Some plants make powerful allies in the battle against harmful insects. Rani Jacobs, who runs a shop in Los Angeles that caters to organic gardeners, suggests a number of helpful plants that can enhance your garden. Some plants are physically compatible. The lettuce appreciates some shade, which the collard provides. Another example of companion planting is training a squash to grow on a trellis to provide shade for lettuce to grow underneath.
Some plants benefit the garden by fixing nitrogen, which enriches the soil.
Peas, beans, clover and alfalfa are all legumes that fix nitrogen.