Companion planting is a system of planting (or not planting) crop near each other. For example, the “3 Sisters” utilizes the complementary natures of beans, squash and corn. Radishes protect cucumbers from pests. Onions stunt the growth of peas.
While companion planting is really really cool, it’s also really really complicated. What can I plant where? What does well with what? Is that going to kill my squash?
There are several good companion planting guides on the Internet, but few as straight-forward and concise as this one from Mother Earth News. Originally published in 1981, it contains companion information for all common vegetables and herbs, as well as the benefits (or drawbacks) of each pairing.