As long as the grass and weeds have not infiltrated the centers of the daylilies, as Charley suggests, you will have to weed them out of the beds and mulch heavily. Every time a weed comes back and through the mulch, you should dig it out and keep the bed mulched. Unfortunately, if grass or weeds are mixed in with existing clumps of daylilies, I would suggest taking them out of the ground, hose off the roots and remove all of the grass and roots, and replanting them.
