Leeks next to cabbage?

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I’m currently working through our entire plant database. The reason is the new Garden Fairy coming in version 2.0. It fills free areas and time slots in the bed, suggests green manure and can even plan a whole garden from the new wishlist if you want. For it to make sensible suggestions, the data behind it has to be right. While testing I ran into inconsistencies in the companion data and in how predecessors and successors are classified. So I’m going through everything again before 2.0 is released.

One case I’ve been sitting with for a while is alliums and cabbage. So onion, garlic, leek and chives on one side, white cabbage, broccoli, kohlrabi and the rest on the other. I’ll take you through how I decided on this pair, and why.

A different answer in every country

What surprised me during the research is that you get a different answer depending on which country you search in.

In the German tables, onion and leek are almost always listed as poor neighbours for cabbage. The reason usually given is essential oils that supposedly harm the cabbage. In English-language literature it’s often the other way round. There, onion and leek are readily recommended as cabbage partners, with their sulphur smell said to keep pests away. But even there opinion isn’t unanimous; for garlic, some sites explicitly advise against it. The French gardening sites I looked at lean towards the German view and advise against cabbage as a neighbour for onion, leek and shallot.

The same plant group, the same smell, and still opposing conclusions. This isn’t a question of right or wrong, and I don’t want to play one tradition off against another here. Each of these garden cultures has gathered experience over decades. But for a database that’s supposed to say the same thing in several languages, that’s a real dilemma. Which value should I trust when experienced traditions contradict each other?

A hunch on the side

When you lay a lot of these tables side by side, something stands out. Within one language area they resemble each other closely. Sure, same topic, same plants, so they should be similar. My assumption is that an original classification was adopted, passed on and at some point became common knowledge. That would also explain why the countries are each so united in their opinion and still contradict one another.

I’m not leaving us out of that either. We researched in all sorts of sources too, so far mostly German-language ones. There’s clearly still some work to do here.

My current approach: first look at what’s actually documented

Since I can’t and won’t decide between the traditions, I went by the scientific evidence and by specialist literature.

The result is sobering. There’s hardly any solid evidence for the frequently claimed protection from the onion smell. Studies on the cabbage root fly even suggest that it isn’t the smell that works, but simply green leaf mass as a distraction, and an onion doesn’t provide much of that. I also found no sound proof of chemical damage to cabbage from alliums.

A look into the books lying next to my desk gives a mixed picture. Gertrud Franck lists cabbage and onions as unfavourable neighbours, based on years of observation in her companion planting garden, and Sister Christa Weinrich sees it similarly. Neither of them gives a reason. Andrea Heistinger calls the pair neither good nor bad. So an observation from long practice stands against a whole language area that says the opposite, and neither side has a mechanism that explains it.

How I’m classifying the companion data now

That’s why we list alliums and cabbage as neutral neighbours. No warning, but no recommendation either. The app shows a short note that this pair is handed down differently in every country.

There is one exception, and that’s kohlrabi and leek. Andrea Heistinger and Sister Christa Weinrich both describe planting kohlrabi between the leeks. The kohlrabi is harvested quickly, and after that the slow-growing leek has the space to itself. That’s a proven practical tip for exactly this pair, so we list kohlrabi and leek as good neighbours.

There’s one point I’m deliberately not counting as a problem, and that’s the frequently mentioned competition for space. The two aren’t planted right next to each other, but with a decent gap. The row spacing means the competition for light and water that some warn about doesn’t even arise. For companion data, the soil is rarely the issue anyway. It’s more about pests, and targeted protection in one direction or the other isn’t documented here.

In the crop rotation it depends on the species

In the first version of this article I described cabbage as a good predecessor for all alliums. That didn’t hold up to closer scrutiny. We have since looked at every allium individually, and the picture is much more mixed.

For leeks, cabbage works well as a predecessor. Andrea Heistinger explicitly names brassicas and potatoes as good predecessors for leeks. She gives no reason; the obvious explanation is that the heavy feeder leek copes very well with a well-supplied cabbage bed.

For garlic, chives, garlic chives and tree onion I found no source that names cabbage as a good or bad predecessor. For garlic, Andrea Heistinger and Sister Christa Weinrich recommend other predecessors, above all legumes. So we list cabbage before these alliums without a rating. For spring onions opinions differ; there cabbage as a predecessor is neutral with a note.

What became of biofumigation

Cabbage and the other brassicas contain glucosinolates. As their residues break down, these form substances in the soil that can inhibit soil fungi. This effect is called biofumigation. In the first version I concluded from this that cabbage residues suppress the cause of white rot in onions and that cabbage therefore paves the way for alliums.

The trials don’t support that for the garden. In pot trials, the number of resting bodies of white rot only fell clearly when cabbage and onion waste made up about half the weight of the soil. In the field the effect was weaker. Amounts like that are never left in the bed after a cabbage harvest. What works in research are green manures grown specifically for the purpose, such as brown mustard, which are chopped at flowering and worked in immediately. That has little to do with the harvest residues of a cabbage bed.

That’s why biofumigation has disappeared from our data as a reason. With it goes the special rule that had excluded kohlrabi, Chinese cabbage and pak choi because of their weak roots.

Why onion and shallot don’t do well after cabbage

For storage onions and their close relative, the shallot, cabbage as a predecessor is actually more of a drawback. The FiBL leaflet on organic onion growing explicitly names brassicas as an unsuitable predecessor. Cabbage leaves behind a lot of crop residue, and the nitrogen that is only released from it late delays the ripening of the onions. Too much nitrogen also makes onions soft and poor to store, and leads to the familiar thick necks.

For leeks and spring onions, which are harvested fresh and not stored, this doesn’t matter. Onions and shallots after cabbage, on the other hand, we list as unfavourable, with a one-year gap.

Conclusion

With our database we’ve ploughed through literature and cross-reference tables too, so we’re part of exactly this system. But I don’t like it when a database sounds more certain than the matter allows. For the Garden Fairy that goes double, because in the end it turns these values into more concrete suggestions for the bed than the previous filters do. For leek and cabbage, the most honest answer on companion data is a neutral “no documented effect in either direction”. There is a usable connection in the crop rotation, but it’s smaller than I first thought. Cabbage is a good predecessor for leeks and a rather poor one for onions and shallots.

Update of 2 October 2026

We have revised this article. In the first version from August, cabbage was a good predecessor for all alliums, justified by biofumigation. Since then we have checked every allium individually against specialist books, advisory services and studies. This showed that ordinary cabbage residues in the bed don’t produce any measurable biofumigation. So we rewrote the crop rotation part: cabbage is now only a good predecessor for leeks, an unfavourable one for onion and shallot, and unrated for the other alliums.

We also corrected two statements. The French gardening sites we checked don’t recommend onion next to cabbage; like the German ones, they tend to advise against it. And in the books by Gertrud Franck and Sister Christa Weinrich, cabbage and onions are indeed listed as unfavourable neighbours. This doesn’t change the neutral classification of the neighbourhood. The only new thing is the exception for kohlrabi next to leek.