They're so ubiquitous we don't give marigolds their due. These are extremely tough little work horses. They do best in full sun and when grown a little on the dry side. If crowded in damp conditions, they can be prone to mildew, but you can avoid that if you give them plenty of air flow. Deadheading will get them blooming repeatedly, but even if you don't bother with it, they will resume blooming soon enough. Another under-rated feature of marigold is their use as companion plants, repelling pests like asparagus beetles, bean beetles, nematodes and even rabbits.