How big are these clumps? We cut our liriope back just before the new sprouts form. We do not use a grass trimmer either. This will cause damage to the plant. There are several tools to cut them back. Just make sure they are sharp. Hand prunners work well for small to medium clumps. For huge areas some use a push mower with bagger set on highest setting and very sharp blades. Before starting with the mower make sure you have clearance from the plants crown.