Plants from the Solanaceae family are informally called nightshade plants. Onions, like red onions, aren’t from the Solanaceae or nightshade family. Nightshade plants comprise potatoes (Solanum tuberosum), tomatoes (Solanum lycopersicum), manyflower tobacco (Nicotiana acuminata) and petunias (Petunia axillaris), grown as perennials at U.S. Department of Agriculture plant hardiness zones 9 through 11. While the potato and tomato are common foods around the world, some within this family, such as the black nightshade plant (Solanum nigrum), are extremely poisonous.
Red Onion Family
Red onions, and all onions, belong to the lily family known as Liliaceae. Red, brown and yellow onions are all considered varieties of the same species, Allium cepa. Every one is grown and selected for different flavors created when cooking. Red onions grow nicely in USDA hardiness zones 3 to 9.