Companion planting: which combinations work, according to the trials

Charts of friendly and unfriendly plants circulate everywhere. Carrot likes leek, tomato hates fennel, French marigold protects everything. Set against published trials, a few combinations hold up. Many have never been tested. Two are best avoided. The mechanism is surprising. Most often, the smell of the neighbour does not protect the vegetable. The green around it does.

Where companion planting charts come from

Most are lists of tradition, copied from one book to the next. The RHS, the British horticultural society, stays cautious. For it, these benefits probably exist, "but not for every crop in every case". It leaves the choice to the gardener.

At the start, the idea was that a strong-smelling plant masks the smell of the vegetable, or puts off the insect that lands on it. The trials looked for this mechanism. They found something else.

Combinations confirmed by the trials

Green around the cabbage: the fly gets lost

At the Wellesbourne horticultural research station, in England, Stan Finch and Rosemary Collier planted cabbages on bare soil and others surrounded by clover. Then they counted the egg-laying of eight pests.

Their explanation comes in three steps. Smell tells the insect that a host is nearby. It then lands on any green surface and avoids bare soil. Once landed, it tastes the leaf with its feet. It has to chain several successful landings on the right vegetable before laying. After a single landing on clover, it starts again from zero or leaves.

The authors warn that you still have to choose the right combinations. The two plants must also be grown in such a way that the cabbage yield stays decent.

Carrots and onions: true, but only at the start

In 1984, two British researchers, Uvah and Coaker, sowed carrots and onions in alternate rows. The carrot fly attacked the carrots less. Thrips attacked the onions less. The effect was strongest with young onions. It almost disappeared once the onions formed their bulb. In the same trial, French marigolds between the carrots changed nothing for aphids.

So the combination helps for a while, without protecting. Against the carrot fly, the RHS tested something else. A net covering the whole bed does better than a barrier. It also advises sowing after mid-May, to avoid the first generation of flies. The detail is in Carrots and leeks against flies.

Diversity in general: fewer pests, a little less harvest

A synthesis of 552 experiments, published in 2011 by the ecologist Letourneau and colleagues, compared single crops and diversified crops. With other plants around, pests were fewer and their predators more numerous. The damage was lower. The yield of the main crop still dropped slightly on average. The companion plants took the place of vegetables.

The Three Sisters: more food per square metre

Maize, bean and squash grown together come from the Haudenosaunee, the Iroquois of the north-east United States. The agronomist Jane Mt. Pleasant calculated the output of the system per hectare. It gives more energy and more protein than each of the three crops alone, or than a mix of single crops on the same area. The maize yields as much as when alone. The bean and the squash yield less, but the protein of the beans and the squash seeds makes up for it. To try them in a cooler climate, read The Three Sisters outside Mexico.

French marigold: yes against nematodes, but not the way it is planted

The University of Florida took stock. The living roots of French marigold release a substance that slows some soil nematodes, including those that make galls on roots. For it to work, you have to grow it densely, for at least two months, on the exact spot of the future vegetable, before planting it. A few plants among the vegetables do not protect. Some varieties even multiply other nematodes. The detail is in French marigolds and nematodes.

Against tomato whiteflies, the RHS cites a study where the limonene of French marigolds kept them away. It immediately adds that mixing the tomatoes with other plants, without marigolds, had a similar effect. We are back to the green.

The table, combination by combination

"Not tested" does not mean false. It means we know nothing about it. The guides Tomato and basil and Combinations to avoid dig into these cases. For nasturtium, read Nasturtiums as trap plants.

The two combinations to avoid

Tomatoes next to potatoes. Both catch the same blight. The RHS writes that the main source of the disease, every year, is the infected potato. Blight overwinters in tubers forgotten in the ground. True, most infections arrive by wind. A few metres are not enough to protect. A patch of diseased potatoes right next to the tomatoes does not help for all that. Also clear volunteer potatoes. Ideally leave four years between two tomatoes or two potatoes in the same place. The moves are in Tomato blight.

French marigolds among vegetables susceptible to nematodes. For the University of Florida, it is "very risky". The nematodes find the roots of the neighbouring vegetable and multiply there. Some varieties of French marigold even feed other nematodes.

How to use it this season

Combinations work when the two plants are at the right stage at the same time. The app gives you the sowing period of each crop in your region, so that the neighbours fall at the same moment. See your dates.

The guide Permaculture, biodynamics, the moon, lasagne beds, the evidence widens the picture. For the order of crops from one year to the next, read Rotation over 4 beds.

Photo: Benjamín Núñez González, Wikimedia Commons, licence CC BY-SA 4.0