Worm bins and vermicomposting: bin size, worms, bedding and feeding

At a glance

Size the bin and the worms from your scraps

Cornell Cooperative Extension suggests weighing your food scraps every day for at least a week before buying worms. NC State's Rhonda Sherman then sizes the bin by surface, not volume: one square foot of surface area for each pound of food scraps produced per week, so 2 pounds a week calls for a bin 2 feet by 1 foot, 8 to 20 inches deep. Red wigglers feed near the surface, which is why New Mexico State keeps its bins 8 to 12 inches deep and Maryland Extension warns that bins deeper than 15 inches get heavy and compact the bedding.

How many worms that bin needs depends on how much a pound of worms eats, and here the sources differ by more than three times. The calculator divides your weekly scraps by each published rate.

Which rate to plan around is a judgment call. A new bin is usually stocked below its full appetite: the EPA starts with about a pound of worms and increases feeding as the population grows, and every source says to add food only after the last feeding is gone. NC State's own starting figure, at least 1 pound of worms per square foot of surface, lines up with its sizing rule: a bin sized for W pounds a week starts with W pounds of worms, which can eat up to 2W.

What a home bin looks like

The EPA's indoor design nests two plastic bins. The working bin, an 18-gallon tub about 15 inches tall with a lid, has two 1-inch air holes about 2 inches below the rim and four ⅛-inch drainage holes near the bottom corners, all covered with glued vinyl window screen; metal screen rusts. It sits inside a shallow bin about 5 inches high, with no holes, that collects the liquid. NC State drains its bins through six ¼-inch holes in the bottom and adds air through holes or vents high on the sides, never in the lid; Maryland drills ¼–½-inch holes in both the bottom and the lid and sets the bin on bricks over a tray. UConn uses a smaller 5–10 gallon bin. All of them want the bin dark: worms are sensitive to light (NC State).

Setting up a bin, step by step

1. Choose the spot. Indoors (a basement or other out-of-the-way space, per the EPA) or in outdoor shade, never in direct sun (NC State). Outdoor bins need insulation in cold weather (EPA). Maryland lists basements, cool garages and kitchens.
2. Drill and screen the holes as described above, and fit a tray or second bin underneath.
3. Make the bedding. Tear black-and-white newspaper into strips about ½ inch wide, or use paper bags, office paper, cardboard, decaying leaves or coconut coir; avoid glossy paper (NC State). Soak it 5–10 minutes and wring it out until only 2–3 drops come out when squeezed (NC State).
4. Fill and add grit. Fill the bin about halfway with fluffed bedding and mix in a handful of garden soil (NC State). The EPA's indoor design uses a 3-inch layer of paper and soil; UConn fills its bin ⅓ to ½ full.
5. Add the worms. Buy red wigglers from a worm grower (NC State) or take them from someone with an established bin (EPA). The EPA warns against invasive Asian jumping worms, which may be sold as "Alabama Jumper" or "Georgia Jumper".
6. Wait, then feed lightly. Let the worms settle for a day (EPA) or a couple of days (NC State), then add a layer of chopped food no more than 1 inch high and cover it completely with 1–2 inches of bedding (NC State).
7. Keep feeding only what they finish. The EPA's indoor guide feeds once a week; Maryland feeds 2–3 times a week, burying scraps in different spots. NC State says worms can go 2–3 weeks between feedings, depending on the population.

What to feed and what to leave out

Woody or dry items such as stems take longer to break down (EPA). Cornell freezes scraps for 24 hours to deal with fruit flies.

Harvesting the vermicompost

Vermicompost starts to show on the bin bottom after a few weeks (NC State) or 6–8 weeks (Maryland). Most sources harvest at about 3–6 months (EPA, Cornell) or about four months (NC State); Maryland harvests every 3–4 months, while UConn harvests once or twice a year. Vermicompost does not need to cure (EPA).

Return the egg capsules: they are lemon-shaped, about the size of a match head, shiny and light brown, and hold 2–7 baby worms (NC State). New Mexico State says the young hatch in about 3 weeks and mature in 4–6 weeks; the EPA increases feeding as the population grows.

Using it

Cornell mixes 1 part vermicompost with 2 parts soil for pots and works a layer 1–2 inches deep into beds; Maryland adds up to ¼ of the volume to potting mix and warns that pure castings can be high in soluble salts, so keep them away from seedling roots. New Mexico State measured an electrical conductivity of 11.70 mmhos/cm in vermicompost against 3.60 in garden compost. Check that no worms or egg capsules go into pots, since red wigglers do not survive typical pot conditions (NC State). For vermicompost tea, NC State steeps 2 tablespoons in 1 quart of water for a day, uses it within 24 hours and keeps it off edible plant parts unless it is certain to be free of pathogens. Outdoor bins, tumblers and the browns-and-greens mix are covered in composting in small spaces.

Fixing common problems

Where sources differ

Worm bin checklist

• Scraps weighed for a week; bin surface sized at about 1 sq ft per pound a week.
• Dark bin with a lid, screened air holes, drainage holes and a tray underneath.
• Shredded non-glossy paper soaked and wrung out, bin about half full, a handful of soil mixed in.
• Red wigglers from a worm grower; no garden, bait-shop or jumping worms.
• Spot in the shade at 59–77°F, never in direct sun or freezing.
• Food chopped, buried under 1–2 in of bedding, next feeding only after the last one is gone.
• No meat, dairy, grease or pet waste; citrus, onion and garlic kept out or minimal.
• Harvest at about 3–6 months; egg capsules returned; castings mixed into soil, not used pure on seedlings.