Companion planting the old way: what holds up about the classic bed partnerships

Ask three older gardeners at the allotment what belongs next to the carrots and you will get the same answer three times: onions. No hesitation, no explanation, everybody just knows. And if you push a little, out comes the sentence that every gardening book has been copying for a hundred years: each one confuses the other one's fly.

I believed that for years without ever testing it. Then I spent two seasons growing one bed half mixed and half in single blocks. There is something to it, but it is smaller than the story promises. The onions helped as long as they were growing strongly. The moment they flopped over in July, the protection was gone.

That is how it goes with most of the old bed partnerships: what generations observed is rarely wrong, but it is almost always incomplete. So let us go through it properly. Which classics hold up, what mechanisms sit behind them, and which neighbours you are better off keeping apart.

Why the old bed partnerships came about

The cottage garden was never a design project. It was a piece of ground that had to feed a family from May well into winter. On that kind of plot you cannot afford tidy single crop blocks standing half empty. So things were stacked: lettuce between the cabbage rows, climbing beans up the fence, onions wherever there was room, often right next to the carrots. The observation came afterwards. If a combination went well two years running, it stayed. The trouble is that this method never separates cause from coincidence. A year with few carrot flies looks exactly like a year in which the onions did the work.

The mechanisms that actually do the work

There is a handful of principles you can genuinely follow in your own bed. Everything beyond that is folklore, or too weak for you to notice.

The classics, checked one by one

Carrot and onion

The carrot (Daucus carota subsp. sativus) is supposed to confuse the onion fly, and the onion returns the favour with the carrot fly. Field trials did show lower infestation, under two hard conditions: the onions had to be in active growth, and the ratio had to tip clearly in their favour. Two to four onion rows to one carrot row, not one row of each in a pretty alternating pattern.

Take the combination by all means, just do not rely on it. What does protect reliably is insect mesh with a 0.8 mm gauge. The coarser nets around 1.3 mm are designed for cabbage white and cabbage root fly; for the slender carrot fly you want the fine version. Two conditions come with it. The mesh goes on from sowing day with no gaps and is weighted down at ground level, and the bed must not have carried carrots or other umbellifers the previous year. Otherwise the overwintered pupae hatch underneath the mesh, and you have shut the fly in rather than out. Details in sowing carrots without carrot fly.

Cabbage and celeriac

Celeriac is said to irritate the cabbage white with its essential oil, and in return the white cabbage shades the soil. Very little of that is documented. The more honest reason the pair looks good: they belong to different families, so they do not share the family specific diseases, neither clubroot nor cabbage root fly. Pathogens with a broad host range, white rot for instance, still hit both. And both are hungry feeders with a similar thirst, which comes at a price: two greedy crops in one bed need a generous dose of compost and 40 cm of space. The safest way to keep the cabbage white off is the mesh, and beyond that it helps to pick the egg clusters off the undersides of the leaves. See protecting cabbage.

Bean and summer savory

Summer savory (Satureja hortensis) is supposed to keep the black bean aphid away. That has never been demonstrated, but it has been observed for centuries. What is certain is the kitchen side of the deal: it is ready to pick when the beans are. Plant it at the head of the row, because in the shade of a vigorous climbing bean it just sulks.

The Three Sisters: the teaching example

Maize, bean and squash are the classic combination in which the load bearing mechanisms mesh most cleanly: the maize supplies the top storey and the climbing frame in one, the bean brings in nitrogen, the squash covers the ground. Scent and trap plants play no part here, but the other three all work at the same time. There is no mysticism in that, just a division of labour.

On yield you have to stay honest: in our climate each of the three gives you more grown on its own than in the group. The gain lies in the total per square metre.

1. Maize first, with a head start

   Sweetcorn goes in from mid May · CW 20, as soon as the soil at 5 cm depth is reliably above 10 degrees, in practice once the mid May cold snap has passed. Plant it as a block, never as one long row, or wind pollination will not work: at least four by four plants, 40 cm apart.

2. Add the bean once the maize is 15 to 20 cm tall

   That takes two to three weeks, so roughly CW 22 to 23. Two bean seeds per maize plant, about 10 cm to the side. Choose an ordinary climbing bean: the runner bean grows so vigorous that it will flatten the maize in a storm.

