When to trim Lirope depends on what type you are growing and your climate. In general  this member of the lily family (it is not a grass but looks like one) should be cut back in winter before the new growth emerges. The Black Mondo Grass lirope grown for the dark strappy leaves may not ever require pruning if the old foliage is not damaged from winter weather. The new growth will just blend in with the old foliage. There are some green liropes used as groundcover in mild winter areas and you can trim these back with a string trimmer or lawn mower set at the highest setting.  Again, do the cutting in the winter when the plants are dormant and before the new growth emerges. This could mean January or as late as March depending on your climate. Keep growing, Marianne Binetti