Article written by Curtis Adams for Houzz
Ornamental grasses are valuable landscape features. They provide three- to four-season interest with their distinctive forms, be they upright towers or arching mounds. Their seed heads often persist into winter, providing garden interest as well as food and habitat for overwintering wildlife.
Additionally, ornamental grasses require very little maintenance. They likely don’t need to be fertilized and only require an annual cutting back and infrequent division to be happy. Early to mid-spring, as new growth begins to emerge, is the prime time to care for your grasses. Here’s what you need to know about caring for perennial grasses.
Tight clumps of ‘Karl Foerster’ feather reed grass act as a translucent screen around a patio. Though a cool-season grass, its seed heads can remain attractive well into winter.
Types of Ornamental Grasses
Ornamental grasses can be broadly grouped into cool-season and warm-season species, which affects the exact timing of their annual care.
1. Cool-season grasses. These grasses begin growing when soil temperatures are from 50 to 65 degrees Fahrenheit (10 to 18.3 degrees Celsius). This typically occurs from late winter into early spring, depending on climate and site conditions. Cool-season grasses include:
- Tuft-forming fescues, like ‘Elijah Blue’ fescue (Festuca glauca ‘Elijah Blue’, USDA zones 4 to 8; find your zone)
- Blue oat grass (Helictotrichon sempervirens, zones 4 to 8)
- Tufted hairgrass (Deschampsia cespitosa, zones 4 to 9)
- Autumn moor grass (Sesleria autumnalis, zones 5 to 8)
- ‘Karl Foerster’ feather reed grass (Calamagrostis x acutiflora ‘Karl Foerster’, zones 4 to 9)
- Eastern bottlebrush grass (Elymus hystrix, zones 2 to 7)
- Inland sea oats (Chasmanthium latifolium, zones 3 to 11)
- Most species of sedge (Carex spp.)
Pink muhly grass is a warm-season grass that puts out clouds of pinkish flowers in late summer through early fall.
2. Warm-season grasses. These ornamental grasses kick off a new season of growth when the soil temperature goes above 70 degrees Fahrenheit (21.1 degrees Celsius), typically in mid-spring to early summer in most areas. Warm-season grasses include:
- Switchgrass (Panicum virgatum, zones 3 to 9)
- Little bluestem (Schizachyrium scoparium, zones 2 to 9)
- Indian grass (Sorghastrum nutans, zones 4 to 9)
- Pink muhly grass (Muhlenbergia capillaris, zones 6 to 9)
- Prairie dropseed (Sporobolus heterolepis, zones 3 to 8)
- Blue grama (Bouteloua gracilis, zones 3 to 10)
- Japanese silver grass (Miscanthus spp.)
- Fountain grass (Pennisetum spp.)
Note: Species of Miscanthus and Pennisetum are considered invasive in various areas in North America. Check with your county agricultural department or native plant society before planting.
How to Care for Ornamental Grasses
The two main maintenance tasks that come with ornamental grasses are cutting back and dividing, or size control.
Cutting back grasses. Ornamental grasses should be cut back annually. After plants go dormant in the fall, prior to the start of growth for the new season, cut them down to between 3 and 12 inches tall. For cool-season grasses, cut back by late winter or in late summer, as plants are coming out of dormancy. For warm-season grasses, cut back by early to mid-spring. Leaving several inches of stubble helps protect the crown of the plant from winter weather and provides a visual placeholder in the garden.
For many years the standard practice was to cut back grasses as they began to die back in the fall. In recent years, however, more and more people are leaving their ornamental grasses up through winter. This adds winter interest to the garden and provides valuable habitat for birds and other overwintering wildlife, including butterfly larvae, birds and bees. In areas that are fire-prone, however, ornamental grasses should be cut back as they dry out in order to reduce fire risk, particularly near structures.
These are tools that can be used for working with ornamental grasses. Central to these are gloves and safety glasses.
Tools and techniques for cutting back grasses. The tools you will need for cutting back depend on the size and types of grasses you are working with.
- Hedge shears can be used; I prefer slightly smaller shears with 8- to 9-inch blades and similarly short handles.
