Beans and onions shouldn’t be planted together. That’s the conventional wisdom. It’s accepted in gardening textbooks, and it’s taught in horticultural schools across the world, but, why can’t onions and beans be planted together?
From my experience, beans and onions work well together, so I was amazed to learn that there was in fact a very specific chemical reason for beans and onions to be planted apart. In this article, I’ll outline the science behind beans’ and onions’ incompatibility, and offer a few solutions for your garden.
Why shouldn’t you plant beans and onions together?
There are three explanations given for why beans and onions are incompatible:
- Beans store nitrogen, so onions bolt (FALSE)
- Onions produce a chemical that stunts beans’ growth (TRUE)
- Beans and onions compete for moisture (TRUE)
Let’s address those theories in order:
- The first idea, that beans are nitrogen fixers, and therefore provide too much nitrogen to onions is misleading. Beans are nitrogen fixers, yes, but they do not share that nitrogen with other plants, so onions actually benefit from this relationship.
- The second, that onions produce a chemical that stunts beans’ growth is true, and most likely where the advice of growing these vegetables apart came from (because there is some weight to this idea, we’ll focus on this in the next part of this article).
- The third, that beans and onions compete for moisture, is true, but again, misleading, as they do compete for moisture, but neither are heavily moisture dependent for their early growth and if you keep them well watered and stop the soil from drying completely, they will thrive. So this is only a half-truth.
Do onions produce allicin, and why does it harm beans?
Alliums release a chemical called allicin. Allicin is an antibacterial, antimicrobial chemical produced as a gas by all members of the onion family. It’s what makes us cry as we cut onions, and why garlic can sting sore hands.
How allicin works to inhibit bean growth
Beans are nitrogen fixers. Any regular allotment gardeners will be aware of nitrogen-fixing, but might not know how it works. Nitrogen-fixing in beans is a process where plants create stores of nitrogen in their roots, stems, or leaves, which help them sustain growth throughout the season.
Nitrogen is only really important for root and leaf development, so fertilizers that are high in nitrogen are typically used for leafy greens, or climbing plants, rather than fruiting vegetables or flowering plants.
By gobbling up nitrogen in the early part of the season, they prevent other plants from using that nitrogen when they need it most – particularly important for annual climbers like beans and peas, which require huge amounts of energy.
Nitrogen-fixing plants include:
- Beans (soy, cowpeas, and all legumes)
All of these plants use a bacteria called urease to process and store nitrogen. If you’re still following the science here, you’ll see the red flag; urease is a bacteria; allicin is antibacterial.
Onions inhibit the growth of young beans and bean seedlings by emitting allicin, an antibacterial gas, which kills the beneficial urease bacteria that beans and peas require to fix nitrogen. This can inhibit the growth of beans in their early stages, and produce smaller, weaker crops.
What nutrients do beans take from the soil?
There is one theory that onions will put on too much top growth if planted near beans because beans are nitrogen fixers, but this is a common misunderstanding of what nitrogen-fixing actually means.
Nitrogen-fixing is often explained as a useful part of rotation planting, where “nitrogen fixers” are planted before “nitrogen hungry” plants. But these plants do not leave nitrogen in the soil unless you dig them through at the end of the season.
Beans store nitrogen in their entire stem, as well as their leaves, while bean roots are the richest nitrogen store in the plant. For no-dig gardens, cutting off bean tops and laying them on the ground to compost over winter will release nitrogen, while the roots will act as slow-release nitrogen fertilizers in spring.
What happens when you plant beans and onions together?
When you plant beans and onions together, it is more than likely that you won’t see any difference in your crops. However, if your onions are under attack by maggots or suffering white rot, they produce higher allicin levels and then onions can stunt the early growth of beans.
This can lead to shorter beans and smaller crops. We noticed last year when panting shallots with beans – as we often do – that the runner beans were significantly shorter than most years.
In previous years, when planting French beans with shallots there was no difference between the plants with shallots at their base and the plants with marigolds at their base. Both sets of French beans grew well regardless of their pairings.
How to plant onions and beans together
I only recently discovered there was an issue with beans and onions at all, following many years of growing French beans and shallots happily alongside each other. But how can you grow beans and onions together if they’re bad for each other?
To grow beans and onions together successfully, you just need to follow the best practice for each plant. Firstly, onions should be planted in the fall (you will always get a better crop from fall-planted onions if you can plant them early where you live).
Secondly, beans can be planted in stages, to provide a longer cropping period, but do not sow them directly in the soil. Onions do inhibit germination, but as long as you start your beans indoors (we use toilet roll tubes which help create a good root run).
In spring (assuming you have a row of onions and a climbing frame for your beans/peas), dig a trench behind your onions and fill it with fresh kitchen waste, straw, and torn newspaper. Plant your young beans into the trench and fill it in with garden compost.
The beans will feed off the decomposing food and paper waste in the trench, directly located under their roots. They will store the nitrogen, and while they may suffer some antibacterial action from the onions, they will ultimately thrive in the higher nutrients.
Plant out new beans every two weeks in spring, and you should have fresh crops as summer, and into early autumn.
My genuine advice to anyone with limited space in their garden is to just try it. Beans and onions will grow together, even if the science is against them. Thousands of allotment gardeners and passionate vegetable growers can’t be wrong, and I have my own experience of successfully growing onions and beans together here too.
Yes, there are problems, but you can grow onions and beans together. In gardens where space is limited and, on no-dig plots, in particular, beans and onions can help each other by regulating nutrients – just make sure your beans are germinated and grown on indoors before planting them out together.