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.

Three countries, three opinions

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 the other way round. There, onion and garlic are recommended as very good cabbage partners, with their sulphur smell said to keep pests away. And in France there’s a third way, because they distinguish within the alliums. Garlic, shallot and leek next to cabbage is considered tricky by many, while the onion is often explicitly recommended.

The same plant group three times, the same smell three times, and three 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 three 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 fits with that. No explicit warning about this pairing there either, but no positive recommendation either. That “neither one nor the other” felt to me like the most honest answer. And I have more confidence in those books.

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.

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.

Crop rotation is where it gets more interesting

Things look different when the two aren’t next to each other but follow each other in the bed. Here there is a documented benefit, but only in one direction: cabbage before alliums.

As their residues break down, cabbage and the other brassicas leave behind substances that are converted into natural, fungus-inhibiting compounds in the soil. This effect is called biofumigation. It can suppress soil-borne pathogens, including the cause of the dreaded white rot. Exactly the effect you want for the allium crop that follows.

Why onion and shallot are an exception in the crop rotation

But there’s a catch, and it affects the onion and its close relative, the shallot, of all things.

The demanding cabbage varieties are often fertilised and leave nitrogen-rich material in the soil. For leek or spring onions this extra nitrogen is no problem. Storage onions and shallots, on the other hand, react sensitively. Too much nitrogen gives them soft onions that store badly and the familiar thick necks. The nice biofumigation benefit and the nitrogen drawback roughly cancel each other out here.

That’s why in the database we treat the onion and the shallot after heavy-feeding cabbage varieties differently from the rest of the allium family. After an early-clearing, undemanding kohlrabi, though, the onion is a good successor again.

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”. The usable connection lies in the crop rotation, and that’s where we map it, where it actually applies.