Another way to grow blackworm (Lumbriculus variegatus) seems to be on Japanese filter mats. I have a piece of that about 40 by 20 cm in a plastic tank outside. The water level is currently 15cm. The worms have settled in the mat, and I can regularly collect a clump of worms from each corner where they also accumulate. The tank has been outside at temperatures from 1 to 20 degrees, so far without problems. The food I use are spirulina tablets, but I will try to replace that by spinach or other leftover green vegetables from the kitchen. I do take care to regularly remove all other organic material than the food. The worms seem to grow larger than in my indoor culture.