Allowing desirable grasses to flourish along roadsides provides a number of benefits to vegetation managers. Photos: Dow AgroSciences
Allowing desirable grasses to flourish along roadsides provides a number of benefits to vegetation managers. Photos: Dow AgroSciences

Weeds and other undesirable vegetation like to make their home along roadsides. But their presence can block motorists' line of sight and promote the spread of invasive species onto adjacent properties. Excessive vegetation also can damage roadbeds by creating erosion, potholes, and flooding due to improper road drainage off the surface.

In most cases, roadside vegetation managers avoid these issues through a combination of mowing and herbicide applications. Many also cultivate desirable vegetation; a healthy grass stand provides benefits beyond erosion control.

Control the right plants

It's crucial to understand the effects of herbicides on grasses. For instance, you can apply only a limited number of herbicides to Bahia grass without causing injury.

A nonselective herbicide may be counter-productive, because it controls all present plants. Thus, the quantity and variety of weeds that invade the area in subsequent seasons may actually increase, since there's no desirable plant stand to resist invasion. If grass density is reduced, weeds begin to germinate and crowd out what grass remains. Aesthetics, motorist security, and the economics of maintaining the roadside begin to take a downturn. This creates an unstable, unpredictable environment that may require more labor to maintain.

A selective herbicide, on the other hand, controls specific targets while leaving the desired vegetation relatively unharmed.

A successful program may consist of a two-phase approach. The first is the “conversion phase” in which herbicides are used to target a mix of broadleaf weeds and woody plants. Applying a proven selective herbicide promotes a low-growing plant cover (grasses) that is resistant to more tall-growing undesirable weed invasion. Once a grass community is established, the “maintenance phase” involves subsequent herbicide applications that are specifically targeted to undesirable species by directed application. (See sidebar on this page.) This phase typically requires less product, labor, and fuel.

Seedhead suppression

Managing grass height is also necessary because overgrown vegetation can limit motorist visibility of roadside hazards. If your vegetation blocks signage or guard-rails, then it's too tall.

In the Southeast United States, roadsides are dominated by warm-season grasses including Bahia and Bermuda. Bahia grass poses the greatest challenge because the fast-growing turf requires frequent mowing. In the Northeast and North Central, the cool-season grass tall fescue also requires frequent mowing, especially in the early growing season.

Selective herbicides can be used in lower doses as plant growth regulators (PGRs) to suppress growth of these grasses. In many cases, this may delay early-season mowing by as much as six to 10 weeks, and reduce the frequency of mowing needed during the season.

Application rates and calibration

When treating roadsides with an herbicide application, consult the product label for proper application rates. Improper application can cause grass injury — whether under- or over-applying. Both can be caused by several factors including improper mixing, lack of calibration, vehicle speed changes, or wind.

Herbicide drift is a concern, as wind can cause a spray swath of 30 feet to be condensed to five feet or less. This usually happens when boom-less nozzles are used on spray trucks. Do not spray if wind speeds exceed 10 mph.

You should also ensure proper spray equipment calibration —which is simply making sure that the correct spray volume is dispersed over a given area. Check your herbicide label for directions.

Applicator training is one way to avoid mistakes associated with calibration. For example, the driver of the spray truck must know the correct speed to travel during application, and the right amount of product must be added to the tank. Dow AgroSciences offers a free online training course at

— Pat Burch ([email protected]) is senior scientist with Dow AgroSciences.


Individual foliar treatments — spot spraying targeted plants with herbicide — are ideal for maintaining a site after an initial reclamation (the conversion phase).

Low-volume foliar treatments are appropriate for stem densities of 1,500 stems or less, and stem heights shorter than 7 feet. High-volume broadcast treatments, usually done with powered sprayers, hoses, and handguns, can treat stem densities of more than 1,500 and heights higher than 7 feet.

During initial reclamation, use a high-volume handgun treatment to knock back high-density areas of brush. During the maintenance phase, follow up with foliar spraying to eliminate undesirables:

  • Spray herbicide directly onto foliage of individual plants.
  • Use spray pressures and techniques that minimize spray drift.
  • Get good coverage on the growing tips and terminal bud.
  • Spray all sides of the target plant to ensure adequate coverage.
  • Apply the herbicide solution at a volume that wets the foliage, but not to the point of runoff.
  • Make treatments during periods of active growth. Early-season applications work best after full leaf-out or target brush. Late-season applications work best before leaves turn and fall colors appear on target brush.