The grass family (Poaceae) is one of the largest and most widespread plant groups on Earth. Asking whether “grass” is an invasive species does not have a simple answer. While thousands of grass species are native and beneficial, a small subset of non-native grasses has become highly problematic invaders globally. These specific species disrupt natural processes, fundamentally altering landscapes in ways that affect both ecology and economics. The distinction lies in understanding the difference between a plant simply growing in a new place and one that actively causes harm.
What Makes a Plant an Invasive Species
The term “invasive species” is a specific ecological designation that goes beyond merely being non-native. A plant is classified as invasive only if it meets two strict criteria: it must be alien to the ecosystem, and its introduction must cause environmental or economic harm. Many non-native grasses were introduced intentionally for agriculture or forage, but only a fraction become truly invasive.
A species that is non-native but causes no discernible harm is considered “naturalized.” Naturalized species can persist in a new environment without displacing native flora or fauna. The harmful impact of an invasive grass is measured by its ability to outcompete native species for resources, leading to a reduction in biodiversity. This ecological disruption often manifests as vast, single-species stands, known as monocultures, which limit habitat for native wildlife and pollinators.
Examples of Highly Invasive Grasses
Several non-native grasses have become notorious for their invasive behavior across North America. Cheatgrass (Bromus tectorum), originally from Europe and Asia, is one of the most widespread and damaging annual grasses in the western United States. It displaces native perennial grasses and is known primarily for its ability to dramatically alter fire regimes.
Johnson Grass (Sorghum halepense), a perennial species introduced from the Mediterranean, was often used for livestock forage. This grass is difficult to control due to its robust root system and resistance to certain herbicides, allowing it to choke out crops and native plants. In the American Southwest, Buffelgrass (Cenchrus ciliaris), an African grass, was introduced for cattle ranching. It now grows in dense stands that shade out other species and consume large amounts of water, causing substantial ecological damage.
Biological Traits Contributing to Grass Dominance
The success of invasive grasses stems from traits that give them a competitive advantage over native species. Many invasive grasses exhibit faster growth rates and high photosynthetic efficiency, allowing them to capture resources quickly. They often have a different phenology, germinating earlier and staying green later, which extends their growing season beyond that of native plants.
Invasive grasses spread effectively both by seed and vegetatively through extensive root structures. Perennial species like Johnson Grass and Smooth Brome develop aggressive underground rhizomes and stolons, allowing them to rapidly colonize large areas. Annual grasses, such as Cheatgrass, produce vast quantities of viable seeds that build up a large seed bank, ensuring continuous re-establishment after disturbance.
A key destructive trait is altering the natural fire cycle, known as the grass/fire cycle. Cheatgrass is a fine-fuel annual that dries out early, creating a continuous layer of highly flammable material. This increases the frequency and intensity of wildfires, which kill native perennial plants and shrubs. The invasive grass quickly recovers from its persistent seed bank, dominating the post-fire landscape. Some invasive grasses also employ allelopathy, releasing chemical compounds that inhibit the growth of surrounding native plants.
Strategies for Control and Ecosystem Restoration
Controlling invasive grasses requires an integrated approach, often referred to as Integrated Pest Management (IPM), which combines multiple strategies over a sustained period. The ultimate goal is not just to eliminate the invasive grass, but to restore a healthy, functioning ecosystem.
Manual and Mechanical Control
These methods are best suited for smaller infestations and involve physical removal, such as hand-pulling or repeated mowing. The goal is to deplete the plant’s energy reserves and prevent seed production. For annual grasses, timing is crucial, as removal must occur before the seeds mature and drop.
Chemical Control
Chemical control, using herbicides, is often necessary for large-scale infestations. Non-selective herbicides like glyphosate are effective but can harm native species. Application must be careful and targeted, often timed for when the invasive grass is actively growing but native plants are dormant.
Biological Control
Researchers are exploring biological control, which involves introducing a natural enemy from the grass’s native range, such as a specific insect or fungus. This method aims to reduce the invasive population naturally.
Ecosystem Restoration
After control measures reduce the invasive population, the site must be revegetated with native species to prevent reinvasion. Restoration success hinges on selecting native plants that are competitive and well-suited to the site’s conditions. This effectively replaces the invasive monoculture with a diverse and resilient native plant community, requiring years of aftercare and monitoring.