile, well-draining potting soil by mixing one-third portion each of rich garden loam, compost and peat moss. Add a 6-inch potful of well-aged manure and a 4-inch potful of bone meal or wood ashes to this mixture. Fill 1 or 2 inches of gravel or river rocks at the bottom of the pot. Pour potting soil to cover half of the pot. Remove the gardenia from its current pot, tap the root ball lightly to discard old soil and place it in the new pot. Hold the plant upright and pour potting soil until the roots are covered.