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Cold-Method Composting A.K.A. Vermicomposting
by Paulette Mouchet

Originally published in The Rose Garden newsletter, September 2002.

composting worm: Eisenia Foetida

Using redworms (Eisenia foetida) to decompose vegetative material is called vermi­com­posting. The resulting material is called castings and is a rich source of nutrients for the garden. Use worm castings like any other compost: spread a thin layer around your plants like organic fertilizer or till up to 20% into the soil. Worm castings are not usually used as mulch because they are so rich.

The following vermicomposting primer was edited from an article written by S. Zorba Frankel and published on the Worm Digest Web site.

A Bin to Call Home

A simple box or bin with a lid and plenty of holes for aeration and drain­age is all you need to get started, or you can purchase one of the many styles available (see descriptions below). You need roughly 1 square foot of surface area per pound of weekly food waste. A fam­i­ly of four produces about 7 pounds of food waste per week and would need a bin with 7 square feet of surface area.

Aeration is Key to Keeping Redworms Happy Plenty of air holes will promote air flow throughout the bin and keep your worms, microorganisms, and other compost critters happy. Your bin can be kept indoors near the source of food waste or in the garage or outdoors. The bedding temperature should be kept between 40 and 85 degrees Fahr­en­heit, and the system will process waste fastest when bedding stays be­tween 65 and 75 degrees Fahrenheit.

Beds for Their Little Heads

Bed your redworms down in any carbon-rich bedding. In combination with ni­tro­gen-rich food waste, carbon-rich bedding provides a balanced diet for the bin ecosystem. Newspaper is an excellent, widely available carbon-rich bedding material. Simply shred the newspaper into strips 1-inch wide or thinner. Don't use cross-cut shredded newspaper as it tends to matt. Coir (shredded coconut husk fiber) is becoming popular and chopped straw or shredded brown leaves also work. Mixing more than one type of bedding helps to promote better aeration. Water your bedding until it's as damp as a wrung-out sponge.

Finding Your Little Waste Managers

According to Dorothy Benoy at Happy D Ranch, you'll need 1 to 2 pounds of redworms per 1 square foot of bin surface area. Worms are available from a variety of local and on-line sources, including Happy D Ranch, for $25 to $30 per pound (including postage), and you should verify the environment the worms were raised in. "Worms grown in paper sludge are cheaper," says Ms. Benoy, "but they don't work in fungal environments like the worm bin." If you purchase a worm bin, the retailer usually supplies the appropriate worms.

Don't confuse vermicomposting redworms with earthworms found in your garden. Earthworms require soil to survive and are not adaptable to your worm bin's environment. Redworms do not require soil and will compost almost anything organic that you feed them. Outside of the worm bin, redworms will only live in garden beds that have been heavily mulched with decaying organic matter.

Bon Appétite, Friends!

It's best to feed your worms lightly for the first couple of weeks. An ecosystem needs to form within the bedding and food waste. As the populations of bacteria, fungi, microorganisms and other critters increases, the bin will be able to process more. You can jump-start the process by mixing a bucketful of material from another worm bin or compost pile into your bedding when you set up your worm bin.

After the start-up period, you can feed your worms daily or even once a week. They'll eat most foods though you'll want to leave out meat, dairy, and fats, which have a tendency to putrefy. As a very rough guide, worms will eat half their weight in food waste (and bedding!) each day, and increase in population to about 1 pound per square foot area.

Because worms have no teeth, they rely on bacteria and fungi to begin consuming the food waste first. The smaller the pieces of food waste, the more quickly they will be available to bacteria and fungi. So, chop your scraps, but don't blenderize them, as a slurry tends to lock out oxygen and cause a stink! Always bury food several inches deep in the bedding and spread it out. Finally, add new bedding now and then to maintain a several-inch-thick layer on top.

worm compost
Finished Vermicompost

Harvesting the Gold

After operating your bin for 3 to 5 months (or more if you prefer dark, very finished castings), it's time to harvest your bin. The castings can be used directly in your garden or on your houseplants, but go lightly as worm castings are a very rich source of nutrients. Dorothy Benoy recommends no more than 20% castings in your planting mix. Mixing equal amounts of coir, topsoil, vermiculite, perlite, and castings creates an excellent potting soil.


If you chose to purchase a worm bin, there are several types of containers available: stacking bins, lateral movement bins, and continuous flow bins. Following is a description of each, courtesy of the Worm Digest Web site.

