I wouldn't recommend cutting back the foliage of the plants.

As for 'nut grass' or 'nut sedge' which I think might be the proper name, it's a diamond-studded-*itch to deal with, and I don't think you can ever get rid of it completely.

To keep it under control, I take a rather narrow trowel, push it into the ground beside the weed, and pry the ground upward just a bit. This helps to dislodge the weed, so that if you're lucky, when you pull it up, you'll pull up some of the runners and those nasty little nuts with it. They can extend for a considerable distance. It's a tedious and back-breaking chore.

There are chemicals that will allegedly kill the nut grass, but they also, as a far as I know, kill other plants as well, including the daylilies.

Nut grass is a tough problem. A number of people use a product called sedgehammer with daylilies. It can be used over the top of daylilies to kill nut sedge. You might try a small spot first to make sure there are no problems at your place.
You can also use round-up to kill it. Cut the bottom out of a milk jug and anchor over the sprout of nut grass so your daylilies wont be hit by any over-spray. It may take a couple of sprays to do the trick.
Chemicals are most effective when the plant is actively growing.
Good luck, Ed