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As the season swings into gear, I am commonly asked the question “Should I mix earthworm species in my worm composting bin?”

Normally, people are interested in mixing red wigglers, European nightcrawlers, African nightcrawlers, and possibly Indian Blues.

I have my own thoughts about that, but before I give you my general recommendations, let’s explore the cases for and against mixing species.

This article is part of a “Vermicomposting 101” series of posts aimed at helping the beginning vermicomposter. The read other “VC101” articles on how to start a worm bin, how to choose worm food, how to maintain moisture, and the differences between composting and vermicomposting, please visit the Vermicomposting 101 section of this site!

Why to Mix Species in Your Worm Bin

You Want To Mix Species to Vermicompost at Different Depths

In general, European nightcrawlers are believed to burrow a little deeper than red wigglers and Indian blues and African nightcrawlers are believed to burrow a little deeper than Euros, so the thinking is that a more uniform processing of waste is more likely with more species.

As I am pretty much a run-of-the-mill red wiggler man, I do not have a particular opinion as to whether this is true. If it is, then this is a fine reason to mix species. But I have also seen red wigglers way deeper in a wet, compacted worm bin than I thought I should find.

Mixed Worm Species Insure Against Swings in Temperature

Each worm species has its own temperature range where it demonstrates its greatest efficiency. European nightcrawlers generally enjoy slightly cooler temperatures than red wigglers would find ideal. And Africans and Indian blues need warmer temperatures than the red wiggler.

So it might make sense to include a variety of species so if you have weather patterns like Philadelphia which goes from well digger’s ass cold to Satan’s armpit-level hot and muggy in an instant.

At either of those ranges, at least one of your species is happier than the others, although it’s worth pointing out that none of them will be happy at the extremes.

Our Diversity is Our Strength!

Like most ecosystems, a greater diversity of organisms portends good things. Much like the temperature argument above, a robust variety of species may withstand shocks to the system better than a single species.

Applying this principle to your choice – or choices – in worm species might make sense if you think several worm species is better than one.

Why Not to Mix Worm Species In Your Worm Bin

Mixing Worm Species Sacrifices Reproduction Potential

Contrary to some wives’s tales, worms cannot interbreed or hybridize.

But this does not mean that worms are unaffected by the population around them.

Worms are excellent self-regulators and will slow their reproduction when the population density reaches a certain level, ultimately maxing out at 2-3 lbs per square foot (maybe a bit more), depending on the conditions in the bin.

Below a certain density – I’m not sure what it is, but I reckon it’s around 1 lb per square foot – the opportunities for a love connection are reduced as well, so if you stock a 4-square foot bin like the Urban Worm Bag with 1 lb of red wigglers and 1 lb of European nightcrawlers, your worm density may be 1/2lb per square foot, but the density of the worms that can actually breed with one another is 1/4 lb per square foot, less than optimum if you’re trying to grow your population quickly.

It’s More Expensive to Buy Multiple Species

Purchasing two breeds normally means two packages from your supplier.

One two-pound package will ship for way less than two one-pound packages. For instance, 2lbs of red wigglers purchased at Urban Worm Company is nearly 30% less than buying one pound each of red wigglers and European nightcrawlers.

My Thoughts

There is absolutely nothing wrong with multiple species in a worm bin, but I can’t find a compelling reason to say you should. Of the arguments for intentionally introducing mixed breeds in your bin, the depth issue is the most convincing.

But I’m not convinced.

The species purported to burrow the most deeply is the African nightcrawler. But it is still an epigeic, top-feeding species and despite its size, I don’t think it is an appreciably better burrower than the red wiggler.

And this may sound like heresy, but the diversity argument isn’t convincing either. A healthy worm bin has huge and diverse microbe population. While diversity is generally a good thing, I don’t consider a different flavor of worm poop to be meaningful diversity.

So while I fully support anyone who is experimenting with different breeds, there’s nothing screaming at me that says it’s a better way to vermicompost and the purported upside to me just doesn’t outweigh the added cost. Take the money you would have spent on two separate pounds of worms and just buy 3 lbs of a single species like the red wiggler for the same price.

If you liked what you read here, I invite you to join my e-mail list below and read the rest of my Vermicomposting 101 Series.

The 2020 Vermicomposter's Starter Guide

Urban worm guide

Your roadmap to a successful beginning in vermicomposting.

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