Question: I have several orange and grapefruit trees in containers, and they look healthy except that their leaves are starting to look yellowish--not the deep green they once were. (The leaves aren't falling off, however.) They have been in a greenhouse through the winter so I know that they haven't gotten too cold. What is causing this, and what can I do?

Answer: Citrus are heavy feeders and will quickly use up nutrients in a container situation, so I suspect your trees simply need some supplemental nutrients. If you can find a citrus fertilizer at your local nursery, use that. Otherwise, any all-purpose balanced fertilizer such as 10-10-10 or an organic equivalent will be fine. Dilute fertilizer according to package directions.