Biggest drawbacks i can see are that you would need to constantly be adding moisture, and critters could get at your worm stock. Maybe the top few inches of the bin being screen and a section of the floor. Covering it with a tarp would trap as much heat as anything else. Wet burlap, canvas, or carpet would help to retain moiusture better.

Bryan

Castings sort of 'knit" together meaning they are highly unlikely to fall through a 1/4" screen. Flow through worm bins, which are designed with an open mesh floor specifically for the purpose of allowing worm castings to fall through the bottom, use 2"x2" welded hog wire as a floor, and this mesh size is often considered too small to allow castings to fall through effectively. Many flow through systems prefer to use 2"x4" hog wire floors.

Consider using a larger mesh for the floor of your design.

Also, in order to ensure the worms are not in the material at the very bottom of the system you will need a minimum depth of about 30 inches in the bed. This provides sufficient space for the worms to stratify, leaving the castings at the bottom worm-free.

The bin you are proposing will definitely be a challenge to keep moist. Moisture retention is a trick in all flow through systems and they are simply open top and bottom. You might consider enclosing your system concept to try and reduce environmental impact on the system, and keeping it in a shed as opposed to open to the great outdoors is definitely a good idea!

Kelly S

When i was designing my first bin, I saw the mesh bottom idea, and thought it would be good to harvest them without the usual troubles. I put a 1/8 holed box on the bottom and a screen under that on top of a bucket. Thinking the castings would drop through that first grid, and if worms got threw, they could be caught by the second screen. NOPE.

The worms not only got threw the first grid, but also the screen!!! i lost one pound, they ALL escaped over night!!! i had tons of dead worms all around it the next day.

I have resorted to just a big old plastic planting pot with holes in the bottom, and a tray to catch tea. And the castings are more of a mud, that would not rain gently through the strainers, and not cute little dry balls that I thought. I doubt that those bins with the migration sections work. But I just moved my finished heap to the side, and added new mix to the other side. I am dying to see if they indeed migrate. I have heard that sometimes they do, sometimes they dont. If they dont ill just mix it all up and wait till its all done and take out half of it to use in the garden, worms and all, and keep half to regen the popluation.

The simplest idea turned out the be the most sucessful. i got a new batch of worms, and i have about ten pounds of castings and the populations seems to be great.

good luck

I have many different types of homemade and manufactured bins. Many are the tiered systems and the worms do migrate to the fresh food, although it all takes time. Most of us human types would like it to go faster.