Companion planting

Companion planting is a method of polyculture used in gardening and small-scale agriculture. It involves planting different crops in close proximity to achieve specific benefits. These benefits include weed suppression, pest control, and improved pollination. It can also provide habitats for beneficial insects and maximize the use of space. By using these methods, farmers may increase their total crop productivity. This approach can also reduce the need for expensive artificial fertilizers and pesticides.

The biological mechanisms behind companion planting are diverse and complex. One primary method is nutrient provision through nitrogen fixation. Legumes, such as clover, host symbiotic bacteria in their root nodules.

These bacteria take nitrogen from the atmosphere and convert it into compounds in the soil. This process provides natural fertilizer for neighboring plants, such as grasses. Another mechanism is trap cropping, where alternative plants attract pests away from the main crop. For example, nasturtium plants may attract caterpillars that would otherwise eat members of the Brassicaceae family.

Plants can also use chemical or physical ways to manage pests. Some plants are allelopathic, meaning they produce chemicals that affect other species. Rye is a cereal crop that can act as a cover crop to suppress weeds. It produces phytotoxic substances called DIBOA and BOA, which inhibit the growth of nearby seedlings. Other plants use aromatic volatile compounds to mask the scent of a target crop. For instance, the smell of onions may deter the carrot root fly. Conversely, the scent of carrots may deter the onion fly.

Another important strategy is host-finding disruption. Insects often find host plants by detecting specific odors in the air. If a target plant is surrounded by "decoy" plants or green ground cover, pests may experience "inappropriate landings." Research showed that 36% of cabbage root flies laid eggs in bare soil. However, only 7% laid eggs when cabbages were grown with clover ground cover. This disruption makes it harder for flying pests to locate their intended food source. Some companion plants also act as insectary plants by producing nectar and pollen. These plants recruit predators, such as the mirid bug attracted to spearmint.

The history of these practices spans thousands of years across many cultures. In Mesoamerica, indigenous peoples developed the "Three Sisters" technique. They grew maize, common beans, and squash together in a functional system. The cornstalk acted as a trellis for the beans to climb. The beans fixed nitrogen to benefit the maize. The wide leaves of the squash provided shade to keep the soil moist.

In China, mosquito ferns have been used with rice for at least a thousand years. These ferns host the bacterium Anabaena azollae, which fixes nitrogen and blocks light from competing plants.

Ancient writers also documented these plant interactions. About 2000 years ago, Theophrastus noted that bay trees and cabbage could weaken grapevines. Pliny the Elder observed that the shade of the walnut tree could poison other plants. In the 20th century, organic farming and permaculture revitalized these methods. Since the 1920s, these techniques have been vital where chemical inputs are restricted. Modern agronomists now suggest a multitrophic approach for sustainable crop protection. This involves managing interactions between crops, companion plants, pests, and natural enemies.

While many interactions are beneficial, they are not always predictable. Some combinations can actually fail to protect a crop. For example, French marigold inhibits the codling moth, which destroys apples. However, it also inhibits the moth's natural enemy, the parasitoid wasp Ascogaster quadridentata. This can result in no actual reduction in apple damage. Similarly, a 2019 study in Brazil found that parsley helped suppress aphids in collard greens. Yet, it also reduced the number of parasitoid wasps because predatory insects ate them. These complex biological webs show why companion planting requires careful study.