Last year's growth on the cool-season grasses should be cut down as soon as the snow is gone. These plants literally "leap" out of the ground in the spring. Many a year I have had lush plants with chopped off leaf tips due to not taking care of this task early enough!
How best to cut back the grasses you ask? Here at Bluestem Nursery we use a sickle (that's Jim cutting back Pennisetum alopecuroides in the picture). One year, we went against our principles and burned the nursery grasses down. However, one must check to see if there are any restrictions in your area before burning. DO NOT burn the dead foliage off of cool season grasses. The growing tips can be damaged.
Heavier grasses can be cut back with a weed eater-type machine, using a blade rather than the nylon line. Be careful not to cut cool season grasses back too far, as they can be damaged beyond repair. Leave approximately 1/3 of last year's growth in place. It will quickly be hidden by the new growth.
We have recently seen a novel approach to warm season grasses. Rather than cut them back first thing in the spring, leave them standing as long as possible. The buff-colored foliage contrasts wonderfully with the new growth of the Spring bulbs.
Whenever you choose to cut back warm-season grasses, they can be cut right down to the ground without any problem. If you leave it real late, just try not to cut off the new leaf tips!
The warm season grasses can be cut down to ground level as long as no new growth has started. If you are late getting to this job, leave 6 - 8" of old growth so as not to damage the new leaves.
Some grasses have very sharp edges. The wearing of gloves is highly recommended.
All this talk of warm and cool season grasses has probably got you asking, "Just which ones are warm and which ones are cool season?" Below is a chart of grasses that Bluestem Nursery carries. Note however that in some climates, Koeleria, Festuca and Helictotrichon are virtually evergreen and therefore will not need to be cut back. Rather than a haircut, just give them a "combing".
Cool Season Grasses
Warm Season Grasses
Note: Luzula, Juncus and some Carex are evergreen (they are not technically grasses)
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Picture and article courtesy of Bluestem Nursery