I'm not sure that there is any safe weed killer to use around daylilies. You can spray the weeds, as long as you make sure that the weed killer doesn't get on the plants. I do this by putting cardboard between the plant and the weeds, but I've never done this around my daylilies. I don't want to take any chances with overspray. Preen is a great thing to use after you get the weeds out, but it won't kill the weeds.
Hopefully someone knows something that is safe to use! I'm an obsessive mulcher myself, and have been very happy with the lack of weeds, but it won't kill them to begin with.
