It’s the 27th of December, and after 10 days away I decided to top up my worm bin with some manure and straw.
When I lifted the lid off to add the material, I immediately saw a bunch of worms at the top (some munching on some well rotted kitchen waste). That’s despite the fact that there’s plenty of material, including some well digested worm compost, for them to burrow into.
It is quite mild at the moment, with lows of about 4°C (39.2°F), but we’ve also had some cold snaps where the weather has been below freezing.
What’s more, at the end of previous winters which have reached -5°C (23.0°F), I’ve always found some surviving worms. That’s despite the fact that my worms are outside with just the roof of my (otherwise open) wood shed to protect them.
While a number of sources advise that compost worms die when temperatures hit zero, there may actually be a good reason why mine have survived.
According to soil expert Bintoro Gunadi, compost worms such as the Red Wriggler can produce an anti-freeze to enable them to survive in the winter. In addition to forming an anti-clotting mechanism in their blood, this anti-freeze also remains in their bedding until late winter or early spring.
Bintoro suggests that the combination of anti-freeze in the worm’s blood, residual anti-freeze in bedding, and organic bedding material allows worms to survive air temperatures as low as -15°C (5°F) and bedding temperatures as low as -5°C (23°F).
I’ve unfortunately been unable to identify the research used by Gunadi, but I did find one study did suggest that survival rates of Eisenia fetida (the commonly used red wriggler) do fall in lower temperatures. I’ve also noticed that while some worms survive in my worm bins, there seem to be fewer in winter than in summer.
On the other hand, our compost worms may be helped because of the high density of worms we maintain in our worm farms. With more worms releasing anti-freeze, they may very well have a greater resistance to the cold.
One of the main reasons for this post is that there is plenty of advice to place worms inside in winter, or to place worm farms on heated pads. (These sources also advise lifting the worm bins off the ground, but mine are already permanently balanced on bricks.)
I think that’s pretty good advice for the diligent gardener who wants to maximize survival rates and/or worm compost production. For maximum reproduction rates, NC State University recommends keeping worms at 60°F – 80°F/15.6°C – 26.7°C.
I think it’s also good advice for people who live in colder climates than mine – anti-freeze or not, I wouldn’t risk keeping my worms at temperatures below 23°F/-5°C, or even long periods of temperature below 32.0°F/0°C.
However, both my experience and some contrary sources suggest that some worms can live through the winter in milder climates – even with occasional colder snaps. Even if most worms don’t survive, their cocoons will, ensuring that new worms will hatch when the weather warms up.
We can also increase the chances of worms surviving by taking steps to protect them from the cold. A simple layer of straw can help, or for greater chances of survival try placing straw bales around your bin.
You could also follow the example of Canadian worm farmer Sam Hambly, as described in Worms Eat My Garbage. Combining styrofoam insulation and a large bin (24″ x48″ x48″) which could hold enough organic material to generate heat, he was able to keep his worms alive through the depths of the Canadian winter.
Tips
Even in a milder climate, try to protect worms from cold temperatures:
- Ensure the bins are raised up on bricks to prevent direct contact with cold ground.
- Ensure there’s enough organic material for worms to burrow away from cold surfaces. Organic materials may also help raise temperature levels slightly as they decompose and generate heat.
- Use a material like straw to insulate the bin and trap heat.
- If possible, move to a sheltered location like a shed for further protection.
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