An increasingly common practice in some areas of horticulture (usually in small-scale situations) is the deliberate establishment of two or more plant species in close proximity, with the intention of deriving some cultural benefit from their association. Such a situation may seem at first sight to encourage competition rather than mutual benefit. Supporters of companion planting reply that plant and animal species in the natural world show more evidence of mutual cooperation than of competition.
Some experimental results have given support to the practice, but most evidence remains anecdotal. It should be stated, however, that whilst most commercial horticulturalists producers in Western Europe grow blocks of a single species, in many other parts of the world two or three different species are inter-planted as a regular practice.
Biological mechanisms are quoted in support of companion planting:
- Nitrogen fixation. Legumes such as beans convert atmospheric nitrogen to useful plant nitrogenous substances (see p366) by means of Rhizobium bacteria in their root nodules. Beans inter-planted with maize are claimed to improve maize growth by increasing its nitrogen uptake.
- Pest suppression. Some plant species are claimed to deter pests and diseases. Onions, sage, and rosemary release chemicals that mask the carrot crop's odour, thus deterring the most serious pest (carrot fly) from infesting the carrot crop. African marigolds (Tagetes) deter glasshouse whitefly and soil-borne nematodes by means of the chemical thiophene. Wormwood (Artemisia) releases methyl jasmonate as vapour that reduces caterpillar feeding, and stimulates plants to resist diseases such as rusts. Chives and garlic reduce aphid attacks.
- Beneficial habitats. Some plant species present a useful refuge for beneficial insects (see p271) such as ladybirds, lacewings and hoverflies. In this way, companion planting may preserve a sufficient level of these predators and parasites to effectively counter pest infestations. The following examples may be given: Carrots attract lacewings; yarrow (Achillea), ladybirds; goldenrod (Solidago), small parasitic wasps; poached-egg plant (Limnanthes), hoverflies. In addition, some plant species can be considered as traps for important pests. Aphids are attracted to nasturtiums, flea beetles to radishes, thus keeping the pests away from a plant such as cabbage.
- Spacial aspects. A pest or disease specific to a plant species will spread more slowly if the distance between individual plants is increased. Companion planting achieves this goal. For example, potatoes inter-planted with cabbages will be less likely to suffer from potato blight disease. The cabbages similarly would be less likely to be attacked by cabbage aphid.
Was this article helpful?
You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!
Get My Free Ebook