Worm Composting in the Desert

I garden a lot where I live. I use worm castings to improve the soil and plant my food crops in raised beds in a covered greenhouse, protected from direct sun and thieving critters of various kinds. I find the combination works great. I can grow a lot of my own food.

One challenge I face is that I live in the Mojave Desert, out near Joshua Tree, CA. The temperatures here are extreme. It gets down to freezing during the winter months, especially at night. Summer temperatures are always over 90 degrees, we often hit 110 degrees, and sometimes up to 120 for a day or two.

Worms can’t take exposure to those kinds of temperatures for more than a day or two. They either die, stop breeding, or they stop eating your compost scraps. Composting worms do best at between 40 and 80 degrees. One solution is to farm the worms indoors, where you can control the temperature better. That wasn’t appropriate for my situation (fruit fly infestations in the house). I tried several different kinds of outdoor worm composting bins, but the summer heat either killed the worms or dried out their worm bed too quickly.

So I decided to exploit one well-known property of the Earth: dig down far enough, and the temperature is more or less stable 365 days a year. Pioneers built root cellars to exploit this fact. So I built a cement-block worm bin that is partially buried. It’s been several years now, and my worms are doing great.

I built the bin using one side of a raised garden bed wall. If you do the same, you will use 7 to 8 fewer cement blocks. If you position it on the side away from the prevailing direction of sunlight in your area, you’ll also find the bin is mostly in shadow during the year. Like the concrete floor, cinder block walls, and multi-part bin cover I built, the shadow can help you achieve a relatively stable temperature environment for your worms.

This bin may work great in other places with extreme climates. Soil is a very poor conductor of heat, but depth seems to be the main variable in how much temperature damping you get. I’d love to hear from anyone who builds this and experiments with the depth of the bin.

Materials:

4 Hinges. I used old door hinges.

Screws (some 1 1/4" and some 3 1/2")

10 J-Bolts with washers and nuts, to hold the 2x6s to the top of the walls

28 8"x8"x16" Concrete Blocks

4-6 8"x8"x8" Half Concrete Blocks

~5 40lb Bags Quickcrete Concrete Mix

~2 40lb Bags Quickcrete Mortar Mix

Four 2’ lengths of 3/8" rebar

2'x2' piece of 1/4" hardware cloth

3 2"x6" Doug fir/pine lumber

4 2x4" Doug fir/pine lumber

4'x12' sheet of ¼” plywood

White Spray Paint

2'x12' Sheet of Corrugated Metal. Aluminum roofing or siding works fine.

Tools:

Angle Grinder with a cutoff wheel, to size the corrugated metal. You can also use tin snips.

Optional: A piece of chicken wire the size of your bin floor

2' Level

Electric Drill

1/2" Drill Bit

Crescent Wrench

Flat Head Shovel

Cement Trowel

Hacksaw Blade

Hammer

Square (rafter square works good)

Circular Saw

Supplies:

5lbs Red Wiggler Worms. You can order these online.

1 bag of Coconut Coir

-Choose a location for your worm bin and mark out the perimeter of the bin with stakes, spray paint or string. The dimensions are your decision, but the perimeter of the bin I built is 6 feet long by 3 feet wide.

-Mark another, “block” line 6 inches around and farther out from your perimeter line. You’ll dig out to this line to give yourself room to lay the blocks underground.

-In the area between your perimeter and block lines, dry-stack two courses of concrete blocks in the shape of the walls. Each block on the second course should overlap 2 blocks on the lower course. Set 1/2" gaps between each block. Take a picture of the dry stacked bin so you can recreate it the same way later.

Move the blocks to the side of your work area. Remove all the soil from inside your previously marked out area to a depth of about 16 inches. Use a scrap length of 2x4 lumber and a level to level out the bottom.

-Mix your concrete and lay some down to about 2 inches thick, covering the whole bottom of the bin including where your first course of blocks will go. Trowel the area relatively flat.

-While your concrete layer is still wet, mix some mortar and start placing your first course of blocks in the pattern you had them in when you dry stacked them. Trowel on 1/2" of mortar between each block and their adjacent blocks. Use the 2' level to make sure each block is level with the other blocks. Use the square to make each wall square with the adjacent wall.

-After finishing the first course, hammer 2' rebar pieces into the hole in the block at each corner. Leave just enough (about 6 inches) sticking up to join the first and second courses.

-Lay the second course of blocks.

-Fill the holes inside of each block with cement and set the J-bolts in the cement. I placed 3 J-bolts for each long wall and 2 on each short wall. Either use scrap pieces of 2x4 with holes drilled in them to hold the J-bolts to the correct depth, or cut the 2x6's that you are going to use for the top plate and drill the holes in the areas you want the J-bolts.

-Pour 1-2 inches of concrete inside the bin. For extra strength, float the chicken wire in the middle for added strength. Smooth and level the floor with a hand trowel.

-When the cement is hard enough (next day) cut your 2x6's that will sit on top of the walls, drill the holes and bolt them down. Screw a 2x4 to your 2x6 along the back wall for your doors to mount to.

-Build 2 door frames with the 2x4s. Cut the plywood to make 2 door frame covers. Cut an opening for a door vent on each piece of plywood. Cover the vent openings with the ¼” hardware cloth to keep out rodents.

-Screw the plywood doors to the frames.

-Use the hinges to mount the doors to the 2x4 along the back wall.

-Optional: Screw or nail 6" wide plywood strips along the upper edge of the door so that the corrugated metal will be pitched to better shed raid and to create a gap that allows more airflow into the bin.

-Screw the corrugated metal pieces to cover the doors (each door has 2 overlapping pieces to cover them). Paint the corrugated metal doors white to reflect the sun’s rays.

-Lay down a 2” inch layer of coconut coir on the bottom of the bin.

-Start adding kitchen scraps. When you’ve got a few inches of kitchen scraps, add your worms.

-Wet the worm bed whenever it seems dry. Cover the top with a couple sheets of wet cardboard during the hottest days of summer.

-Install a plywood divider down the middle of the bin. Drill 1" holes in it so worms can migrate from one side of the bin to the other. Fill one side of the bin full of food scraps, and add the worms in that side. While they’re working on that pile, begin filling the other side with coir, soil and food scraps. Once they have finished devouring everything on the first side, your worms will migrate to all the new food. Makes harvesting their castings very easy.

-To make it easier to work in the bin, install a piece of chain on each door and a hook somewhere to hold the door up. You could also just prop the door up with a stick.

-For better insulation inside the bin, pile soil up to almost the top of the 2nd course of blocks.

-If you live somewhere where frost heaves are an issue, I would set up an actual gravel+rebar footer under the bin walls and use more rebar in the wall itself.

-Sprinkle food grade diatomaceous earth around the inside of your bin to keep roaches from taking up residency.