Antique Gardenia Has Black-Green Leaves

Q: We have an antique gardenia that normally has full-size leaves and big fragrant blossoms in the spring. I was inspecting it the other day and noticed the leaves looked weird. Their color is as much black as green. 

A: I would wager that you had gardenia whiteflies in abundance on your shrub last year. The black leaves are covered with sooty mold which grows on the undigested sap excreted by the sucking insects last summer. The black film hinders sunshine from penetrating to the gardenia’s leaf surface. Your situation is not an emergency but if you let it continue it will weaken your gardenia. The easiest way to control whiteflies is by applying a systemic insecticide, like imidacloprid, after the blooms fall. If you want to kill the insects immediately, use any garden insecticide, including insecticidal soap and neem. Aim the spray at the underside of leaves since that’s where the insects live. The black film on the leaves will gradually disappear in a month or so.