When you have an older gardenia, it is not uncommon for the leaves to yellow before new growth and it should not cause much concern. When numbers of the leaves on a younger gardenia weaken and yellow, you do have a problem. Leaf yellowing is called chlorosis. It has several possible causes. Environmental or cultural stressors such as insect infestation, too much or too little water or light and poor drainage can all cause leaf yellowing. The plant may also have a nutritional deficiency.
When flower buds fall off before they bloom it can be disheartening. Bud drop is a common problem with gardenias. As with leaf yellowing, the cause is usually environmental or cultural. The balance of light, water, air and soil might be the problem. The gardenia might be infested with thrips, aphids or whiteflies. Nematodes might be eating the roots. It the temperature drops suddenly, the buds might drop, too. Excessively hot weather can cause the bud drop.
Yellow Leaf Solution
The solution to both yellowing leaves and bud drop is to get back to basics in care of the plant. Run down the checklist of watering and feeding requirements for your gardenia and make sure to follow them. Make sure to keep the gardenia moist but not saturated. For yellowing leaves, the soil may need to be acidified. Add supplemental aluminum sulfate, iron sulfate or wettable sulfur. Use iron chelate on the yellowing leaves directly. Lower the soil pH with an acidulating fertilizer.
Bud Drop Solution
When the buds are dropping, make sure the gardenia is exposed to proper temperatures. They do best when the daytime temperatures are between 68 and 74 degrees Fahrenheit. At night, they are comfortable at around 60 degrees Fahrenheit. Quick changes in temperature can cause leaf drop. If nighttime temperatures drop suddenly, cover the shrub to keep it warmer. When bud drop is a problem, pay close attention to watering, making sure not to overwater. The gardenia needs full sunshine each day. If you have planted it in too shady of an area, move it to a better lighted location.