Ornamental grasses differ from turfgrasses in that they’re not meant to be mown. Instead, they grow into distinctive shapes — tufts, sprays, and stands, or flowing, shimmering sweeps. Instead of drooping, many species remain upright and attractive through cold weather and even in snow.
Many ornamentals are bunch or clump grasses, meaning that their roots don’t put out rhizomes or stolons (horizontal shoots which can start new plants) but depend on seeds to reproduce. Clump grasses used in turf, such as fescues, can blend together into a smooth, continuous surface, but ornamentals tend to produce tight, well-defined bunches. An individual grass plant does get gradually larger as new shoots grow around the base of the parent plant, but each individual, one might say, retains its integrity.
Turf grasses generally get mown before going to seed, but ornamentals are another story and have gained a reputation for being invasive. However, many believe that most problems with invasiveness occur when the problem plants are not suited to their new environment. Take a non-invasive prairie grass, give it a longer season or too much water, and suddenly you’ll have something quite different, and probably unwelcome, all over your yard.
Some of the worst offenders, like ribbon grass are in fact running grasses, not bunch grasses. (Running grasses are also known as “creeping” grasses, but in the case of ribbon grass, “running” appears to be more accurate.) Frequently chosen because it spreads so rapidly, ribbon grass is extremely difficult to control and often turns out to be more trouble than it’s worth.
The clumping grasses can be kept in check in several ways. The first and most important is to buy a grass appropriate for your region, and to tend it so that it will not produce excessive seeds. Don’t over-water, and don’t grow a cool-season grass in a warm season district, or you may have problems. Native Plants almost always work well.
When you set the grasses, plant them through weed-cloth, which you can then cover with mulch. The fabric will keep out weeds and seeds — including those of the grasses. This won’t keep seeds from drifting on the wind, but it makes all the difference in the immediate vicinity.
Finally, the coup de grass for seedy invaders: buy a sterile strain that can’t spread — no matter how many seeds it produces.