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What is companion planting?
Planting two or more crops or other plants together in the hopes that they will help each other grow and thrive is known as companion planting.
Companion planting works because plants can have all manner of effects on their surrounding environment - they may beneficially change the soil's chemistry, for example, and prevent other plants or animals from encroaching on their territory. Many provide benefits that encourage other plants to grow.
What are examples of companion planting in action?
Many gardeners take advantage of simple companion planting without even realising it. For instance, they may grow shade-tolerant plants like ferns under the branches of a large tree.
A good example of typical companion planting is the 'Three Sister' system used by Native American groups in North America and involving the planting of three primary food crops - maize, squash and climbing beans.
Each species would provide benefits for the group. The beans, climbing the maize stalks, removed the need for beanpoles. The beans in turn provided nitrogen to the soil ensuring its fertility, while the creeping ground squash kept out weeds, helped to retain moisture in the soil, and had a prickly stem that discouraged pests.
Why companion plant?
Companion planting can help establish a healthy, lush garden, and the best argument for the technique is that of variety; instead of having blocks of a single plant in your garden, a variety of plants mixed together can help attract a year-round range of helpful pollinators and deter unhelpful pests.
Your garden will also be more varied and attractive to the eye!
What evidence is there that it really works?
There hasn't been much scientific study on companion planting. Most planting combinations have been taken from long-established traditions (the agricultural techniques of indigenous peoples such as the Native Americans, for example).
A lot of our information also comes from trial and error. Some combinations really do seem to work; others are a little less consistent.
But the beneficial strategies used by plants are no fable. Using toxins to keep away predators is a good and well-documented strategy - you don't see very many birds or insects gobbling down the garlic patch! Many deep-rooted trees help transfer ground water closer to the surface, benefiting nearby smaller plants, and nitrogen-fixers like beans improve the soil both for themselves and for other plants.
I want to give this a go! What combinations can I use in my own garden?
It's important to first investigate what and how much you should plant together, and some of your plantings may end up being be trial and error. Traditionally basil is supposed to help protect tomatoes from flies - but in practice you may need to plant an enormous amount of basil to have any real effect.
Make sure that if you are planting any toxin-releasing species, you aren't doing it near something vulnerable to natural chemical warfare! Planting cabbages near tomatoes or potatoes near pumpkin may reduce growth in the latter crops, for example.
Strong-smelling herbs, such as mint, chives and dill can help to keep away insects from the garden, and caterpillars can be lured away from your lettuces by planting a 'trap crop' of nasturtiums close by (caterpillars love the colourful nasturtiums and will ditch lettuce for them in a heartbeat!).
A comprehensive list of companion plants - what goes together, and what doesn't - can be found at Sustainable Gardening Australia.