Plants that keep pests away, and how far they really go

There is a version of this advice that goes around every spring. Plant marigolds and your aphids vanish. Put a pot of basil on the table and the flies leave. Grow lemongrass and the mosquitoes give up on your whole street. It would be lovely if any of that were true at the strength people claim, and the honest answer is that some of it works, some of it works only in very specific conditions, and a fair bit of it is folklore that has been repeated so often it now sounds like science.

We are not trying to talk you out of growing these plants. Most of them are worth having anyway, because they smell good, they feed bees, and several of them go into supper. We just think it is more useful to know what each one actually does, so you plant it in the right place and are not surprised when the aphids arrive regardless. Here is what holds up.

What companion planting can and cannot do

The idea behind companion planting is that some plants protect their neighbours, either by repelling a pest directly, by confusing it, or by pulling in the predators that eat it. All three of those things are real. The problem is scale. A single plant in a large garden produces a small amount of volatile oil that disperses within a metre or so, and the pest that is hunting by sight rather than smell does not care at all.

Where it does work is at close quarters and in numbers. A row of aromatic plants growing right against a crop, a dense band of flowering plants that keeps ladybirds and hoverflies on site, a trap plant sacrificed on purpose. Our longer piece on companion gardening and why people do it covers the practice itself. This article is about the individual plants and how far each one really goes.

Marigolds, the one with real evidence behind it

Marigolds are the classic recommendation and they are also the one with proper research attached, though not for the reason most people think. The evidence is not about aphids or whitefly. It is about root knot nematodes, the microscopic soil pests that make knobbly galls on tomato and carrot roots and stunt the whole plant. Certain Tagetes species produce compounds in their roots that suppress those nematodes in the soil around them.

The catch is in the method. A review in Applied Soil Ecology on using marigold as a cover crop against plant-parasitic nematodes found the effect depends on growing a dense stand for a couple of months and then working it into the soil, and that it is species and cultivar specific. Three marigolds dotted along a bed will not clear a nematode problem. If you are refreshing a container of soil that grew a sickly tomato crop, though, a season of marigolds in that pot is a genuinely sensible thing to do, and they are cheerful while they are at it.

Nasturtiums are a trap, not a repellent

Nasturtiums get sold as a pest repellent and they are close to the opposite. Aphids adore them, and so do cabbage white caterpillars, which is exactly why they are useful. Grown at the edge of a bed or in a pot beside your beans, they draw the pests onto themselves and hold them there where you can see the problem and deal with it in one place. NC State Extension has a good profile of Tropaeolum majus if you want the growing detail.

Two things make trap cropping work. You have to accept that the trap plant will look terrible, because a nasturtium covered in black aphids is doing its job. And you have to actually intervene, either by hosing the colony off, cutting the worst stems and binning them, or leaving them long enough for ladybirds to move in. A trap crop you ignore just becomes a nursery that feeds the rest of the garden.

Basil, and the aphids that ignore it

Basil earns its place next to tomatoes for reasons that have very little to do with pest control and everything to do with how good the two taste together. There is some evidence that its volatile oils interfere with how certain insects locate a host plant, but the effect is close range and short lived, particularly on a windy Highveld afternoon when everything disperses.

Grow it because it is one of the easiest and most rewarding things you can put in a pot, and put that pot within arm's reach of where you cook. Basil hates cold, so on the Highveld it is a spring to autumn plant and it will collapse with the first proper frost. Our guide to the five easiest herbs to grow in pots has the practical side, and a light open mix like Bark Outdoor Potting Soil suits it far better than heavy garden soil.

Lavender and rosemary near the doors

These two are worth having for real, just for slightly different reasons than advertised. Both carry strong aromatic oils, and both are convincingly disliked by some flying insects at close range, which is why a pot of either beside a doorway or a seating area is more useful than the same plant at the bottom of the garden. Brush past a rosemary bush and you release a cloud of oil, which is when the effect is strongest.

They also happen to be perfectly built for South African conditions. Both want full sun, sharp drainage and far less water than people give them, and both sulk badly in a pot that stays wet. If you have killed lavender before, it was probably drowned rather than starved, and our piece on growing lavender that lasts goes through the drainage and pruning that keeps a plant going for years rather than months. The flowers are also excellent bee forage, which matters more to your vegetable yields than any repellent effect.

