Companion planting is the practice of growing crops that are natural allies side-by-side. It’s a tradition based on many years of observations by dedicated gardeners, though in a few cases science backs up the practice, too. The most famous example of companion planting is the “Three Sisters” that Native American farmers planted together—squash, corn, and beans. The three vegetables grow together perfectly: The corn acts as a trellis for the beans, the beans return nutrients to the soil, and the broad leaves of the squash literally “squash” down weeds while locking moisture into the soil, explains The University of California Master Gardeners. But what about plants that are natural enemies in the vegetable garden? Here are seven combinations companion planters say are no-gos.