I do worm composting and I have noticed that when I use my worm castings the seeds that have been in the plants that I feed my worms sprout in what ever I use my castings in. So I get tomatoes and melon plants growing every where. I have used store bought worm castings before and this did not happen. Why would it happen when I use mine but not the store bought castings? Do large worm composting operations sterilize their castings before selling them?
Or is it just because I don't sift my castings enough ?
Your suggestion about freezing the worm castings, would this prevent this from happening.
I've experienced the same thing. I just throw stuff into the bin without any processing or freezing; perhaps the commercial producers grind the food up before composting. This would break up the hard shells of the seeds so the worms could get at the insides.
When I get seedlings sprouting right in the bin, I rake them into the bedding and this disturbs them enough that they usually rot and provide more food. I never bother to sift the castings before using them.
Freezing? It might kill some seeds that are from temperate or tropical regions.