Mint, useful but keep it contained

Mint is genuinely disliked by ants, and a crushed handful rubbed along a windowsill or a doorframe will move an ant trail for a while. It is also one of the plants most likely to take over a bed and never leave, because it spreads by runners that travel under paving and come up somewhere you did not plan.

Grow it in a pot. Always. Not a pot sunk into a bed, because the runners will find the drainage hole and escape, but a pot standing clear of the soil. It is happy in half sun, it wants more water than the other herbs, and it will need dividing every second year when the roots fill the container. That is not a flaw, it is just how mint works.

Wild garlic, the local option

Tulbaghia violacea, the wild garlic or society garlic you see in verges all over Gauteng, deserves more credit than it gets. The whole plant smells strongly of garlic when disturbed, it is used traditionally against a range of ailments, and gardeners have long planted it as an edging where they want something aromatic between a path and a bed. SANBI's profile of Tulbaghia violacea covers its range and its uses properly.

What we like about it practically is that it asks for nothing. Full sun, poor soil, long dry spells, light frost, it carries on flowering through most of the warm months and it multiplies steadily. Garlic chives do a similar job in a smaller space and go into the kitchen as well. Neither will clear a pest outbreak, but both keep something aromatic and flowering where the useful insects can find it, and our notes on planting for bees explain why that matters more than repellence.

Lemongrass and the mosquito claim

Lemongrass and citronella grass get sold on the promise that a pot on the patio keeps mosquitoes away. The plant does contain citronellal and related compounds, and those compounds genuinely are repellent. The problem is that an intact leaf releases almost none of it into the air. A review in Malaria Journal on plant-based insect repellents and how they are tested is clear that the useful protection comes from oils applied to skin, and that it is short lived even then.

So the honest version is this. A lemongrass plant standing quietly beside your chair does very little. Crush a few leaves in your hands and rub them on the table edge and you will notice a difference for maybe half an hour. It is still a good plant to have, it makes excellent tea, it looks handsome in a large pot, and on the Highveld it will need protection from hard frost. Just do not buy it as a mosquito solution.

When the pests arrive anyway

They will, because that is what a garden is. The realistic approach is to make the pests easy to spot and easy to deal with early, which means checking undersides of leaves once a week rather than waiting for a plant to look sick. Most container pest problems on a patio come down to aphids, mealybug, red spider mite in hot dry spells, and the occasional caterpillar, and all four are manageable if you catch them in the first fortnight.

Healthy plants also shrug off far more than stressed ones. A herb that is root bound in a small pot, in exhausted soil, watered erratically, is the one that gets covered in aphids while the plant next to it is fine. Our quick guide to potted plant pests covers identification and the low intervention fixes that work in containers, and a trough of mixed herbs like the Curo trough gives roots enough room that the plants stay tough enough to look after themselves.

Plant the marigolds, grow the nasturtiums, keep the lavender by the door and the mint in its own pot. You will get a garden that smells wonderful, feeds bees, and has fewer pest crises than it otherwise would, which is a fair return even if none of it is magic. Our herb and vegetable pots are the sizes that keep these plants growing strongly, which is the real pest control.

Frequently asked questions

Do marigolds actually repel insects?

Not reliably. The good evidence is for suppressing root knot nematodes in soil, and it needs a dense stand grown for a couple of months and then dug in, rather than a few plants scattered through a bed.

Does lemongrass keep mosquitoes away?

Only when the leaves are crushed, and only briefly. An intact plant standing on a patio releases very little of the repellent oil into the air, so it will not clear an area of mosquitoes on its own.

What is a trap crop?

A plant grown deliberately to attract pests away from the crop you care about. Nasturtiums are the classic example for aphids and cabbage white caterpillars, and they work only if you then remove or treat the infested plant.

Which pest repelling plants grow best in South African pots?

Rosemary, lavender, wild garlic and garlic chives all handle full sun, poor soil and dry spells. Basil and mint need more water, and basil will not survive a Highveld frost.

Should I stop using pesticides if I plant these?

Pest plants reduce pressure rather than remove it. Weekly checking, healthy roots in good potting soil and early intervention do more for a container garden than any single companion plant.