Roadside Use of Native Plants
Incorporating Grasses into Clear Zones — Clear Zones As Grasslands
Bonnie L. Harper-Lore, Federal Highway Administration
The clear zone is "the roadside border area, starting at the edge of the traveled way, that is available for corrective action by errant vehicles" (1991 AASHTO Guide for Transportation Landscape and Environmental Design). The designed width of the accident recovery area is determined by the speed limit. No objects, including trees of more than 4" diameter, are allowed within this zone.
As a logical result, the clear zone (not the entire right- of- way) of a roadside is generally maintained as grassland that can be mowed to prevent encroachment by potentially hazardous trees and shrubs. It has become common practice to keep these zones of roadsides mowed continually. The consequences include: high maintenance costs, mower/vehicle accidents, monocultures of grassy vegetation, and a "front lawn" expectation from the traveling public. In the urban environment, that expectation relates well to adjacent designed and manicured landscapes.
In the rural environment, beyond the city limits, that expectation does not relate to the adjacent landscape. I submit that, since clear zones are not scientifically proven, common sense would tell us that planting large trees and shrubs near the traveled way is asking for trouble. However, rural recovery areas can be safe as well as interesting, diverse and require less maintenance attention if they are planted in native grasses and forbs. Incidental consequences include: regionally recognizable vegetation, a seasonally dynamic landscape, habitat for small mammals, songbirds and insects, deep- rooted erosion control, water quality improvement, and preservation of our natural heritage. If highway users understand this unmowed grassland and its environmental ramifications, they are likely to be supportive. That has been the experience of States like Iowa, Wisconsin, Illinois, Minnesota, Utah, Oregon and Florida. Using native wildflowers and grasses in clear zones makes common sense!
All States have some native grassland community (some called prairie or meadow) within their State. Those grasslands include both native grasses and wildflowers (forbs). Using what is native to your State makes ecological sense. But natural regions change within a State due to climate and geology. The State of Texas has 26 natural regions. You understand why one grass seed mix might not be successful in all 26 regions. The likelihood of finding one grass seed mix for landscaping, erosion control, mitigation, or revegetation purposes is low. This native grassland can be achieved without planting in many regions. By allowing existing seed in the roadside seed bank and adjacent seed sources to grow, a native landscape can be encouraged. However, segments of roadside must be analyzed before using a hands-off approach to vegetation management. Many roadsides and adjacent lands have been highly disturbed and have no native seed in the soil. To allow these areas to grow up will only result in stands of nonnative invasive species, many of which are on noxious weed lists. Care must be taken in this approach. On the other hand, if native grass and forb seed does exist and would flourish without mowing, you have discovered the cheapest way to revegetate your roadside with native plants. The only caution is to watch for noxious weed invasion during the early stages, and to control those weeds quickly or they will persist and compromise the natural vegetation.
