The argument for companion planting is not entirely straightforward because there are two quite different approaches to the art of choosing a companion.
Traditional Companion Planting
This is the folk wisdom approach and has been built up and – to a certain extent – verified, over centuries of agriculture. An example of this kind of companion planting is the use of the Three Sisters method inNnative American agriculture to grow beans, squashes and corn by planting the seeds in the same hole. The theory is that the corn grows tall and sturdy and makes a support for the climbing beans which add nitrogen to the soil which feeds the corn and the squash which spreads out around the base of the other plants and makes shade with its large leaves to conserve moisture as well as offering a spiky protection from slugs and snails.
It sounds ideal, but it’s worth remembering that this system was developed in a tropical climate so the corn started much earlier than it does in a temperate one. Here the beans almost always grow faster than the corn! Also, the corn used in Native American growing is dried to make flour, not sweetcorn, so it has a different growth pattern. It is possible to use the Three Sisters method but the results are nowhere near as good as those achieved in the climate and with the seeds for which it was developed.
More usefully, growing French marigolds near food crops has generally been found to reduce pests – this is because their roots exude a substance which spreads through the soil in their immediate vicinity, and which is toxic to certain damaging nematodes. However you do need to plant the marigolds densely for this to work. In addition, whitefly hates the smell of French marigolds and will not come near tomatoes interplanted with the flowers. Beans, on the other hand, can fail to thrive if planted near them.
Strong odours can repel certain pests – so a garden path or border planted with creeping thyme or lavender that is regularly trimmed may deter slugs and snails because crawling over or through it can confuse their food sensors located along their ‘foot’. Both tansy and pennyroyal repel ants by creating a strong fragrance that interferes with the chemical compounds ants use to give each other orders. In all cases though, you need to plant your companion at about the same density as your crop – a few plants scattered around will not achieve the required effect.
Sensitive Crystallisation Planting
This New Age system is based on the work of Dr Ehrenfried Pfeiffer in the 1930s. It is essentially a process of making a chromatogram (separating the components of a mixture and displaying them in a coloured pattern) of different plants. Dr Pfeiffer mixed plants together and then made the chromatograms – he decided that mixtures of plants which formed clear and vivid chromatograms were mutually beneficial, while ones that were dull or cloudy were damaging to each other.
This is simply not proven by any scientifically verifiable method and the brightness of a chromatogram doesn’t bear any resemblance to the way the plants grow together and the suggestions Pfeiffer made were not based on watching the plants grow so he had no evidence one way or the other about the actual performance of the plants. While this system is still used by some biodynamic gardeners who believe in the ideas of Rudolf Steiner, several research studies, including one by a senior researcher at Washington State University, have found no support for the Pfeiffer companion planting suggestions.