Hi,

Thanks for your question!

You are absolutely correct that plants use their foliage to photosynthesize and convert sunlight into nutrients, and that in the fall plants start to draw nutrients down into their bulbs, fleshy rhizomes or roots to be used for spring growth. (Hostas and day lilies grow from rhizomes, which are roots that grow laterally, and grasses have roots that grow in all directions.)

You may have noticed that hosta leaves aren't very attractive after they've been through a frost. You can cut the hosta leaves off after the first frost or in the spring after the snow is gone. Ornamental grasses should be cut back in late winter or early spring. 

You can do pollinators (specifically, the mason bee) a favor by leaving day lilies as they are. The dried stems are hollow and provide potential nesting habitat for next year's bees. New day lily growth will hide (or mostly hide) the dried stems.