Ornamental grasses are popular additions to landscapes, valued for their texture, movement, and winter interest. The truth is that all ornamental grasses expand, but the degree and method of that expansion vary dramatically based on the species’ fundamental growth habit. Understanding this biological difference is the primary factor for successful long-term gardening with these plants.
Running Versus Clumping Growth
The two primary growth habits dictate how a grass will spread, classifying them as either clumping or running. Clumping grasses, also called bunch grasses, expand slowly outward from a central crown. This expansion is predictable and manageable because the plant’s root system remains dense and compact. Examples include Feather Reed Grass (Calamagrostis) and many varieties of Miscanthus.
The rhizomes, or underground stems, of clumping grasses are short and tightly packed, causing the plant to increase its diameter by only a few inches each season. Over time, the center of the clump may become woody and less vigorous, signaling the need for division. This localized growth pattern ensures the grass remains confined to its planting area.
Running grasses are known for their aggressive spread. These species utilize elongated, horizontally growing underground stems called rhizomes. These rhizomes can travel significant distances beneath the soil surface, sending up new shoots far from the parent plant.
Certain types of bamboo and some species of ribbon grass (Phalaris) are classic examples of this running habit. The speed of spread can be considerable, making these grasses excellent for erosion control or groundcover where containment is not a concern. However, when planted near garden beds or lawns, their rapid expansion can quickly become a maintenance issue.
Identifying Grass Type Before Planting
Determining the growth habit before planting is the best strategy for avoiding future containment problems. The most immediate source of information is the plant label, where you should look for specific terminology. Terms like “clump-forming,” “tussock-forming,” or “non-spreading” indicate a manageable habit.
If the label uses words such as “running,” “spreading,” “creeping,” or “rhizomatous,” the plant will require proactive containment measures. When purchasing grasses without detailed labels, the plant’s genus and species name can often provide a clue. For instance, most species in the Phyllostachys genus are runners, while Festuca species are reliably clumping.
A visual inspection of the plant in its pot can also reveal its tendency. Clumping grasses exhibit a dense, tightly packed crown where all the shoots emerge closely together from the center. Running grasses, particularly those established for a while, may show evidence of new shoots emerging near the edge of the pot or outside the main root ball. Examining the soil surface for thin, horizontal underground stems breaking the surface is a definitive sign of a running habit.
Strategies for Controlling Spread
Managing the expansion of an ornamental grass depends entirely on its growth type. For aggressive running grasses that spread by rhizomes, a physical barrier is the most effective containment strategy. This barrier, often made of heavy-gauge plastic or metal edging, must be buried deep enough to block the subterranean stems.
For highly aggressive types, like running bamboo, the barrier should extend 18 to 30 inches deep. It must also protrude an inch or two above the soil line to prevent rhizomes from growing up and over the barrier. Routine inspection and digging of any runners that escape the perimeter are necessary to maintain control.
Clumping grasses do not require a subsurface barrier but need periodic division to manage their size and health. When the center of the clump begins to die out, typically every three to five years, the entire plant should be dug up. The woody, unproductive center is discarded, and the vigorous outer sections of the crown are separated and replanted.
Beyond vegetative expansion, both running and clumping grasses can spread through self-seeding when mature seeds drop and germinate nearby. To prevent this secondary spread, seed heads should be removed, or deadheaded, before they fully mature. This action must be performed before the seeds are viable, usually when the seed heads have lost their color but are not yet dry enough to shatter.