According to WSU, prune ornamental grass in the spring and how early depends on their cool-season, warm-season, or evergreen classification.
My cool-season grasses, such as the tall Calamagrostis acutiflora, Karl Foerster feather reed grass, are perennial and go dormant in the winter and start growing late winter or early spring again. In my zone 7 growing climate, high temperatures of 60-70° in March and April is the prime growing season for this grass. Knowing the grass’s seasonal growth pattern will inform you whether it is warm or cool-season grass or an evergreen. Pruning should occur before they start growing again.
Since grasses reach their peak glory in the fall, it would be such a shame to prune them and miss out on their beauty over the winter! So to you enthusiastic Edward Scissor Hands in the fall, I say, “Lay your hedge trimmers down! Let the grasses be!”
Even with the snow, frost, and ice, grasses continue to add height and contrast to the landscape; then, when the sun melts the snow, the feathered tips dry out and move again with the air around them. The grasses remind us of last summer’s beauty and what’s to come in the new year.
Besides their simple maintenance, ornamental grasses add a gentle pastoral effect to the classic flower beauties around our house. When the roses, daylilies, and lavender bloom have dried up and gone dormant, the grasses maintain their surreal beauty throughout the winter.
I also have warm-season grasses such as Pennisetum orientale, Fountain grass. These grasses don’t begin growing until later in the spring, when the soil warms from longer, warmer days. As you can see, I have a lot of these beauties! Thankfully, because I have a mix of cool and warm-season grasses, I don’t have to prune all of them at the same time!
In this early summer photo, the Fountain grass has not developed plumes yet because it’s a warm-season grass and is beginning its growing season. As the mounds grow, so does their girth. Since fountain grass gains size over the years, I like to save money by starting them from seed or purchasing small plants.
At the time of this writing, February 6th, I’m mentally gearing up for pruning the cool-season grasses at the beginning of March. I’m doing my core exercises in preparation, so one day of hedge trimmer work won’t put me in the hospital. I’m gathering my tools so that I’m ready to get the work done on day one. And I’m watching the weather forecast so that I’m looking beyond my pruning day for good weather that will warm up the crown of the plant so that it resumes growing as soon as possible.