Leeks are pretty foregiving. I let a bunch of leeks go to seed last year, cut them off at the base, and new leeks sprouted from the roots (not from the seed, which I saved). I'm transplanting and separating them next week, maybe week after. Leeks don't require a lot of mulch to overwinter in my zone. Some of the green tops will look a bit ratty come Spring, but the white parts are just about unaffected.