Late winter or early spring is the ideal time to cut back and clean up ornamental grasses. Plants grow better when the previous year’s foliage and flowering culms are removed; and they look better and grow sooner.
This is not an option for most of us, but in a large collection, such as the Minnesota Landscape Arboretum, it works well. We obtain an annual permit, burn when the winds are minimal and the snow is gone and plants have dried usually in early April in Minnesota. The burn is finished in just a few hours, much less time than cutting back the plants by hand. Burning releases nutrients back into the soil and it invigorates the plants; warm season grasses respond well to a spring burn. Panicum, miscanthus, pennisetum, little bluestem, prairie cordgrass are warm season grasses that benefit from a spring burn.
Warm Season Grasses
Because burning is rarely an option, cutting back the grasses is the next best maintenance practice. Cut back to 6 inches or less. Most warm season grasses do not start to grow until late spring; they look totally dormant at cleanup time, with no sign of green foliage. Cut back as far as you can to the ground. The growing points are at the crown, or root-shoot junction, which is low in the ground. You can use hand pruners, or for a group of grasses, use an electric hedge trimmer, as shown here. You can also re-cut the stems into shorter sections, for easier handling and faster decomposition, shown in the second photo.
Cool Season Grasses
Cool season grasses such as feather reedgrass, blue oatgrass, blue fescue, and tufted hairgrass grow early in the spring (and can be damaged if you are able to burn) also benefit from a spring cleanup. If these cool season grasses are growing and green, care must be taken to not damage the new green growth in spring cleaup.
Blue oatgrass, shown at left is starting to grow amid a lot of dead foliage from last year. Cut back or rake out dead growth from these plants very early in the spring, perhaps even in February or March, as soon as the snow allows access to the plants. Some of these cool season grasses, most sedges are cool season growers, some are evergreen or semi-evergreen and you can easily see what foliage needs to be removed in early spring, as the next photo shows.
The grass foliage and stems you remove are a good addition to your compost pile. One year, my compost pile even started smoldering with the heat caused by the fine grass stems decomposing!
It is not a good idea to cut back the grasses in the fall, this seems to predispose them to winter injury, especially in colder climates. The winter foliage is usually attractive and can provide cover and food for birds. Clean up can be done any time after the majority of the winter is past, and you can access the plants (for those of us in the north, this means no snow is in the way).