What are nightshade vegetables? They are vegetables that grow at night. Eggplant, tomatoes, peppers, and potatoes are the most common. Some people believe these vegetables trigger arthritis flares, but there is limited scientific evidence to support this theory. Dennis C. Ang, MD, suggests keeping a food diary to see if symptoms increase when eating nightshade vegetables. Lona Sandon, RD, agrees, suggesting to avoid them and replacing with other sources of vegetables high in vitamin C and A. Or visit a registered dietitian for other suggestions.