A worm blanket covers the worm bed. Usually, it lays directly on the bed. There may or may not be a lid above the blanket, topping and closing the bin .
A worm blankets are many different things. It can be inorganic like a plastic or can be organic material such as cloth, paper, jute, hemp, or the common favorite – cardboard. There have been metal, mineral and vegetable worm blankets. I like to use actual blankets picked up cheap from the thrift store.
The worm blanket; protects the worms, helps to control moisture, blocks flies, makes the bin look better, and generally improves the function of the bin. The blanket can be; thick or thin, sturdy or flimsy, temporary or permanent, edible or not. The different kinds of blanket are better or worse at these functions and you get to decide which is most important.
Worms like worm blankets. Very often, you will find worms hiding in, clinging to, or clambering on or inside the blanket. Some breeders say the worms crawl up to do their mating dance. Certainly, the blanket is a well oxygenated place to slither or rest.
‘Organic’ materials are carbon rich such as newspaper, cardboard, or cotton/wool cloth. All of these soak up moisture from the bedding. Over time they become slimy, rot, and merge into the bedding. People like them because they are cheap. Worms like to crawl up into them probably because they are moist and full of tasty bacteria. But then worms crawl up into just about anything. Some worm bin sellers supply a hemp or sisal mat. Over time the merchant will be happy to sell you replacements as the worms eat the material. All of these allow the bed to breathe. They allow some moisture to escape which can be a good thing.
Non-organic are usually plastic. Can be a plain plastic sheet or maybe bubble wrap. These don’t rot. They hold down the moisture, sometimes too well. They can be folded back to cover only part of the bed for fine tuned moisture control. The edges allow gaps that let the bed breathe. Polyester blankets are completely breathable and hold in heat when that’s important. I’ve seen shade cloth used and of course my Worm Filter (https://oldtomswormery.com/product/worm-filter/) fits here.
Notice how holding in moisture is a common trait? The blankets helps keep the bedding moist but also helps keep the bin walls dry. Wet walls allow wandering worms! With each of the blanket types, you adjust by folding the sheet back or adding extra layers. The trick is to allow oxygen access. So, best effect is moisture held in while oxygen flows freely.
Worm blankets also help to block flies. Flies must find damp food to lay their eggs. If your bin contains fly eggs, you will get flies when you open the bin. When flies can’t find damp bedding, they can’t lay eggs. Anything dry and fluffy on top of a worm blanket will block those pesky flies. I maintain a dry fluffy layer of shredded newspaper on top of a Worm Filter. The Worm Filter is a plastic mesh that doesn’t wick moisture. The mesh gives just enough protection to the newspaper so it stays dry for a good long time. Any blanket can be covered with a dry layer.
So try different types of blankets to see which works best for you.