Gardenias are perennials grown as garden or house plants. Plants will grow 2 to 8 feet tall with a spread of up to 8 feet. The plants have 3- to 5-inch, dark, glossy leaves and waxy white flowers. Native to South Africa and China, gardenias need the proper care in order to achieve healthy flower buds.
Bacterial spot is most severe when temperatures are between 72 and 82 degrees Fahrenheit. Pinhead-sized brown or black spots appear on the leaves of the gardenia, with yellowing of the leaf. Flowers buds that haven’t opened yet will turn black. Eventually the buds and leaves affected by bacterial spot will fall from the plant. Avoid overhead watering to minimize exposure to the disease. Infected plants should be removed so that nearby plants cannot become infected.
Dry, hot weather conditions will cause the buds to turn black and drop from the gardenia. When the conditions are hot and dry, the gardenia cannot absorb water as quickly as needed to keep the plant healthy. Gardenia plants lose water through the leaves in hot, dry conditions and the buds and leaves will suffer. The only way to prevent the buds from turning black and dropping is to check the soil daily and keep the plant soil moist at all times.
Thrips and aphids are sucking insects that will suck the juices from leaves and flower buds causing the flower buds to turn a brown-black and leaves will turn yellowish in color before dropping for the plant. Use a pyrethroid or organophosphate insecticide to get rid of aphids, but insecticides are not as affected for thrip removal in the long term. If you use an insecticide formulated to get rid of thrips, you will need to treat plants more than once to completely get rid of thrip adults. Orthene insecticide is beneficial in treating thrips, but you can only apply the insecticide when the plant is in bud or the spray will burn the flower petals.
Allowing the gardenia soil to dry excessively between each watering will cause the flower buds to turn black and drop from the plant. Sudden changes in temperatures or cold temperatures will cause buds to die and turn brownish-black. Ideal temperatures for gardenias is between 70 and 75 degrees Fahrenheit during the day and night temperatures about 10 degrees Fahrenheit cooler.