Ornamental grasses are well established as important elements for the perennial bed or border. These grasses grow heartily in warm weather, Then the blades dry. Many gardeners leave the "straw" to add structure to the winter garden. Come spring, new green grass blades grow from ground up. If the straw was left standing all winter, it should be removed post haste. Larger grass clumps may need to be divided as well.
Grasses that have browned in the fall have died down to their hardy roots. Over winter the initially interesting grassy bits get literally weatherbeaten. You may remove all brown material in late winter or spring. Try using a pair of heavy scissors on smaller grasses, or a serrated knife it the scissors won't cut it. Be ready to escalate to a hedge trimmer on bigger, tougher species. Some grasses will self sow terribly. Gather any seed heads or remaining seeds you see. Dispose of all of this by bagging or burning. (If you have experience with the grass and are confident that it does NOT self sow, use the grass as mulch or compost.) Burning the straw where it stands is another way to get rid of the dry material. Those dry stems flare up, but without much excess heat, so neighboring juicy plants are usually unscathed. You might not like the temporary "scorched earth" effect, but it will be hidden by new green grass blades. (I trust you to use judgement about risk of brush fire!)
On the left- This Northern Sea Oats is dead in the center and on one side, and shows growth only along the opposite edge.
On the right- This clump of the same grass looks uniformly vigorous. At about eight inches across I could divide it to propagate, but I don't feel it must be done this year.
Divide and conquer
After getting rid of the straw, look at the condition of the grass clump. Is it still the right size for the spot? Is it showing any new growth? All ornamental grasses can be divided in spring when they begin to grow. Clumps of grass grow gradually out from the center by rhizomes. The center of the plant therefore is the oldest part. Interior stems can become crowded, woody and choked by thick blades all around. Eventually the grass may die out in the center, or the clump simply become too large for the place you initially planted it. Loosen up your muscles and get ready for a wrestling match. And consider the goal: will you replant here, or elsewhere, or pot them? This is a great time to add soil amendments if needed, or pot up plants to share.
The grass roots must stay moist while you work. Get a bucket of water ready, or some way to cover the roots as you work, to keep them from drying in the sun and breeze. Dig around the clump and pry it up. Grass roots are usually fine, wiry, and dense. The clump may be very heavy when you first unearth it. Shake or wash soil from the root area to lessen the load and let you see the roots. Now choose a weapon. Often the fastest way to break up the grass clump is simply to attack it with an axe or sharp shovel. Some tough clumps respond to "persuasion" with a fine tooth saw. The gentler way to divide the clump is to use two garden forks to pry it into pieces. Whichever method you use, break up the large mass into smaller ones and expect some loss. Pull or cut off any obviously dry and unthrifty sections, too.
Divide, plan B
Some gardeners divide grasses by prying out chunks from the still-rooted mass. This method may seem easier, as you won't have to lift the entire heavy rootball at once. Lift it all by chunks, or leave some in place. Fill the holes you leave with soil. The grass will spread into the empty space.
On the left- Two new divisions of a Pennisetum. One is much browner and has lost most of its roots in the process of being split. I'll trash that one, and use the one with plenty of green growth supported by an appropriate amount of roots.
On the right- Keep the roots moist while you work with grass divisions. When replanting, look for the "crown," where roots end and greenery begins. Cover the roots with soil but take care not to bury the leaf blades at all.
As you work and create new divisions, dunk them in the bucket of water, or cover them. Dispose of the waste: chunks that lost most of their roots, hunks with little green new growth, blades lacking roots and vice versa. It can be composted (slowly, as a brown). Amend the planting area if needed. Replant the divisions. Give the new root mass adequate room and always plant up to, but do NOT bury, the junction between root and blades. Press the soil in. Water. You're done.
Bluestem Nursery, http://www.bluestem.ca/