It’s easy to keep Worms warm in your worm farms, whatever design you have. Worms like to be warm, they breed faster, eat more and grow fatter when they’re warm.
Worms and bacteria generate heat as they go about their lives. Capturing this heat and keeping it in the Worm farm is the easiest way to keep the farm warm. A simple blanket or sack that covers the Worm farm is the easiest way to do this. I favour a sack inside the farm, over the top layer of activity. Sacks are good because they absorb moisture and are porous, allowing oxygen and worms to pass through. It also breaks down slowly too.
You want something that is openly woven so that moisture doesn’t build up to too high a level. The Worm farm is a living organism, or at least a system of living organisms, afterall.
A layer of straw or hay over the top layer of scraps also generates heat and traps it. It breaks down over the winter due to bacterial and Worm activity, so adds to your castings. Lucerne is the best to use, its high level of protein and other nutrients fatten up the Worms nicely,
Mixing bran into the kitchen scraps is another way to generate warmth. As the bacteria in the Worm farm attack the bran, they generate heat, as they do when they’re reproducing, The Worms love it too.
Add Bokashi scraps or Bokashi bran , or just spray the liquid periodically. The masses of bacteria in these materials are anaerobic and start attacking the oxygen deprived areas (there are a few of them in a worm farm, but not many). They also die in huge numbers in the presence of oxygen in a healthy, well ventilated and mixed bed. This sounds bad but is a good thing. It provides a feast for the bacteria, and insects, especially the Worms in the farm. They are more active and generate more heat.
Hot water bottles, wrapped in cloth so the Worms don’t come into contact with the hot surface, can be used in very cold areas. We haven’t experience of things getting that cold at Ligaya Garden, but the principle is sound.
Moving the farm inside protective shelter is another technique. Moving it away from cold wind and into an area of still air stops any heat generated being blown away.
If you’ve got time, you could move the farms into the warm Sun when it shows itself, then move it into shelter when the Sun goes down.