Among the vegetables Elizabeth grows in the community garden are tomatoes, cabbages, peppers and cauliflower.
This question is probably just a test to see if you've been doing the assigned reading, as it doesn't really address the meaning of the garden or the vegetables themselves; I don't really see a purpose to naming the vegetables themselves.
At this point in A Question of Power, Elizabeth is attempting to combat her mental illness by demonstrating her sanity, in part by working in the garden. However the garden serves as an allegory for her mind, and her efforts to care for it and keep it thriving in an orderly way, with healthy vegetables replacing the wilted and rotting imports (akin to her illness) that the town was used to. In a sense, though, Elizabeth is still separated from other people, just as the garden is fenced off; her existence is not like that of anyone around her, just as the garden is a very specific and, in a way, unnatural arrangement.