The vegetation planted and managed along highways, often referred to as “highway grass,” is a carefully engineered feature of the transportation system, not merely a decorative element. This vegetation is located within the highway right-of-way, the strip of land acquired for construction and maintenance. The grasses, shrubs, and occasional trees are selected and maintained for practical reasons that ensure the long-term integrity and safety of the roadway.
The Engineering Functions of Roadside Vegetation
Roadside vegetation plays a fundamental role in protecting the physical structure of the highway from environmental degradation. The root systems of grasses and other ground cover are highly effective at controlling soil erosion, especially on cut and fill slopes and embankments prone to washing away. By stabilizing these surfaces, the vegetation maintains the designed slope geometry, which is necessary for the road’s structural stability.
The cover also manages water flow and drainage, a major engineering concern. Healthy grass improves the soil’s infiltration capacity, slowing and absorbing rainwater runoff. This reduces the amount of water reaching the pavement and sub-base. Managing moisture prevents soil saturation, which increases the shear strength of embankments and reduces the risk of landslides or slope failures.
From a safety perspective, the vegetation acts as a clear zone or safety buffer for vehicles that leave the main roadway. In median strips, strategically planted shrubs or dense grasses can mitigate the glare caused by the headlights of oncoming traffic, improving driver visibility at night. Maintaining low-growing, well-defined turf ensures that road signs, guardrails, and the edge of the pavement remain clearly visible to drivers.
Types of Plants Used in Right-of-Way Management
The selection of plant species for highway use is based on strict criteria related to survival and function in a harsh environment. Plants must be highly tolerant of poor, compacted soils, extreme temperature fluctuations, and periodic drought conditions. A robust and deep root system provides the necessary soil stabilization and resistance to erosion on slopes.
Highway managers utilize a mix of regionally appropriate native species and highly adapted non-native plants that have proven their hardiness. Common choices include various fescues, perennial ryegrass, and Bermuda grass, which are selected for their fast establishment and ability to form a dense turf. Certain plants, like Crownvetch, are occasionally planted for slope protection due to their aggressive, mat-forming growth habit.
The preference is given to species with rhizomatous root structures, which better withstand the physical stress of repeated mowing and maintenance activities. In some regions, state mandates encourage the planting of native wildflowers and grasses to support local ecosystems and pollinators, such as Monarch butterfly habitat programs. These plantings are integrated with traditional grasses to fulfill functional requirements while increasing ecological value.
Maintenance Strategies for Highway Grass
The appearance of unevenly cut or unkempt highway grass is often a result of a highly differentiated maintenance strategy designed for efficiency and cost savings. Transportation departments employ varied mowing schedules based on the location’s proximity to the road and its functional purpose. Areas immediately adjacent to the road shoulder are mowed more frequently to ensure clear visibility and to control the invasion of undesirable, tall-growing species.
In contrast, the broader, outer portions of the right-of-way are often mowed only once or twice a year, or sometimes not at all, to reduce costs and foster a natural, low-maintenance environment. Rural mowing standards may dictate a minimum cutting height of six inches to protect the grass crown and help the plants tolerate drought conditions. This less-intensive approach allows desirable native species to thrive without constant intervention.
Beyond mechanical mowing, chemical control measures are routinely employed to manage weeds and invasive species that could compromise the integrity of the road structure or spread to adjacent private lands. Herbicides are used strategically to control aggressive or difficult-to-mow plants, such as Johnson grass, which mowing alone cannot effectively eliminate. These integrated vegetation management programs combine specialized equipment and precise timing to preserve the functional benefits of the roadside turf.