Will Onions Kill Worms?

Question from Peggy

I put a small container of red worms in my outdoor compost. I went searching for them and I see no worms. Could the onions I had in it killed them?

Hi Peggy,
In all honesty, it is VERY unlikely that you would have killed off your worms simply by having onions in the outdoor composting system. There are numerous other potential reasons for the disappearance that are far more likely in my mind.

I’m not sure what type of system you added them to – but assuming there is any possibility of escape (eg. it is an open heap, or a bin with an open bottom etc), the worms may have simply moved on if the habitat wasn’t to their liking. Outdoor composting bins/heaps need to be set up in much the same way as an indoor worm bin if you want the worms to thrive in them. In my experience, a lot of people tend to use outdoor composters more like garbage cans for dry yard waste than actual composting systems (not suggesting you are doing this, of course).

Temperature could be playing a role as well. If this IS an enclosed bin of some sort, and temps are fairly warm in your region, it may have overheated and killed off the worms. Anything above 90 F (in the system itself – which will often be warmer than ambient) can kill off most composting species.

Getting back to the onions (for the benefit of others reading this)…

The oils in raw onions could definitely irritate or even harm your worms if you add a large quantity of them at once – but in a system with enough safe habitat material, it’s unlikely this would ever cause serious issues (assuming here you’re producing a “normal household” amount). Cooked onions don’t seem to create any issues at all in my experience.

Hope this helps!

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    • Dave Pawson
    • April 21, 2015

    Ah. Thanks for posting the question (and response). We bought a bag (60lbs?)
    and have the remainder left, now starting to shoot with our spring.
    I was thinking of adding them to an outdoor compost bin. I’ll perhaps let them rot down first now, before adding them.

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