Companion planting in gardening and agriculture is the planting of different crops in proximity for any of a number of different reasons, including pest control, pollination, providing habitat for beneficial insects, maximizing use of space, and to otherwise increase 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 forest gardens in Asia, and thousands of years ago in Mesoamerica.

History

In China, mosquito ferns (''Azolla'' spp.) have been used for at least a thousand years as companion plants for rice crops. They host a cyanobacterium (''Anabaena azollae'') that fixes nitrogen from the atmosphere, and they block light from plants that would compete with the rice. Companion planting was practiced in various forms by the indigenous peoples of the Americas prior to the arrival of Europeans. These peoples domesticated squash 8,000 to 10,000 years ago, then maize, then common beans, forming the Three Sisters agricultural technique. The cornstalk served as a

trellis Trellis may refer to: Structures * Trellis (architecture), an architectural structure often used to support plants (especially vineyards) * Trellis drainage pattern, a drainage system Technology * Trellis (graph), a special kind of graph used ...

for the beans to climb, the beans fixed nitrogen, benefitting the maize, and the wide leaves of the squash plant provide ample shade for the soil keeping it moist and fertile.

Practice

More recently, starting in the 1920s, organic farming and horticulture have made frequent use of companion planting, since many other means of fertilizing, weed reduction and pest control are forbidden. The list of companion plants used in such systems is large, and includes vegetables, fruit trees, kitchen herbs, garden flowers, and fodder crops. The number of interactions both positive (the pair of species assist each other) and negative (the plants are best not grown together) is also large, though the evidence for such interactions ranges from controlled experiments to hearsay. For example, plants in the cabbage family ( Brassicaceae) grow well with celery, onion family plants ('' Allium''), and aromatic herbs, but are best not grown with strawberry or tomato.

Mechanisms

Companion planting can operate through a variety of mechanisms, which may sometimes be combined. These include pest control, pollination, providing habitat for beneficial insects, and maximizing use of space; all of these may help to increase crop productivity. excerpted from

Nutrient provision

Legumes A legume () is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant. When used as a dry grain, the seed is also called a pulse. Legumes are grown agriculturally, primarily for human consumption, for livestock fo ...

such as clover provide nitrogen compounds to neighbouring plants such as grasses by fixing nitrogen from the air with symbiotic bacteria in their

root nodule Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known a ...

s. These enable the grasses or other neighbours to produce more protein and hence to grow more.

Trap cropping

Trap cropping uses alternative plants to attract pests away from a main crop. For example, nasturtium (''Tropaeolum majus'') is a food plant of some

caterpillar Caterpillars ( ) are the larval stage of members of the order Lepidoptera (the insect order comprising butterflies and moths). As with most common names, the application of the word is arbitrary, since the larvae of sawflies (suborder Sym ...

s which feed primarily on members of the

cabbage family Brassicaceae () or (the older) Cruciferae () is a medium-sized and economically important family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family. Most are herbaceous plants, while some are shrubs. The leav ...

(brassicas); some gardeners claim that planting them around brassicas protects the food crops from damage, as eggs of the pests are preferentially laid on the nasturtium. However, while many trap crops divert pests from focal crops in small scale greenhouse, garden and field experiments, only a small portion of these plants reduce pest damage at larger commercial scales.

Host-finding disruption

Flying pests are far less successful if their host-plants are surrounded by other plants or even "decoy-plants" coloured green. Pests find hosts in stages, first detecting plant odours which induce it to try to land on the host plant, avoiding bare soil. If the plant is isolated, then the insect simply lands on the patch of green near the odour, making an "appropriate landing". If it finds itself on the wrong plant, an "inappropriate landing", it takes off and flies to another plant; it eventually leaves the area if there are too many "inappropriate" landings. Companion planting of clover as ground cover was equally disruptive to eight pest species from four different insect orders. In a test, 36% of cabbage root flies laid eggs beside cabbages growing in bare soil (destroying the crop), compared to only 7% beside cabbages growing in clover (which allowed a good crop). Simple decoys of green cardboard worked just as well as the live ground cover.

Pest suppression

Some companion plants help prevent pest insects or pathogenic fungi from damaging the crop, through chemical means. For example, the smell of the foliage of

marigold Marigold may refer to: * Marigold (color), a yellow-orange color It may also refer to: Plants * In the genus ''Calendula'': ** Common marigold, '' Calendula officinalis'' (also called pot marigold, ruddles, or Scotch marigold) * In the genus ' ...

s is claimed to deter aphids from feeding on neighbouring plants. A 2005 study found that oil volatiles extracted from

Mexican marigold Mexican marigold also known as , or Aztec marigold is a native flower to México and was first used by the Aztecs and is used in the Mexican holiday "Dia de los muertos" or day of the dead. Day of the dead originated from Aztec mythology to honor th ...

by vacuum distillation reduced the reproduction of three aphid species ( pea aphid,

green peach aphid ''Myzus persicae'', known as the green peach aphid, greenfly, or the peach-potato aphid, is a small green aphid belonging to the order Hemiptera. It is the most significant aphid pest of peach trees, causing decreased growth, shrivelling of the ...

and glasshouse and potato aphid) by up to 100% after 5 days from exposure. Another example familiar to gardeners is the interaction of onions and carrots with each other's pests: the onion smell puts off

carrot root fly The carrot fly (''Chamaepsila rosae'') is a pest of gardens and farms, and mainly affects the crop of carrots, but can also attack parsnips, parsley and celery. It is a member of the family Psilidae (order Diptera). Larvae Crop damage is cause ...

, while the smell of carrots puts off

onion fly ''Delia antiqua'', commonly known as the onion fly, is a cosmopolitan pest of crops. The larvae or maggots feed on onions, garlic, and other bulbous plants. Morphology and biology The onion fly has an ash-grey body and resembles a housefly. Th ...

Predator recruitment

Companion plants that produce copious nectar or

pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

in a vegetable garden ( insectary plants) may help encourage higher populations of beneficial insects that control pests.

Protective shelter

Some crops are grown under the protective shelter of different kinds of plant, whether as wind breaks or for shade. For example, shade-grown coffee, especially ''

Coffea arabica ''Coffea arabica'' (), also known as the Arabic coffee, is a species of flowering plant in the coffee and madder family Rubiaceae. It is believed to be the first species of coffee to have been cultivated and is currently the dominant cultivar, r ...

'', has traditionally been grown in light shade created by scattered trees with a thin canopy, allowing light through to the coffee bushes but protecting them from overheating. Suitable Asian trees include '' Erythrina subumbrans'' (tton tong or dadap), '' Gliricidia sepium'' (khae falang), '' Cassia siamea'' (khi lek), '' Melia azedarach'' (khao dao sang), and '' Paulownia tomentosa'', a useful timber tree.

Approaches

Companion planting approaches in use or being trialled include: *

Square foot gardening Square foot gardening is the practice of dividing the growing area into small square sections, typically on a side, hence the name. The aim is to assist the planning and creating of a small but intensively planted vegetable garden. It results ...

attempts to protect plants from issues such as weed infestation by packing them as closely together as possible. This is facilitated by using companion plants, which can be closer together than normal. * Forest gardening, where companion plants are intermingled to simulate an ecosystem, emulates the interaction of plants of up to seven different heights in a woodland.. See also

References

{{Permaculture Sustainable gardening Permaculture Crops Biological pest control Sustainable technologies Chemical ecology