Companion planting is the planting of different crops in proximity (in gardening and agriculture), on the theory that they assist each other in nutrient uptake, pest control, pollination, and other factors necessary to increasing crop productivity. Companion planting is a form of polyculture.
Companion planting is used by farmers and gardeners in both industrialized and developing countries for many reasons. Many of the modern principles of companion planting were present many centuries ago in cottage gardens in England and home gardens in Asia.
For farmers using an integrated pest management system, increased yield and/or reduction of pesticides is the goal. For gardeners, the combinations of plants also make for a more varied, attractive vegetable garden, as well as allowing more productive use of space.
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In China, the mosquito fern has been used for at least one thousand years, as a companion plant for rice crops. It hosts a special cyanobacteria that fixes nitrogen from the atmosphere, and also blocks out light from getting to any competing plants, aside from the rice, which is planted when tall enough to stick out of the water above the azolla layer.
Companion planting was practiced in various forms by Native Americans prior to the arrival of Europeans. One common system was the planting of corn (maize) and pole beans together. The cornstalk would serve as a trellis for the beans to climb while the beans would fix nitrogen for the corn. The inclusion of squash with these two plants completes the Three Sisters technique, pioneered by Native American peoples.[1]
Companion planting was widely touted in the 1970s as part of the organic gardening movement. It was encouraged for pragmatic reasons, such as natural trellising, but mainly with the idea that different species of plant may thrive more when close together. It is also a technique frequently used in permaculture, together with mulching, polyculture, and changing of crops.
Nasturtium are well-known to attract caterpillars, therefore, planting them around vegetables such as lettuce or cabbage protects them from damage, as egg-laying insects will tend to prefer the nasturtium.[2] This practice is called trap cropping.
Marigolds assist crops suffering from aphids (greenfly among others) through their smell deterring to aphids and attracting hoverflies (a predator of aphids).
There are a number of systems and ideas using companion planting.
Square foot gardening, for example, attempts to protect plants from many normal gardening problems by packing them as closely together as possible, which is facilitated by using companion plants, which can be closer together than normal.
Another system using companion planting is the forest garden, where companion plants are intermingled to create an actual ecosystem, emulating the interaction of up to seven levels of plants in a forest or woodland.
Organic gardening often depends on companion planting for its best performance, since so many synthetic means of fertilizing, weed reduction, pest control, and other garden needs are forbidden.
There are many beneficial weeds, which can be allowed to grow alongside plants, imparting the same kinds of benefits as mixing cultivated crops.
Recent studies on host-plant finding have shown that flying pests are far less successful if their host-plants are surrounded by any other plant or even "decoy-plants" made of green plastic, cardboard, or any other green material.
The host-plant finding process occurs in phases:
Thus it was shown that clover used as a ground cover had the same disruptive effect on eight pest species from four different insect orders. An experiment showed that 36% of cabbage root flies laid eggs beside cabbages growing in bare soil (which resulted in no crop), compared to only 7% beside cabbages growing in clover (which allowed a good crop). Simple decoys made of green cardboard also disrupted appropriate landings just as well as did the live ground cover.[3]
Companion plants can benefit each other in a number of different ways, including:[4]