in gardening and agriculture is planting of different crops in close physical proximity, on the theory that they will help each other. It is a form of polyculture
Companion planting is used by farmers and gardeners in both industrialized and developing countries. Many of the modern principles of companion planting were present many centuries ago in the cottage garden
. Although there is a wealth of information on its historic use, there remains limited available scientific research on the effectiveness for each of these interventions.
For farmers, these techniques are used in Integrated Pest Management, and systems can be set up to allow the farmer to have more yield and/or reduce pesticides. In the developing world, tropical crops are used instead of temperate ones and provide NGOs and other organizations a tool for alleviating poverty.
For gardeners, the combinations of plants also make for a more varied, attractive vegetable garden.
It can also be used to mitigate the decline of biodiversity.
Companion planting was widely touted in the 1970s as part of the organic gardening
movement. It was encouraged not for pragmatic reasons like trellising, but rather 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
, and changing of crops
One traditional practice was 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.
are well-known to attract caterpillars
, so planting them alongside or around vegetables such as lettuce or cabbage will protect them, as the egg-laying insects will tend to prefer the nasturtium. This is called a trap crop
Crops that suffer from greenfly and other aphids may benefit from the proximity of marigolds: These smell bad to aphids and attract hoverflies, a predator of aphids, and are also said to deter other pests. A more complete list of plants that deter insects is listed below.
The use of plants that produce copious nectar and protein-rich pollen in a vegetable garden (insectary plants) is a good way to enhance the population of beneficial insects that control pests. Some insects in the adult form are nectar or pollen feeders, while in the larval form they are voracious predators of pest insects.
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:
- The first phase is stimulation by odours characteristic to the host-plant. This induces the insect to try to land on the plant it seeks. But insects avoid landing on brown (bare) soil. So if only the host-plant is present, the insects will quasi-systematically find it by simply landing on the only green thing around. This is called (from the point of view of the insect) "appropriate landing." When it does an "inappropriate landing," it flies off to any other nearby patch of green. It eventually leaves the area if there are too many 'inappropriate' landings.
- The second phase of host-plant finding is for the insect to make short flights from leaf to leaf to assess the plant's overall suitability. The number of leaf-to-leaf flights varies according to the insect species and to the host-plant stimulus received from each leaf. The insect must accumulate sufficient stimuli from the host-plant to lay eggs; so it must make a certain number of consecutive 'appropriate' landings. Hence if it makes an 'inappropriate landing', the assessment of that plant is negative, and the insect must start the process anew.
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.
This is one of the reasons why monoculture is counter-productive: pesticides effectively immunized the pests more and more, generation after generation, while still providing ample shelter and food for these.
Source: Horticulture Research International, Wellesbourne : “Insects can see clearly now the weeds have gone.” Finch, S. & Collier, R. H. (2003). Biologist, 50 (3), 132-135.
Companion plant categories
Companion plants can benefit each other in a number of different ways, including:
- Flavor enhancement — some plants, especially herbs, seem to subtly change the flavor of other plants around them.
- Hedged investment — multiple plants in the same space increase the odds of some yield being given, even if one category encounters catastrophic issues
- Level interaction — plants that grow on different levels in the same space, perhaps providing ground cover or working as a trellis for another plant
- Nitrogen fixation — plants that fix nitrogen in the ground, making it available to other plants
- Pest suppression — plants that repel insects, plants, or other pests like nematodes or fungi, through chemical means
- Positive hosting — attracts or is inhabited by beneficial insects or other organisms which benefit plants, as with ladybugs or some "good nematodes"
- Protective shelter — one type of plant may serve as a wind break or shade for another
- Trap cropping — plants that attract pests away from others
- Pattern disruption — with monocultural crops, pests can quickly and easily spread from one plant to the next. Companion plants interrupt this spread.