3. Squash into the gaps, then harvest one at a time

   A trailing variety, not a bush type, otherwise you lose the ground cover. One single plant is enough for four square metres, because it genuinely needs that trailing space to itself. Harvest in sequence: beans from August, the maize once a squeezed kernel releases milky sap, the squash shortly before the first risk of frost.

Bad neighbours: here the old lore is more reliable

With the good partnerships the evidence is thin. With the bad ones it gets better, because the mechanisms are harder.

Same family side by side. The clearest case of all. Potato and tomato are both nightshades and share blight, kohlrabi and radish are brassicas like white cabbage and share clubroot and cabbage root fly. Put them next to each other and you have built the pathogen a bridge.

Onion and bean. The one incompatibility from the old list that you really do see: beans next to garlic or onions often stay pale and short. The usual explanation, sulphur compounds interfering with the nodule bacteria, is plausible but not proven. For the bed it is good enough: keep at least a metre between them.

Fennel. Florence fennel (Foeniculum vulgare var. azoricum) has the worst reputation in the bed. In laboratory trials fennel extracts inhibit germination and early growth in a long list of species, beans and tomatoes among them. For the bed the observation is enough to act on, but it has never been cleanly quantified. Give the fennel its own corner. On growing it: Florence fennel.

Several hungry feeders in a tight space. Cabbage next to squash next to potato goes wrong because all three are fighting over the same nutrients. That is not an incompatibility, that is a shortage.

Planning beats copying a table

Gertrud Franck developed her system from the 1940s onwards and published it as a book in 1980. It is more practical than any companion table: fixed rows about 50 cm apart, sown across with spinach in spring as a marker crop, ground cover and later mulch, with the main crops then going into every second row. Translated to your own bed that means distributing the plant families first and separating the relatives, then putting one tall, one medium and one low crop into each block: climbing bean, kohlrabi, lettuce. That gets the companion planting sorted, but not yet the crop rotation.

Common questions about companion planting

What can I plant next to tomatoes?

Basil, parsley, nasturtium, lettuce and dwarf beans all work well around the edge. What does not belong next to tomatoes: potatoes, because both are nightshades and share blight, and fennel, which slows growth down. Leave room either way, tomatoes want air around them.

Do onions really help against carrot fly?

A little, but not enough to rely on. Trials measured lower infestation, though only while the onions were actively growing and only when several onion rows came to one carrot row. Once the foliage flops over in summer the effect is gone, and the second generation flies from July. The only reliable protection is insect mesh with a 0.8 mm gauge, laid from sowing day and weighted down tightly at ground level. One thing matters here: the bed must not have carried carrots the previous year, or the overwintered pupae will hatch underneath the mesh.

Why do onions and beans not get along?

It has been observed for generations: beans next to onions or garlic stay small and pale. The most common explanation is that sulphur compounds from the alliums interfere with the nodule bacteria on the bean roots. That has never been cleanly proven, but the rule still serves you well: keep at least a metre between them.

When do you plant the Three Sisters?

The maize goes in from mid May · CW 20, as soon as the soil at 5 cm depth is reliably above 10 degrees, so after the mid May cold snap. The climbing bean follows two to three weeks later, around CW 22 to 23, once the maize is 15 to 20 cm tall. The squash goes in at the same time as the bean. Start the bean too early and it will overgrow the maize and drag it down.

Does companion planting replace crop rotation?

No. Companion planting organises what grows side by side in one season, crop rotation organises what stands on the same ground in successive years. Soil borne problems like clubroot, nematodes or tired soil can only be eased by moving things over time. A perfectly mixed bed that carries cabbage for four years will still get sick.

A bed does not remember, so you have to

The reason the old bed partnerships survive so stubbornly is simple: nobody ever wrote down whether they worked. All that was left was the feeling that the year with onions between the carrots was somehow better.

That is where a garden diary comes in. Note down in autumn which combination stood in which bed, how much damage you had and what the harvest was like, and after three years you have data from your own soil. In Gartenkern the plantings hang off the bed and stay there across the years, photos and notes included. When you plan in February you can see in black and white what stood in that spot. That is the most honest version of the old knowledge: do not copy it, observe it yourself.