Here in Seattle, red worms (as well as slugs, earwigs, and more) will be found in the protected parts of many different plants, brassicas, lettuces, leeks, etc. The only way to keep them out is to grow in worm-free soil, i.e. pots or beds with artificial soil well isolated from natural soil dweller populations. Worms will travel large distances over bare concrete to enter pots through unscreened drainage holes. That is all quite a bother, so perhaps it would be better just to resign oneself to always disassemble and examine garden produce before use.
