Native to China, gardenia is an evergreen shrub best known for its fragrant flowers. Cultivars range in height from to 2 to 15 feet and produce narrow, glossy leaves and single or double blossoms. The 2- to 4-inch wide flowers bloom in a variety of colors, including white and yellow. With proper care and growing conditions, gardenias bloom abundantly. However, temperature, insect pests and other factors can interfere with bud formation and blooming.
Damage from insect pests, especially aphids and thrips, can cause bud drop. Aphids are small, pear-shaped insects which feed on fluids from buds and other plant parts. Thrips are so tiny that they often go undetected as they feed on the undersides of leaves and buds. According to the University of Hawaii Cooperative Extension Service, imidacloprid insecticide, horticultural oil or soapy water are effective against aphids. To control thrips, use orthenone or pyrethroid insecticides.
Insufficient lighting may cause failure to bloom. If you live in a cool climate, place your gardenia in an area that receives full sun exposure. In a hot, dry area, your plant will bloom best in filtered shade. In a shady area, place the plant as far as possible from competing shade tree roots. Gardenias fare poorly in heavy shade, which prevents proper leaf and bud formation.
Root injury is a common cause of bud drop in gardenias. When transplanting, handle the root ball carefully to avoid damaging the roots. Use well-drained soil, and allow the top inch of the root ball to remain above the soil. To avoid root rot, do not allow the soil to become waterlogged. Dry, cracking soil can also damage roots. Instead of cultivating the soil around the plant, add a layer of mulch to suppress weed growth.
Fluctuations in moisture levels after the plant has budded can cause bud drop. During hot dry periods, water your plant on a regular basis to keep the soil evenly moist but not soaked. Do not allow the soil to dry out between watering sessions. Use a drip irrigation system to provide consistent moisture levels for your plants. The University of Florida Cooperative Extension Service recommends a 2- to 3-inch layer of mulch to maintain soil moisture.
Weather conditions can affect a gardenia’s ability to bloom. A sudden temperature drop, as well as moisture loss during hot, dry weather, can result in bud drop. Nighttime temperatures play a crucial role in bud formation. According to the University of Hawaii Cooperative Extension Service, plants exposed to nighttime temperatures below 60 degrees will develop yellowish leaves and fewer flowers.