Worm castings, or vermicompost, is one of the best fertilizers and soil amendments you can use in your container garden. A simple worm composter or worm bin can be made for less than $20, and you may even already have everything you need! Ready-made worm bins and worm trays can be purchased; these are somewhat easier to use and have convenient features, but a home-made bin is almost as easy to use and harvest.
First, decide how large of a bin you need. If you have access to a pound or more of kitchen scraps a day (vegetable matter, fruit peels and cores, bread, cereal, grains, pulverized egg shells, tea bags, coffee grounds and filters), consider making a large bin (2' x 3' or larger) or multiple bins. A good rule to estimate the size of your bin is 1 square foot of surface area for every pound of food scraps per week. These instructions are for a fairly small bin, but can easily be scaled upward as needed. Just remember that your bin will be considerably heavier when it's time to harvest your worm castings, so don't make it so big that you won't be able to handle it.
2 opaque (non-see-through) plastic storage bins with lids, 8 to 10 gallon size. Note: taller bins are better than short and flat bins. If possible, choose bins roughly as tall as they are wide. The bins need to be the same shape so they can fit inside each other.
2 bricks, 4 pot stands, 3-4 old mugs, or similar items to use as a bin stand
Newspaper, torn into 1/4” to 1/2” wide strips. Note – black ink only, no colored print or paper!
untreated cardboard, cut to size of the bin opening
Drill with 1/4” and 1/16” drill bits
Step 1: Drill 1/4” holes in the bottoms of both storage bins. Make them evenly spaced, and about 2” to 3” apart. For an 8-10 gallon bin, drill about 20 to 25 holes. Add more holes for a larger bin. Remove any hanging bits of plastic and ensure all holes are open so the bins can drain, and so the worms can crawl between the bins when it is time to harvest the castings.
Step 2: Drill 1/16” ventilation holes around the top edge of both bins, about 1” apart. Drill a second row of holes 1 1/2” below the first on the two long sides of the bins.
Step 3: Drill 30 to 40 evenly-spaced 1/16” ventilation holes through the top of ONE of the bin lids (the other lid will be used to catch any liquid that drips from the bottom of the bin, so no holes in that one!).
Step 4: Shred or cut newspaper into 1” strips, and soak in water to moisten. Ring out excess moisture, and place in the bottom of one of the bins. 'Fluff up' the wet strips so there is room for the worms to crawl around among them. Keep adding strips until they are 3 to 4 inches thick in the bottom of the bin. Sprinkle in a handful or two of garden soil or fine sand – worms need the grit to help digest their food. You can also sprinkle in some dead decaying leaves, if you like. Note: if you don't have newspaper, use decaying leaves, strips of untreated corrugated cardboard, shredded computer paper, composted cow, horse or rabbit manure, dries grass clippings, chopped hay, coconut coir, or peat moss sprinkled with a little crushed limestone.
Step 5: Add your worms. You can purchase worms at many garden centers and through the internet in most states, or you can gather them from someone you know with a worm box. Regular garden worms may adapt well to your worm composter, but there are many types or worms and, unless you are a worm expert, it's hard to tell what type you have. It's best to buy them, and if you do, Eisenia Foetida is the best type for home worm bins. Definitely DO NOT use night crawlers – these worms like to burrow very deep in soil (up to 3 feet) and will not adapt to your shallow bedding and food scraps environment. A mixture of all sizes and ages of worms will contain about 1000 worms per pound; full-grown breeders will yield about 500 worms to a pound.
Worms eat about ½ of their body weight in food scraps a day. If you average ½ pound of food scraps a day for your worm bin, you'll need about a pound of worms to keep up.
Step 6: Cut a piece of untreated cardboard to the inside size of your bin and wet it down so it is damp but not soggy. Place it over your worms and their damp bedding.
Step 7: Assemble your bin system in a well-ventilated area indoors or outside in the shade. Worms prefer temperatures between 55 and 75 degrees F, and they need moisture and air (oxygen) to survive.
Put the un-drilled lid upside down on the floor, to act as a drip-catcher.
Place the bricks, pot stands, mugs, etc on top of the lid, to act as supports for the bin(s) and allow space for drainage.
Place the bin that contains your worms and their bedding on top of the supports, making sure any drainage will land in the lid on the floor.
If you have food scraps ready, gently bury them in one corner of the bin, under the dampened cardboard.
Place the lid with drilled ventilation holes on top of the bin.
And now you let the worms do the work! Add your kitchen scraps in a different location in the bin each day or week, depending on your schedule. Moving in a spiraling pattern around the bin makes it so the worms don't have far to crawl from one meal to the next. If you will be away from home for a long period of time, give your worms a little extra food before you leave. If you will be away longer than three weeks, arrange to have someone feed your worms at least once every two weeks while you are away.
If the materials in the bin ever appear to be drying out, mist the mixture lightly with a spray bottle. The worms need moisture to survive, but too much water will drown them.
When the bin is full and all food scraps have been eaten, it's time to encourage your worms to move to the second bin.
Prepare this bin as you did the first in step 4, with the damp bedding and handful of dirt.
Remove the lid from the bin containing your worms and worm castings, remove the cardboard cover, and place your second worm bin directly on top of the worm castings/compost. Add fresh food scraps into the bedding of the new bin. Put dampened cardboard over the new bedding and food scraps, and put the ventilated lid on the new, second bin. Wait several weeks for the worms to make their way up to the second bin. (You can check the bottom bin periodically to see if any worms remain.)
Once most (or all) of the worms have moved, remove the bottom bin and place the second bin directly onto the original supports. Run your fingers or a cultivator through the compost and worm castings to be harvested, and place any stray worms you find into your new, active bin. Any worms still left will be a nice addition to your garden.
Mix the worm castings into your garden, spread on the top of your containers, or add to your potting soil mixes. Wash out the plastic tub and set aside to swap in when your now-active worm tray is full.