How to Start a Worm Bin for Apartment Composting

You do not need a yard or an outdoor compost pile to turn kitchen scraps into rich material for your containers. A worm bin fits under a sink or in a closet, and University of Maryland (UMD) Extension describes vermicomposting as easy and inexpensive to start, possible indoors in a small space, and odorless when it is working well.

1. What vermicomposting is

Vermicomposting uses redworms to digest food scraps and bedding. UMD explains that over a period of months, the worms produce vermicompost: worm castings (manure), decomposed bedding, and lots of worms and tiny organisms. The finished material is dark, crumbly, safe to handle, and an excellent soil conditioner and source of plant nutrients.

2. The right worms

Not every worm will survive in a bin. UMD says redworms, also called red wigglers or manure worms, are well suited because they thrive in confinement and tolerate a wide range of conditions. The two most common species are Eisenia foetida and Lumbricus rubellus, sold online.

UMD warns that common garden worms and nightcrawlers quickly die in a worm bin, and that redworms do poorly in average garden soil. Digging worms from a park will not work.

How many to buy: UMD says you need at least one pound of redworms (about 500 to 1,000 worms) for a small bin. Each worm is 2 to 4 inches long and can eat about its own weight in raw organic matter per day.

3. The bin

UMD's design is a plastic storage bin with a lid, about 21 inches long, 15 inches wide, and 5 to 8 inches deep, roughly 2.2 square feet of surface. Its setup steps:

1. Avoid deep containers (over 15 inches). They are heavy, and the bedding compacts, making it hard for worms to find food.
2. Drill 1/4 to 1/2 inch holes in the bottom and the lid for airflow. UMD notes rigid plastic can crack, so use a sharp bit and drill slowly.
3. Raise the bin on bricks or scrap wood so air reaches the bottom.
4. Set it on a tray to catch leachate, the nutrient-rich liquid that drains through.

UMD says a bin this size can handle 2 to 3 pounds of kitchen scraps a week.

4. Bedding

Bedding gives worms the cool, moist environment they need. UMD recommends hand-shredded newspaper (color pages included) or corrugated cardboard torn into thin strips. To prepare it:

• Soak 4 pounds of bedding in a bucket with 1.5 gallons of water for a few minutes.
• Drain off the excess water.
• Place the bedding loosely in the bin.

Redworms breathe through their skin and must stay moist. If the bin dries out, UMD suggests misting the inside with a spray bottle.

5. Where to keep it

UMD gives a temperature range of 55°F to 75°F, where redworms eat and reproduce most efficiently. They are sensitive to light, so keep the bin covered and out of direct sun. UMD lists basements, cool garages, and kitchens as good locations; in an apartment, a cabinet or closet that stays in that temperature range works.

6. What to feed, and what not to

Penn State Extension's list of foods worms like:

• Fruit and vegetable trimmings (salad scraps without dressing)
• Potato peelings, in moderation
• Coffee grounds, filters, and tea bags (remove staples and plastic strings)
• Small amounts of cereal, bread, and pasta, without sauce or butter
• Occasionally, very finely pulverized eggshell

Penn State's never list: meats, eggs, fats, oils, dairy products, or citrus peels.

UMD's feeding method: collect scraps in a lidded container, then feed 2 to 3 times a week by burying them under the bedding in different spots. Smaller pieces are digested faster.

7. Fixing common problems

From UMD's troubleshooting list:

• Fruit flies: always bury food beneath the bedding. Flypaper on the lid underside or a yellow sticky card near the bin can help.
• Odors: usually from adding too many scraps at once. Remove rotting food and skip feeding for a week.
• Too wet: bedding compacts, smells, and air cannot flow. Check drainage holes, stir, and add fresh dry bedding.

UMD also notes worms will not crawl out of the bin unless it becomes too dry or too wet.

8. Harvesting the compost

UMD says it takes 6 to 8 weeks for worms to produce a noticeable amount of vermicompost. Every 3 to 4 months, you can harvest using one of its methods:

• Side-to-side: stop feeding for a few weeks, rake the compost to one side, add fresh bedding to the other, and feed only on the new side. Within a few months the worms move over and the old side can be harvested.
• Light method: dump the contents into piles on a plastic sheet in a bright room. The worms dive to the bottom, so you can skim finished compost from the tops and sides.

9. Using it in containers

UMD's guidance for container gardeners: mix vermicompost into potting mix for indoor or outdoor containers at no more than 1/4 by volume. Pure worm castings can be high in soluble salts, so use them sparingly and keep them away from direct contact with seedling roots.

10. Key takeaways

• Use redworms (red wigglers), at least one pound for a small bin; garden worms and nightcrawlers die in bins.
• UMD's bin is about 21 by 15 inches and 5 to 8 inches deep, with air holes, raised on supports over a drip tray, and handles 2 to 3 pounds of scraps a week.
• Bedding is shredded newspaper or cardboard, soaked and drained; keep the bin moist, covered, and between 55°F and 75°F.
• Feed fruit and vegetable scraps and coffee grounds; never meat, eggs, fats, oils, dairy, or citrus peels.
• Harvest every 3 to 4 months and use vermicompost at no more than a quarter of a container mix.