Worm Towers & Worm Stacking Systems

“I was wondering about worm bins. The tower variety. It seems to me that the lower trays get very complicated when I add enough bedding to cover the food. Is this a problem? I have 5 trays and they say I can add up to 8. What do you think? Thanks.” ~ Helen

Hi Helen,

I’m not sure if you meant “compacted” when you said “complicated’, but either way I figured this would be a good opportunity to chat about stacking worms systems. Truth be told, I actually recently ordered my very first multi-stray bin so I certainly have some thoughts to share.

Just in case some of you are unfamiliar with the stacking worm bin (or at least are not 100% sure how they work), we’ll talk first about the basic principles involved. Stacking (or ‘tower’) systems, as the name implies, are made up of a series of vertically interlocking trays – typically 3-5.

Each tray has a mesh bottom which allows for the upward migration of worms and the downward drainage of excess liquids (typically some sort of reservoir at the very bottom). When you first start out you simply have the first tray in place with the lid over top.

Just like any other worm bin, a mixture of bedding & food materials are added, along with some composting worms. Given the mesh bottom, it’s not a bad idea to lay newspaper layers (or a piece of cardboard) over it before adding materials and worms – this helps to keep the worms from venturing down into the reservoir.

The basic idea behind these systems is that as each tray fills up, you then add a new tray (with new materials) above it, thus encouraging the worms to move out of their castings. By the time you have added and filled the last tray, the the first one should (in theory) be completely free of worms/cocoons and ready for use.

This is the major advantage of continuous-flow bins. Once you have been through 1 cycle (have harvested your first tray of castings), you should then have a fairly steady supply of the material. Unlike the contents of a sealed worm bin which locks in lots of moisture, the castings in a stacking system will likely be better oxygenated, drier, and higher quality in general.

One of the limitations I see with most small-scale stacking systems currently on the market is the small size of each tray.

This is particularly important with the first tray since the worms don’t have anywhere else to go (assuming you cover the bottom mesh) if conditions are not to their liking. Aside from providing limited refuge for the worms, this also makes these systems much more prone to fruit fly invasions (in ‘regular’ bins it’s generally easier to suppress populations with ample quantities of bedding material).

My new (wooden stacking) system seems especially prone to fruit fly invasions, since unlike the plastic models that have tightly fitting trays, this system has plenty of small spaces for flies to get in and out. As per usual, preventing fruit fly populations from getting established in the first place is always the best approach. Make sure you microwave or freeze all fruit rinds and peelings that you put in the system since these can have fruit fly eggs buried in them.

Regarding the ability to add more trays, the main benefit would likely be that the castings tray (lowest one) would contain materials that are even more mature (and ready for use) since it would be allowed to sit while 3 additional trays have been added and filled (assuming you use 8 trays).

The downside of this approach, aside from likely creating a awkward, heavy system, is that some of the lower levels will become compacted thus impeding the airflow up through the composting mass.

One other thing to keep in mind about these systems. While it is often advertised that these systems can support large quantities of worms, due to the total volume of the system, this is in fact a little misleading since in actuality the worm population will essentially be moving upwards as more materials are added, and by the time you’ve added the last tray there won’t likely be very many worms left in the lower trays.

The population is essentially limited by the surface area of the system (which is actually quite small for tower bins).

Despite some of the limitation of the stacking tray worm bin however, these systems are still fun the play with and certainly offer some advantages over the more basic worm bins. I’m very interested to start testing out my new multi-tray bin and will certainly share my experiences with it in future newsletters.

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