Density: Worm Reproduction’s Forgotten Factor

Whether you just want to increase your vermicomposting capacity, you are planning a vermicomposting or vermiculture business or have one in operation already, you’ve probably asked yourself the following question.


“How can I make my worms reproduce faster?”

I’ve noticed most attempts to answer this question center on very common guidelines concerning moisture and temperature. No doubt, these two factors are crucial.

But it’s just as important to keep worm density in mind. Speaking from experience, I can attest that maintaining a nice 60-70% moisture level and 77 degrees but stocking a 2.5 square foot grow bin with a half pound of worms is going to give you disappointing results if you’re trying to grow your worm herd quickly.

What’s the Right Density for Worm Reproduction?

Vermiculture experts recommend a stocking density of at least 1 lb per square foot for better results. This density allows for good opportunity for, ahem, “rendezvous” while allowing space for an increase to an optimal 2 lb per square foot stocking density.

To be honest, without actually removing and weighing your worm biomass, I don’t know how to tell you exactly what 1 lb per square foot looks like in your bin. But you can always start fresh with a new bin making sure to stock at the minimum density for your container.

I took this more deliberate approach this spring and the results have been wonderful, maybe too wonderful. Not only am I splitting grow bins much faster than I anticipated, I am also adding the excess biomass to my vermicomposting beds and am now processing aged horse manure at a rate so fast that I cannot harvest the castings quickly enough given my rudimentary equipment and limited time.

In short, my attention to stocking density and willingness to experiment with more and more worms has led to much faster reproduction than I was getting before.

Cross-Species Effects on Worm Reproduction

There is one caveat though. If you are unwittingly stocking your grow bins with worms of different species, you will be achieving less-than-optimal reproduction. It’s pretty common that you are ordering red wigglers (eisenia fetida) from certain suppliers and receiving a mix of reds and Indian Blues (peronyx excavatus) or European Nightcrawlers (eisenia hortensis), which will appear the same to a beginner.

As there is no interbreeding between species, if you stock your bin with ½ lb per square foot density with a 50/50 mix of reds and blues, your effective worm density for reproductive purposes is only ¼ lb per square foot. Your vermicomposting performance will still seem unchanged with a mix of species but your reproductive performance will suffer.

So you should err to the high side when stocking your bins unless you’re sure you’ve got all the same species.

Ok, I Got It. Now When Do I Split My Bins?

This leads into the question of when to remove excess worms from your grow bins, either to sell or start a new bins with. As I mentioned earlier, it takes someone with much more experience than I have to say “It looks like I’ve got about a pound and half per square foot of worms in here” and even then, it’s going to pretty inexact.

But a telltale sign that Larry Shier on Facebook pointed out – which I’ve observed this summer – is that when individual worm size starts to shrink, it may be time to split your bins. What look like a bunch of juveniles may actually be a bunch of small adults (adults are identified by the presence of a clitellum) who, due to limited space, aren’t attaining their potential size.

A reduction in size certainly accompanies and I suspect possibly precedes a reduction in cocoon production and worm reproduction.

One way I like to harvest the excess worms is to place a decent chunk of melon or pumpkin facedown on the surface of the grow bin and return the next day. I remove the food and typically find a solid layer of worms lined up for the buffet. I do this with 5 or 6 grow bins at a time and literally just grab a handful from each and move them to a new grow bin, again making sure that the new bin gets enough worms to meet my ½ lb per square foot standard.

Another way is to fill an onion bag full of the aforementioned and worm bait and bury it within the first 3 inches of your bin.  Give it a day or two, return to your bin, observe a nice cluster of worms and simply remove the bag and either A) empty the contents into another bin or B) just place the bag itself in another bin.  Once the worms finish getting what they want in there, they’ll find their way out.

9 thoughts on “Density: Worm Reproduction’s Forgotten Factor

  1. Good points in this article! I have been amazed at the difference density has on worm size. I have seen it work both ways. When I moved some small worms to a new bin, within 2 weeks the worms were noticeably larger. And likewise, when I started a grow bin with my largest worms, after three months and lots of new worms, there are no longer any of the really large ones. Self-regulating in size is a VERY accurate description of composting worms with regards to worm density!

  2. Great article Steve! Thank you for sharing your knowledge and experience with your readers. I look forward to more 🙂

    ~Pauly P

  3. I am not a mathematician by any means often 1+1 doesn’t equal 2 in my world. I found myself around 3 am staring at this question on a spreadsheet while pondering the next day’s worming activity. If a pound of worms eat a 1/2 a pound a day and it took them five days to digest that weight isn’t there an equation in there revealing how many pounds of worms are there. And if you know the pounds of worms you get a rough estimate on how many worms you have. I am approaching the relm of the brain dead and the interior clock rings around 6 am, so I’m off for a nap.

  4. Thank you, great information. Can you suggest a red worm seller who ships to Colombia, SA? Thank you in advance!

    1. I’m sorry Dalila,
      US suppliers cannot ship worms overseas and even if we could, they would not survive the transit, I’m afraid!

Leave a Reply

Your email address will not be published. Required fields are marked *