Vermicompost 101 — Worm Housing

Cursory web searches for “vermicomposting” turn up images of stacked bins, but worms are not picky and thrive in many configurations. We can quickly satisfy what the worms need and then concentrate on what system is best for the worm farmer.

The Worm Real Estate Market

Red wrigglers are near-surface-dwelling earthworms, staying in the top 8 in of soil. Their preferred habitat is in decaying organic matter, not the soil, which does not hold much nutrition for them. They like some moisture, but not too much (as they breathe through their skin, they can drown if submerged under water). They dislike light and movement. Their ideal temperature range is 55–77°F. If we meet these conditions, the worms don’t care if they’re living in a plastic tub, reclaimed barn-board bins, an expensive designer composting palace ($375 from Williams-Sonoma) or a literal hole in the ground.

Considerations for the Would-Be Worm Farmer

Worm farmers should consider the size as well as location and mobility needs of their system. The general rule of thumb is 1 ft² of surface area for each weekly pound of food scraps (or 0.2 m²/kg/week). Individual mileage may vary, but a family of two might be looking at 4 lb/week so a composting system with 4 square feet of surface area would be needed.

In warmer climates, worms will stay active living outside in the shade, although their productivity slows down outside their ideal temperature range. In colder areas, outdoor worms need protection from freezing weather or the ability to move their bin inside, preferably to a basement location.

The size and mobility of a system are related: a big system may be too heavy and cumbersome to move. For those that want to move their bins inside for the winter but need a bigger system, a series of bins might be a good answer. Otherwise, permanent outdoor (but covered) locations give the most flexibility for bigger systems.

Those who can get by with a smaller system (say, under 6 ft²), have the most options: mobile bins or outdoor builds.

Mobile Bins

Bins are essentially stacking boxes with screened bottoms. These can be made out of plastic, wood, or any other material that can withstand constant moisture. The bins are rotated to create a slow-moving treadmill for the worms. Bedding and food scraps go in the bottom bin with the worms. As they finish eating the scraps and the bottom bin fills up, a second bin is placed on top. The bottom of the new bin must rest on the surface of the worm castings. Then new scraps are put in the top bin and the worms crawl through a screen attached to or holes drilled in the bottom. The bottom can be removed as the worms migrate, the castings taken out and then this bin can become the next top level. The worms keep moving upward in an endless cycle of bins and food scraps.

Bins must maintain the worm ideal conditions. It should be taken inside if the average daily temperature drops below 55°F and when outside, it should be in the shade — those in hot areas might not be able to put them outside in the hottest part of the summer. Moisture can become an issue for plastic bins, as the walls hold it in. Ventilation is needed, either in the form of drilled holes in the tops of the sides or on the lid that covers the system to keep it in darkness.

These can be built at home for tens of dollars. You can see my recent build of a plastic bin in this post or the wooden bin I built a few years ago in this post. These are also available from a variety of retailers — I have not used these and list them for illustrative purposes, not as a recommendation. The Can-O-Worms is a simple design and has the best name I’ve come across; a friend had one and recommended it to me. The Worm Factory seems to be a souped-up version of the Can-O-Worms. Or you can spend a lot more for the Williams-Sonoma solar assist composter.

Permanent Outdoor Systems

If you have enough vegetable matter and the inclination, a larger, permanent, outdoor worm system may be an option. These systems can be large with a greater capacity to devour rotting material and a huge worm population. These generally come in four configurations: migrating windrows, raised or sunken beds, worm tunnels, and flow-through systems. Windrow systems are long, straight mounds of rotting scraps on one side and finished worm castings on the other. The worm farmer scatters new scraps on one side and worms migrate that way to feast. As they move towards the scrap side, they leave behind castings. Over months, the scrap side grows and the casting side can be harvested. In this way, the windrow migrates slowly across the space. These are usually open-air systems so probably not the best thing next to your (or neighbor’s!) house (as seen in this video from Northwest Redworms).

Raised or sunken beds are what they say: a rectangular construction that holds a mass of worms. Both can be built with wooden, cinder-block, or brick sides and sunken beds can also have PVC or pond-liner sides placed against the walls of the dug-down pit. Both should be one or two feet in depth or height and open to the soil below. This allows microorganisms and mycelium to grow up through the mass and help the decomposition and farmers need not worry about losing their worms as they don’t generally like to dig down too deep. In cold areas, these should be deep enough to prevent complete freezing during the winter and lids, as well as straw insulation can help keep the worms happier in cold months. Placement is an issue for these permanent installations and shade, damp mulch, and/or lids are necessary to keep the bed from overheating in the summer. These can take on a larger amount of scraps (depending on the size) than mobile systems, but are generally smaller than windrows. You can see an example in this post or more information here. Sunken beds can also be created with partitions separated by perforated walls. One chamber can be filled with fresh scraps over time and the worms will chow down. As this chamber fills, the neighboring chamber can then be filled and the worms will finish in the first chamber and migrate to the second, leaving finished castings behind for harvest. See a fuller discussion at the end of this page.

Worm tunnels are smaller systems than the other permanent ones. They are essentially a tube or hatch built into the ground into which scraps can be thrown and worms living in the ground can then access the tubes. The idea is that the worms crawl in, eat the food and crawl out to deposit their castings in the surrounding garden beds. A neat idea, but the least-commonly used of those described here. See further information at Deep Green Permaculture and here.

Flow-through systems are the closest to treadmills for worms. Usually contained in large buildings, these systems use a conveyor belt that can be turned by cranks to move a mound of composting mass from one end of the space to another. Worms work on fresh scraps on one end, and as space runs out to add more vegetable matter, the conveyor belt is turned, dropping finished castings off the other end and exposing more belt space for additional material. Another type of flow-through system uses gravity: large upright containers receive organic material in the top and the bottom has an opening to release finished castings. This type of flow-through system has been miniaturized into mobile bin as well (an example here).

This is one article in a series of posts about vermicompost, or using worms in compost systems. Follow this link to see all posts in this series.

Check out these other resources:
Sustainable Table
Red Worm Composting
Composting Junkie
DIY Natural
Worm Composting HQ
City of Euless, Texas


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