Ornamental grasses are perennial plants highly valued in landscape design for the texture, movement, and vertical structure they provide. They are categorized as either warm-season or cool-season varieties, influencing their growth patterns throughout the year. Gardeners frequently wonder if these plants can withstand a significant trim during the warm summer months. Understanding the biological cycle of these plants is necessary to determine the best approach to summer care.
The Primary Rule: Why Full Cutbacks Are Detrimental
Attempting a full cutback of ornamental grasses during the summer growing season severely damages the plant’s long-term health and vigor. During high heat and sunlight, grasses rapidly photosynthesize, converting solar energy into sugars. This stored energy is crucial for the plant’s survival through winter dormancy and for initiating robust new growth when spring returns. Removing the majority of the green foliage eliminates the plant’s primary energy-producing mechanism, essentially starving the root system.
A sudden, severe reduction in leaf surface area forces the grass to expend its stored reserves to regenerate new blades, placing the crown under immense stress. This stress can weaken the plant’s defenses, making it susceptible to disease or environmental pressures like drought or heat. For warm-season grasses, which are at their peak growth in summer, a full cutback is particularly damaging because it interrupts the period intended for maximum energy storage.
Ornamental grasses that are cut down completely in summer often enter a state of shock, leading to stunted or significantly delayed recovery. The resulting appearance is typically an unsightly patch of brown stubble, which offers no aesthetic value for the remainder of the season. This drastic removal of photosynthetic material can also prevent the grass from adequately hardening off before the first frost, jeopardizing the perennial’s ability to survive the cold.
The crown of the grass, located just below or at the soil surface, contains the meristematic tissue responsible for all new growth. When the tall blades are present, they help shade and protect this delicate crown from the intense summer sun and high soil temperatures. Interrupting the energy flow forces a biological reset that the plant is not biologically programmed to handle during its active phase. This major setback means the grass will likely have a sparse or weak return the following spring.
Essential Summer Grooming
While severe pruning is detrimental in summer, lighter maintenance, often referred to as grooming or tidying up, is acceptable and beneficial. This approach focuses on removing only the dead or damaged portions without significantly reducing the overall green photosynthetic mass. The goal is to improve the aesthetic quality of the grass while keeping the plant healthy and fully active.
One common practice is deadheading, which involves removing the spent flower heads or seed plumes once they have faded. This action improves the plant’s overall look by eliminating browning or drooping material that detracts from the plant’s form. For certain species, such as feather reed grass (Calamagrostis), deadheading can also prevent unwanted self-seeding.
Gardeners can selectively remove individual blades of grass that have completely turned brown or yellow, often found at the base of the clump. Use sharp, sterilized hand shears or pruners to make clean cuts on the dead material. This selective removal improves air circulation within the clump, which can help mitigate fungal issues in humid summer conditions.
Brown tips on otherwise green blades can be trimmed off using sharp scissors or shears, making a subtle angled cut to blend with the natural shape of the leaf. When performing this light trimming, it is important to follow the natural contour of the blade or plume, making the cut inconspicuous. The cumulative amount of material removed should be minimal, never amounting to more than fifteen percent of the total foliage mass.
The Proper Season for Major Pruning
The correct time for a major cutback depends on whether the variety is warm-season or cool-season, but generally falls outside the summer window. Gardeners delay this major pruning to enjoy the visual interest the dried foliage provides throughout the winter months. The stiff, upright stalks and plumes often catch snow and frost, adding texture and movement to the otherwise barren winter landscape.
For warm-season grasses, the preferred time for a full cutback is late winter or very early spring, just before the new shoots begin to emerge from the crown. This timing allows the plant to use the old foliage as a protective layer, insulating the sensitive crown tissue during the coldest part of the year. Cutting the grass down to approximately four to six inches above the ground clears the way for vigorous, clean growth, which starts rapidly once temperatures rise.
Cool-season grasses, which tend to green up much earlier in the spring, can be cut back in late fall after they have gone dormant or in early spring. These varieties often benefit from a lighter cut, perhaps reducing the height by only one-third or one-half, depending on the species and its appearance. Performing the major cutback during the dormant season ensures that the plant’s stored energy reserves are not depleted, setting it up for a healthy year of growth.