There are two options to consider when maintaining ornamental grasses: cut them back prior to new growth or remove dead blades in spring to allow for new growth.
While removing dead blades in the spring is self-explanatory, if you do decide to cut them back it’s important to understand which type of grass you’re cutting.
In general, ornamental grasses are classified as either cool-season or warm-season grasses.
Cool-season grasses put on most of their growth in spring before temperatures begin to reach 75°and in the fall when temperatures cool down. They generally maintain good color through the summer but won’t grow much when it’s hot.
Since they tend to look good even as the weather cools, you can leave their foliage in place until early spring. Be careful to keep about one-third of the plant in place when do you trim them.
Warm-season grasses start growing mid-to-late spring or early summer. Their major growth and flowering happens when the weather is hot. They will usually turn shades of brown during the winter, at which point you can trim them back almost any time. Keep in mind they are slow to establish and will truly start to grow in their second and third years.
If you like to tidy your garden in the fall, trim warm-season grasses so they are just a few inches tall. You can also leave the dried grasses and seed heads in your garden for winter interest. However, not all ornamental grasses look good through the winter. Fall is a great time to trim back those that don’t. If you leave the trimming until spring, try to cut them down to the ground (you can leave a couple of inches) by late spring before new growth begins. Don’t worry, they will grow back just fine!