Stacking Bins

Stacking tray systems have multiple nesting trays with grid bottoms that sit above a liquid collection tray. These systems offer a harvesting method aimed to minimize hand-sorting of castings from worms and decomposing organic matter. Worms are fed in one tray until it is full, then the next (empty) tray is set on top. Worms move upward toward the available food. When the uppermost tray is full, the lowest can be removed, emptied, and the castings used.

A common misconception about stacking units is that each tray is a working tray, and that the systems have much greater surface area in a vertical stack than they actually do. This is not so. Redworms prefer to work in the most microbially-active areas of the bin, usually within 6 to 8 inches of the surface of any composting material, thus the lower trays in these systems are left behind by the vast majority of redworms.

Possible challenges to stacking tray systems

  1. Stranded Worms. Although trays nest into each other, the composting material in the tray below continues to settle over time, while trays sit on tiny "shelves" and do not continue to move downward. This may leave a gap that worms are unable to cross. Follow the instructions carefully to avoid adding a new tray too early, and check to see if worms have been left stranded. Adding a small pile of shredded newspaper can serve as a "bridge" for worms to move upward.
  2. Drownings. Quite a few users report finding some worms in the liquid in the collector tray at the bottom of this type of bin. To avoid drownings, some people drape landscape fabric over the top of the collecting tray.
  3. Heavy Trays. When full, a single tray is fairly heavy, and lifting or moving the entire stack can be a very hard job.

Advantages of stacking tray systems

  1. Ease of Harvesting. If the design is working well, by the time you need a fresh tray to add on top of the stack, most of the worms will have moved out of the lowest tray. With the exception of a few stragglers, they have sorted themselves.
Lateral Movement Bins

Lateral movement vermicomposting systems are nothing new, being commonly used in the 1970s in homemade worm beds. Because it was (and is) a simple method for separating worms from castings, many vermiculturists would divide their worm beds in half, usually by placing a piece of screen or a board between the left and right sides of the bin. One side would be fed until the worms had processed the majority of the material. The system was then fed on the other side of the divider, which was eventually removed, allowing the worms to move into the newer material over a period of weeks. Once the worms had transferred to the newer material the divider was replaced and the finished castings removed, without taking significant numbers of worms with it. The empty side could then be rebedded awaiting the next castings harvest.

One of the great advantages to lateral movement systems is that the decomposing material is left in the system for an extended period of time, allowing it to be thoroughly worm worked and "cured". This ensures a nicely matured finished product. It also ensures that the worms will remain comfortable in the system as there is a significant quantity of castings in which they can harbor if the fresh material is not microbially active enough to be ideal.

Possible challenges to lateral movement systems

  1. Size. Some units are small and if not insulated, it is best to bring them indoors during cold weather.
  2. Odors. If the system is overfed there is the potential for odors.

Advantages of lateral movement systems

  1. Neat and Tidy. The units are compact and attractive, well designed for indoor use.
  2. Ease of Harvesting. About the easiest we've ever seen!

Continuous Flow Bins

Continuous flow bins are relatively deep containers with raised floors made from widely spaced welded wires. The system is generally bedded by laying several sheets of newspaper over the wire floor, on top of which is spread roughly 6 inches of damp bedding. The newspaper sheets, which eventually decompose, serve to prevent bedding from initially falling through the floor until the worms have had an opportunity to work through it. Worms are added to the system and food waste is added gradually, layered with bedding material, just the same as in the top-feeding method of system management. The system is continually fed until the bin is nearly full. The worms move upward through the feedstock/bedding layers. When fed at the appropriate rate, they concentrate some four inches below the surface, where microbial activity is highest. If the system is overfed, the worms will spread throughout the bin material, reducing the efficiency of the system.

Once the bin is nearly full, castings are removed from the bottom each time the system is fed, establishing an equilibrium. Some continuous flow systems have a metal bar that sits on top of the wire floor. Castings are removed by pulling the bar across the floor, which causes a thin layer of castings to fall through. Some systems suggest scraping a small rake across the bottom of the floor to dislodge the castings.

Possible challenges to continuous flow systems

  1. Cavitation. Castings need to remain in contact, or a very short distance above, the grate for harvesting to work correctly. Sometimes the castings stick together, away from that grate. This can often happen when the system is drier than recommended. The user needs to learn to adjust the system's moisture well.

Advantages of continuous flow systems

  1. Ease of Use. These systems accept feedstock from the top of the bin, at a height that is convenient for many users. Continuous flow designs enable the user to remove castings from the system without having to separate it from the worms.

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