- For well-rooted grasses I like to use a curved, serrated blade like that on a Nisaku sawtooth sickle (the cutting, tearing action can uproot shallow grasses).
- Powered hedge pruners work well for larger jobs.
- If you don’t want to bend down so much, you can use a string trimmer; for tougher grasses, use a brush cutter.
- A reciprocating saw fitted with a long pruning blade will also make quick work of a stand of ornamental grass. You can also use this saw to divide clumps of grasses, but cutting through soil can dull the blade quickly.
- Tie larger bunches of grasses together into a bundle with string prior to cutting to help with cleanup.
- Watch out for any hidden posts or plant supports in the grasses to avoid doing damage to you or your tools.
- Since many ornamental grasses have sharp edges or points, wear gloves and safety glasses when handling them.
- If you live in an area where there are no hazards from fire spreading, you can use fire to clean up some of the more compact warm-season or prairie grasses in late winter or early spring. (Make sure the fire department has a low fire hazard sign posted.) Fire is a natural process for returning nutrients back to the soil and controlling plant pathogens, but it is not without its risks. Rather than setting an entire bed ablaze, I’ve used a garden torch to burn specific areas, making sure the fire is extinguished before moving on. I also keep a source of water handy, just in case. If you have any doubts, check with local authorities before doing any burning, or use another method.
Dividing ornamental grasses. Ornamental grasses benefit from division when they lose their vigor or become too big. The best time to divide plants is when they are beginning an active growth phase. For warm-season grasses this is in mid-spring, as the soil temperature moves near 70 degrees Fahrenheit (21.1 degrees Celsius). For cool-season grasses, growth begins in late winter to early spring when soil temperatures are from 50 to 65 degrees Fahrenheit (10 and 18.3 degrees Celsius). Cool-season grasses can also be divided in the fall as the soil temperatures drop and root growth resumes. Do not divide grasses when they are flowering.
- Grass clumps that are riddled with dead zones can be dug up and divided. Discard the dead portions and replant the healthier parts to their original depth. Digging down 12 to 18 inches will get you enough root mass to make successful divisions for larger grasses. Smaller grasses do not need to be dug so deep.
- A shovel with a sharpened blade and a strong handle will allow you to dig up most grasses. In tough cases you can use an ax to divide clumps while they are still in the ground.
- Each division should have at least three healthy stems (culms).
- When dividing grasses, do not allow the exposed roots to dry out. If not replanting right away, divisions can be put into old nursery pots and covered with a little soil or put into plastic bags and sprayed with a little water to keep roots moist. Store divisions in a cool, shady spot until they are replanted.
These large masses of Miscanthus could take over this border if they are not occasionally reduced by digging out some of the mass.
- Clumping grasses have a tendency to die out in the center of the clump. These plants can be rejuvenated by dividing the clump and adding back fresh native soil (grasses do not need enriched soil to grow well). One option is to dig out the center using a post hole digger. If desired, the center can be filled in more quickly by putting a healthy division back into the center of the mass, along with fresh soil.
- Another option for large clumps is to cut out several pie-shaped wedges of the grass and backfill with native soil. This keeps the clump in its original position, reduces the amount of disturbance and provides lots of fresh space for the grass to grow into.
- While not absolutely necessary, cutting back grasses beforehand makes these digging chores easier. This allows you to see what you’re doing and provides easier access to the soil.
Little Bluestem is a warm-season grass that thrives in poor soils. In rich soils it tends to flop over, rather than holding its upright form.
Fertilization. Most ornamental grasses don’t need fertilization, preferring rather lean soils. Unlike a lawn grass, which is cut regularly and needs to have constant feeding to regrow, overly fertilized ornamental grasses tend to grow quickly and flop over, thus losing the tight upright form expected of many of these.
Invasiveness and spread. It’s important to know how a grass spreads before you plant, as some species can be aggressive, spreading through seed, stolons or rhizomes. Many exotic grass species, such as Chinese silvergrass (Miscanthus sinensis), are considered invasive in various U.S. regions due to their aggressive spreading and ability to escape into the wild and take over.
Most of the common ornamental grasses are clump-forming and are easier to control.