One thing you need to know before cutting back the dry, dead stems of ornamental grass is whether the grass is a cool season or warm season grass. If you don’t know what particular variety of ornamental grass is in your yard, observing its growth habits will tell you whether it is a cool season or warm season grass.
Cool season ornamental grass begins to produce its new growth quite early in the spring, soon after temperatures begin to stay above freezing. Cool season grasses flower by early summer, making them good additions in the short growing season of a northern garden.
Warm season grasses begin to grow much later in the spring, sometimes so late that you may begin to wonder if they made it through winter. Warm season grasses begin flowering later in the summer and into the fall.
Some of the cool season ornamental grasses are fescue, ribbon grass (Phalaris), feather grass (Stipa), northern sea oats and tufted hair grass. Warm season ornamental grass includes both little and big bluestem, Japanese blood grass, maiden grass (Miscanthus), fountain grass (Pennisetum) and hardy pampas grass (Saccharum).
The spent flowers and seed heads of ornamental grass, along with the dried foliage, can add interest to the landscape throughout winter. Although the dead foliage of either cool season or warm season grasses could be cut back in late fall, many gardeners enjoy the beauty of the foliage throughout winter.
Unless the plant becomes too shabby over winter, trimming back the dead stems of cool season ornamental grass can wait until the first balmy, late winter or very early spring day. As soon as any snow melts and the ground begin to thaw, cool season grasses should be cut back. Waiting too long may risk damaging the new shoots that will begin to emerge as soon as the weather begins to shift toward spring.
Cut back cool season grasses so about a third of last year’s growth remains. Be careful to not cut back a cool season ornamental grass too far, as this can seriously harm the plant. Resist the temptation to burn off the dead foliage of a cool season grass, as this will also damage the growing tips. Don’t worry that these remaining dried stems will be unsightly, because the bright new spring growth will soon hide it nicely.
Warm season grasses can be left standing later into the spring while you take care of more urgent gardening tasks. Providing you don’t wait so long that the new foliage is already emerging, warm season grasses can be cut back to the ground. If you can already see some new green growth emerging as you prepare to trim the plant, just cut above it, being careful to not damage the new growth, otherwise all season long the plant will look like it has a crew cut.