Shenandoah switchgrass (Panicum virgatum ‘Shenandoah’) is a highly popular ornamental grass, prized by gardeners for its upright, vase-like form and its striking burgundy-red foliage that intensifies in the fall. The parent species, switchgrass, is a native component of the North American tallgrass prairie, yet this cultivar often sparks confusion among homeowners. Many gardeners describe its tendency to spread as problematic, leading to the question of whether it is an ecologically harmful invader or simply a vigorous grower. Understanding the precise difference between an aggressive plant and an invasive one is necessary to correctly classify the behavior of ‘Shenandoah’ in a garden setting.
Defining Invasive Versus Aggressive
The distinction between an aggressive and an invasive plant is based on its origin and its impact on the ecosystem. According to the United States Department of Agriculture (USDA), an invasive species must meet two criteria: it must be non-native to the ecosystem, and its introduction must cause or be likely to cause economic or environmental harm. This harm typically involves outcompeting native flora, reducing biodiversity, or altering natural habitats.
Conversely, an aggressive or vigorous plant is defined by its growth habit, not its origin or ecological impact. This type of plant spreads rapidly or self-seeds readily within a cultivated space, often crowding out neighboring plants. Aggressive plants can be either native or non-native, but they do not cause widespread ecological damage outside of a localized area. The term “aggressive” describes a gardener’s management challenge, while “invasive” refers to a formal ecological threat.
The Specific Behavior of Shenandoah Switchgrass
The species Panicum virgatum is native across much of North America, from the Atlantic coast to the Rocky Mountains. Because ‘Shenandoah’ is a selected cultivar of a native species, it cannot be classified as “invasive” within its native range under the standard ecological definition. Its spread is driven by its method of reproduction, not by an ability to cause ecological harm.
‘Shenandoah’ is primarily a clumping grass, also known as a bunchgrass, which maintains a tight, upright form. However, like the straight species, it does spread slowly via short, underground stems called rhizomes. This rhizomatous growth causes the clump to expand laterally over time, leading to its reputation for being aggressive within a border. The new shoots that emerge from these short rhizomes are the reason the plant seems to “creep” into nearby garden space.
The cultivar ‘Shenandoah’ was selected for its compact habit, making it less rambunctious than the wild form of switchgrass. While it produces airy, reddish-pink flower panicles that hold seed heads, its ability to successfully self-seed is generally limited. Cultivars often do not come true from seed, meaning any seedlings that sprout may revert to the appearance of the wild species. Therefore, the plant is correctly characterized as vigorous or aggressive, but not invasive.
Practical Management and Control
Managing the vigorous nature of ‘Shenandoah’ switchgrass is a routine gardening task that focuses on containment and rejuvenation. The plant’s deep, fibrous roots are beneficial for erosion control, but they also anchor the plant firmly, requiring effort for removal or division. The most effective method for controlling its size and maintaining its vigor is periodic division.
Gardeners should plan to divide the established clumps every three to five years to prevent the center from dying out and to control its lateral spread. Division is best performed in the early spring just as new growth begins to emerge, using a sharp spade to cut manageable sections from the outer edges of the clump.
The entire clump should be cut back severely, to just a few inches above the ground, in late winter or very early spring before new shoots appear. This annual pruning encourages the fresh, vibrant foliage that gives ‘Shenandoah’ its celebrated color.
For long-term containment in a mixed bed, leaving adequate space around the clump is the simplest solution. Since the rhizomes are short, a physical barrier is usually not necessary unless the grass is planted in a very small, confined area. Recognizing the plant’s aggressive growth habit allows gardeners to place it strategically where its size and spreading nature can be used as a strong, architectural focal point in the landscape.