In nature, plants often form communities with other species, even engaging in behavior that attracts certain species able to provide nutrients, protection or repair that the community needs. For centuries, farmers and gardeners have recognized that some species provide benefits to another when grown together and have tried to replicate these natural synergies in their own gardens. Cilantro, an early-summer herb used for its leaves and its seeds—called coriander—provides benefits to other plants grown in its vicinity.
Beneficial insects, such as small wasps and ladybugs, act as natural predators of pests like aphids that afflict garden plants. However, because the mouthparts of many beneficial insects are so small, many flowers are too large for them to reach the nectar they seek. Plants with clusters of small flowers—like cilantro, which grows bunches of tiny, white flowers in late June—attract beneficial insects that will protect nearby plants as well.
If you've ever stood downwind from a growing clump of cilantro, you know that it has a very strong and distinctive odor, one that some people find unpleasant. Bugs, it seems, do as well. Coriander seeds are used in some natural insecticides, and the plant itself repels aphids. "Rodale's Ultimate Encyclopedia of Organic Gardening" recommends pairing cilantro with spinach. The former will repel aphids that attack the tender young spinach leaves.
The National Sustainable Agriculture Information Service recommends achieving higher yields by planting species together that complement each other spatially. For example, cilantro grows rapidly into a tall, narrow plant. It can be used to provide shade or wind protection to smaller, shade-loving or tender plants. NSAIS, for example, notes that tall plants can disorient squash bugs and prevent them from finding squash and pumpkins.
Cilantro matures rapidly and, by late June, is ready for harvest. If left alone, the plant will yellow and drop its seeds, continuing to provide shade or a windbreak if you are using it for that purpose. However, when planning your garden, consider that the place you allot for cilantro will be available again by the middle of the summer, in time to sow the seeds for your autumn vegetables.
Gardeners classify plants as heavy, medium and light feeders depending on how many nutrients the plant drains from the soil during a growing season. Cilantro, like most herbs, is a light feeder. Sally Jean Cunningham, author of "Great Garden Companions," recommends rotating heavy feeders with legumes that restore nutrients and light feeders that will give the soil a chance to recover its nutrients.
Cilantro helps anise germinate and form stronger heads on the plant. Avoid planting cilantro with fennel, as it will inhibit the formation of the seeds, one of the prized parts of the plant and, of course, essential for fennel's ability to self-sow.