Companion planting that works in South Africa, in beds and in pots

Companion planting is one of those garden ideas that gets passed around as a chart. Plant this next to that, never plant this near that, and your vegetables will look after themselves. Some of it is genuinely useful and backed by research. A fair amount of it is tradition that nobody has ever tested, and a small part of it is simply wrong. Since most of the people asking us about it are trying to fit a few crops into a trough on a patio, it is worth being clear about which is which.

The underlying idea is sound enough. A garden bed with one crop in it, planted in a neat block, is the easiest possible target for a pest that eats that crop. Mix things up, add flowers, keep the soil covered, and the whole system becomes harder to exploit and more welcoming to the insects that eat the pests. That much is not controversial. The trouble starts when the general principle gets turned into a list of specific pairings that promise more than they deliver.

What companion planting actually means

Strictly, it means growing two or more species close together so that at least one benefits. The benefit can come in several ways. One plant may shade or shelter another, or hold the soil, or fix nitrogen, or draw pests away from a crop, or feed the insects that hunt those pests. Interplanting also breaks up the visual and chemical signals that flying pests use to find a host plant, which is why a bed of mixed planting often takes less damage than a monoculture next door.

None of that requires magic between root systems. It is mostly ordinary ecology, and once you see it that way the useful decisions get much easier. You are not looking for plants that like each other. You are looking for plants that want the same conditions and that together make life harder for pests and easier for pollinators.

The claims that hold up

Mixing crops rather than planting blocks does reduce pest pressure in field trials, and researchers have gone further and built deliberate push and pull systems where one plant repels a pest and another draws it away. Work published in the Journal of Pest Science on combining intercropping with semiochemical releases to control aphids shows the effect is real and measurable rather than wishful thinking.

Flowers among the vegetables are the other reliable win. Nectar and pollen keep hoverflies, lacewings, parasitic wasps and bees around, and the larvae of several of those insects eat aphids in serious quantity. That principle is well established in horticultural research, and in South African gardens alyssum, borage, cosmos and any of the daisies do the job happily. We wrote more on this in our piece on planting for bees.

Marigolds and nematodes, with the fine print

This is the classic claim and it is largely true, but not in the way most people apply it. Marigolds do suppress root knot nematodes, and the University of Florida's extension service has a thorough summary of how marigolds are used for nematode management. The catch is in the detail. It works best when marigolds are grown as a dense cover crop over a whole bed for at least two months and then turned into the soil, not when three plants are dotted along the edge of a tomato row.

Species matters too. African and French marigolds, meaning Tagetes erecta and Tagetes patula, are the ones with the research behind them. Our own khakibos, Tagetes minuta, is in the same genus and has a long local reputation for the same effect, though it is an aggressive self seeder and most gardeners are trying to get rid of it rather than plant it. In a pot, the nematode benefit is mostly irrelevant anyway, since you started with clean potting soil. Marigolds still earn their place there for the flowers and the insects they bring.

Basil and tomatoes, the pairing worth doing

Basil and tomatoes in one pot is the companion pairing we recommend most, and for reasons that are practical rather than mystical. They want identical conditions, full sun, steady water and rich soil, so neither compromises for the other. The basil sits low and shades the soil surface, which cuts evaporation noticeably in a Highveld December. And you use them together in the kitchen, so having them in one trough by the back door makes sense.

On the pest question, the evidence is more interesting than the folklore. Research in the International Journal of Pest Management on testing basil as a banker plant in tomato crops found basil useful mainly for supporting populations of beneficial insects rather than repelling pests by scent. So plant them together, and expect a better ecosystem rather than a force field. Our notes on easy herbs to grow in pots cover the watering rhythm that suits both.

Alliums, and what they do and do not do

Onions, garlic, chives and our own wild garlic all carry strong sulphur compounds, and there is decent evidence that these confuse or deter some pests, particularly aphids and certain flies. Chives around roses is an old habit that seems to help a little. Wild garlic is tough, indigenous, flowers for months and shrugs off drought, which makes it a much better border plant here than most imported alternatives.

What alliums will not do is protect a bed from everything. They also compete badly with beans and peas, which is one of the few classic incompatibilities that has some support. Keep them apart and you have lost nothing. Plant chives in a small pot of their own and move it around, which is what we do.

The claims that are mostly folklore

The long charts of enemy plants deserve real scepticism. Most of them trace back to a handful of mid century books and have never been tested properly. The idea that a plant a metre away can stunt your cabbage through the soil is, in the great majority of cases, not something anyone has shown. Where plants do suffer next to each other, the reason is usually much simpler, namely one of them is taking the light, the water or the root space.

That is why we would rather you thought about it as competition than as chemistry. If a plant is doing badly next to a neighbour, look at its shade, its size and how thirsty it is before reaching for a folklore explanation. Our article on plants that actually help with pest control takes the same approach.

Pairings we plant in troughs

A trough changes the calculation because you have one shared body of soil and one watering schedule. Everything in it has to want the same treatment. Basil, cherry tomato and a few marigolds is our standard sunny trough. Lettuce, spring onion and coriander works beautifully in a cooler, partly shaded spot, and the lettuce shades the soil for the coriander which stops it bolting quite so fast. Thyme, oregano and rosemary belong together in their own trough because they all want it dry, and they will die slowly if you plant them beside anything that needs daily water.

Give a trough decent depth and fill it with Bark Outdoor Potting Soil mixed with a little Bark Compost. A Curo trough takes a mixed planting comfortably without the plants fighting over root room, and because it is cast from our Fibre Concrete Mix it stays put in wind and handles frost without flaking. There is more on crop by crop depth in our guide to growing food in pots, and the herb and vegetable range shows what sizes suit what.

How to try it without wasting a season

Change one thing at a time and watch. Put flowers among the food this year and see whether aphid outbreaks calm down. Next season, try two crops in one trough instead of one. Keep a rough note of what got eaten and when, because your own garden will teach you more in two seasons than any chart will.

The gardeners who get the most out of this are not following rules. They have simply stopped planting in blocks, kept something in flower most of the year, and stopped spraying every insect they see. That combination does more for a vegetable patch than any specific pairing.

If you want to try mixed planting without digging a bed, a trough is the easiest way in. One deep container by the kitchen door, planted with a crop, a herb and a few flowers, will teach you the whole idea in a single summer.

Frequently asked questions

Does companion planting really work?

Parts of it do. Mixing crops instead of planting blocks reduces pest pressure, and flowers among vegetables genuinely bring in pollinators and predatory insects. The long charts of plants that supposedly harm each other are mostly untested tradition.

Do marigolds keep pests away from tomatoes?

Marigolds suppress root knot nematodes in the soil, but mainly when grown as a dense stand over a whole bed and then dug in. A few plants along a row will not do much beyond attracting beneficial insects.

Can I plant basil and tomatoes in the same pot?

Yes, and it is one of the better pairings. They want the same sun, soil and water, the basil shades the soil surface, and both are useful in the kitchen at the same time.

What should not be planted together in a trough?

Anything with different water needs. Mediterranean herbs like rosemary and thyme want to dry out between waterings, so they suffer badly next to lettuce or basil that need daily moisture.

Which flowers are best among vegetables in South Africa?

Alyssum, borage, cosmos, marigolds and any of the daisies all flower long and hard here, and they keep hoverflies and lacewings around, whose larvae eat aphids.