Companion plants for courgettes, and why space matters more

"What should I plant next to courgettes?" The answers circulate as charts: courgettes "like" sweetcorn, beans, borage and nasturtiums; they "hate" potatoes. Those lists have been copied for decades, are almost never sourced, and they miss the main problem.

With a courgette — zucchini, if you are reading this in North America — the question is not who gets along with it. It is where to put it. A mature plant is bulky and grows fast, throwing large leaves up on thick stalks. From midsummer onwards, everything around it lives in its shade. That physical constraint comes before any question of companionship.

A plant that settles the space before it settles its neighbours

Start with a rough figure, presented for what it is: common practice, not a measurement. Usual planting distances run to about a metre, roughly three feet, between bush courgettes, and more for trailing varieties. Plenty of gardeners plant at two feet because the young plants look absurdly small in late spring. By August those same plants overlap, the foliage dries slowly after rain, and powdery mildew moves in.

Space you give the courgette is space you take from its neighbours. A pairing is therefore never good in the abstract: it is good if the companion tolerates shade, if it occupies the bed before the courgette claims it, or if it grows tall enough not to be buried.

Lettuce and radishes, for instance, do very well beside a courgette, but only in spring while the plant is small. That is temporary use of ground you are about to lose, which is perfectly sensible as long as you harvest in time.

The three sisters: a real pairing, on strict conditions

Sweetcorn, pole beans and squash make up the best known association in vegetable growing, and one of the few whose logic is explicit. The corn acts as a support for the beans; the beans, like all legumes, associate with nitrogen-fixing bacteria; the squash covers the ground and suppresses weed seedlings. It is a documented agricultural system, long practised by Indigenous farmers across North and Central America, not a magazine invention. But it is nearly always presented without its conditions, and without them it disappoints.

The first is area. The three sisters are grown as a block, not a row: sweetcorn is wind-pollinated, and a short line of plants gives you cobs with missing kernels. You need a square, several rows deep.

The second is staggering the sowings. Beans sown at the same time as the corn climb a stem that is still too soft and flatten it. The usual practice is to sow the corn first, then add the beans once it stands a foot or so high.

The third is variety choice and what you are actually after. An over-vigorous bean smothers the corn, and modern sweetcorn is often too short to serve as a pole. More to the point, the system aims at total yield from an area, not at the best possible harvest of one crop: the squash there gets less light than it would on its own.

Pollination: where the harvest is really decided

This is the most useful thing in this article, and the part the charts almost always leave out. A courgette carries separate male and female flowers on the same plant. The female is easy to spot: at its base sits a miniature fruit, already formed, which will only swell if pollen is carried to it from a male flower. Cucurbit pollen is heavy and sticky, so it does not travel on the wind — insects move it, mainly wild and honey bees. That is an established mechanism, not a belief.

When pollination fails, the symptom is distinctive: the small fruit yellows, softens at the tip and drops off. Many gardeners read that as disease or drought; more often it is simply a fruit that never set. It also happens early in the season, when the plant is producing male flowers only, which is normal and passes.

This is where flower pairings earn their place. Sowing nectar-rich flowers around the plot in a succession that keeps something in bloom — borage, phacelia, calendula, cosmos, plus dill and coriander allowed to bolt — increases visits from pollinators and beneficial insects. Nobody can honestly promise you a specific percentage gain without stating trial conditions, but the link between insect activity and fruit set in cucurbits is not in doubt.

Failing all that, hand pollination works: pick a male flower in the morning, strip the petals, and dab the pollen onto the stigma of a female flower. It is the one action here whose effect you can see for yourself within days.

Nasturtiums and flowers: what is plausible and what is not

Nasturtiums are routinely recommended as a trap crop for aphids. The principle is genuine and used in commercial agriculture: a more attractive plant concentrates the pest and draws it off the crop. Nasturtiums do get heavily colonised by blackfly.

The caveat deserves saying plainly: a trap crop attracts, it does not kill. The arrangement only works if you watch the nasturtiums and cut out the worst-infested stems, or if you let predators — ladybirds, hoverflies, lacewings — build up on them. Otherwise you have simply created an aphid reservoir.

The related claim that nasturtiums "repel" insects, or that a given herb drives pests away by smell, is far more weakly supported. Those statements mostly derive from tests on concentrated essential oils, which bear no relation to what a whole plant releases in a garden. Grow nasturtiums, but do not treat them as protection.

What to keep at a distance

Potatoes. This is the least controversial exclusion. Both crops are hungry and bulky, and they compete for water and nitrogen at the same moment; earthing up and then lifting the potatoes also disturbs the courgette's shallow roots. Not antipathy, then, but competition compounded by incompatible soil work.

Other cucurbits planted tight. Cucumbers, melons, pumpkins and winter squash share the same diseases, powdery mildew first among them. Grouping plants susceptible to the same fungus under dense foliage that slows air movement makes it easy for infection to spread from one to the next. Grow several by all means, but space them out.

Anything needing light at ground level. Basil, summer lettuce, carrots, beetroot: too slow, and covered over before they deliver.

A last word on Florence fennel, cited everywhere as a universally bad neighbour. Allelopathy — one plant inhibiting another through compounds it releases — is a real phenomenon, described in several species; in fennel's case in the vegetable garden, the effect is reported more often than it is measured. Since fennel grows perfectly well on its own, the doubt is reason enough to keep it apart.

How Marmanda handles this

Marmanda works out sowing and planting windows and placements from your location and a description of your garden: area, aspect, what is already growing. Every fact it shows comes with its source and its limits, and a popular pairing resting on thin evidence is labelled as exactly that. Where no usable data exists for a pair of crops, Marmanda leaves the box empty.

In short

Sow nectar-rich flowers, in succession, because courgettes depend on insects to set fruit. Grow nasturtiums if you are willing to tend them. Try the three sisters if you have a block to spare and want total yield. Keep potatoes away, and loosen up the cucurbits.

And give each plant the room it will need in August, not the room it takes up in May: no pairing makes up for a foot and a half of missing spacing.