Vegetables Companion Planting Guide provides information on how to plant different vegetables together to help or hinder pests. Alliums such as onions help fruit trees, nightshades and brassicas, and are helped by carrots. Brassicas help potatoes and cereals, and are attracted to by beneficial insects but avoided by mustards and nightshades. Beans help corn and spinach, and are attracted to by beneficial insects but avoided by tomatoes, chili peppers and alliums.