Driving Out Roadside Weeds
Weed control along roadsides can be more complicated than it needs to be because roadside groundcover is often uncomfortably positioned between functional and aesthetic concerns. Roadside vegetation needs to stabilize soil and prevent erosion under the gaze of potentially thousands of motorists each day. This large audience often puts highway managers in the mindset that roadside vegetation should be treated as an ornamental planting. However, efforts to make the roadside look like a front lawn are often detrimental and complicate the objective of managing weeds.
FUNCTION OVER FORM
The primary function of roadside plantings is to stabilize the highly disturbed soils that are left after a road is built. These soils are typically compact, infertile and often have only a thin layer of topsoil placed at the completion of grading. Roadside plant material must therefore be adapted to poor growing conditions. Another function of roadside vegetation is to provide a groundcover that is easy to maintain. A roadside that can be maintained as much as possible by a one-size-fits-all program is the goal. Lastly, the roadside vegetation should be aesthetically acceptable. Arguably, it's a matter of saying that the roadside shouldn't look bad, rather than saying it should look good. You shouldn't be worried about mow-striping your interchanges or planting wildflowers if motorists can't see road signs or oncoming traffic at intersections.
Your weed management program should be based on the principles of Integrated Pest Management (IPM). In a nutshell, IPM is a structured approach to common sense pest management. In an IPM-based program, you must establish goals: 1) develop a list of pests; 2) establish pest thresholds to initiate treatments. You then use all the tools at your disposal in a coordinated effort, keep accurate records of what you did and check to see if it worked. If it worked, you try to make it work better. If your approach did not work, you fix it.
The most important functional reasons for controlling roadside weeds will vary depending on whether you are dealing with primary or secondary roadways, as well as the environment you are working in. Secondary roadways have narrower rights-of-way, so weed encroachment into the motorist's line of sight is more of a concern. Additionally, the narrower right-of-way makes it more important to prevent brush establishment by controlling it as it initially encroaches, rather than trying to eliminate it after it is established. An important aspect of a roadside IPM program is being a good neighbor. Therefore, controlling noxious and invasive weeds to prevent their spread to adjacent private property is a key responsibility.
Primary roadways have higher speed limits, more traffic and wider rights-of-way. Mowing frequency is higher and covers a wider area than on secondary roads. Therefore, the top priorities of a weed control program on primary roadways is preventing brush encroachment into the vehicle recovery area adjacent to the roadway and preventing the spread of noxious and invasive weeds. An additional concern is maintaining the integrity of the roadside groundcover. For example, in the Northeast, it is not uncommon for crownvetch to grow in roadside turf areas. If left unchecked, particularly in areas with a low mowing frequency, the crownvetch can displace the turf. This may seem like an acceptable scenario in a low maintenance setting, until some truly undesirable weed infests the crownvetch patch. If it is a broadleaf weed, you now have a broadleaf weed in a broadleaf groundcover. This is a much more challenging weed control scenario than trying to remove broadleaf weeds from a grass groundcover.
Aesthetic concerns will also play a role in roadside weed control. The appearance of the roadside needs to have a lower priority than safety issues and keeping noxious and invasive weeds from spreading to adjacent private property, but it is part of being a good neighbor, and will foster more public good will towards your agency.
WEED CONTROL METHODS
Weed control methods can be broadly grouped into four categories: biological; mechanical; cultural; and chemical. Classical biological control is the use of one organism to control another. Because biological control is most specialized, typically focusing on managing a single species, it is the least-used method in a roadside setting. Examples of biological control in the roadside setting include past releases of a weevil (Rhinocyllus conicus) to feed on musk thistle (Carduus nutans), and the release of leaf-feeding Gallerucella beetles to feed on purple loosestrife (Lythrum salicaria).
Mowing can be regarded as both a mechanical and cultural approach — it can be used to defoliate weeds and to maintain the integrity of roadside turf. Mowing reduces the vigor of all plants by removing leaf tissue. A properly implemented mowing program will cause less injury to turf compared to many roadside weeds and provide a net reduction in weeds. When a roadside turf is cut fairly high, a uniform-appearing cover can be achieved while removing minimal leaf tissue. This maintains turf vigor and competitiveness, which reduces the ability of mowed-off weeds to regrow. Mowing the turf too short, too often reduces the integrity of the turf and creates opportunities for continued weed infestation.
Chemical management of roadside weeds involves both non-selective and selective herbicides. Non-selective herbicides are an integral part of maintaining the narrow margin of bare ground adjacent the roadway to provide water movement off the pavement to the ditch and preserve line of sight. Non-selective applications to maintain bare ground are made to guide rails, barriers, shoulders and around sign posts.
On an acreage basis, broadleaf herbicides play the bigger role in selective control. Such applications must address everything from annual seedlings popping up in the turf to encroaching brush. Because of the vast acreages on roadsides, it is especially important to maximize application efficiency. A tank mix of herbicides that control a broad range of weed species is the best approach. The composition of the mixture will depend on the weed species you expect to encounter and the amount of inhibition of grass growth you can tolerate. If you live in a part of the country where brush encroachment is an issue, you should also have a brush-specific program in place. Therefore, your roadside weed control program should serve to maintain areas that are largely clear of brush — rather than to serve as the primary brush control program. Ideally, the rates you select should be appropriate for herbaceous weeds and small encroaching brush that is beginning to encroach. Check the label for general-use considerations, restrictions and limitations and useful tank-mix partners.
Common herbicide ingredients used for broadleaf weed control in residential turf include 2,4-D, 2, 4-DP, clopyralid, dicamba, MCPP and triclopyr. All of these herbicides are classified as auxin-type herbicides. They injure plants by interfering with the plant's ability to control its growth, resulting in twisting of stems and curling of leaves. They are fast-acting, and do not affect grasses when used at label rates. Carfentrazone is a new selective, broadleaf herbicide for turf use. Unlike auxin types, it inhibits a key enzyme in chlorophyll production causing cell membrane disruption. Carfentrazone is touted as fast-acting, hence the brand names QuickSilver and SpeedZone. Herbicides for selective control of broadleaf weeds that would be limited to roadside or other non-crop settings include picloram, chlorsulfuron and metsulfuron. Picloram is also an auxin-type herbicide, but is a restricted-use material and has significant soil activity. The primary uses for picloram are noxious weed control in rangeland areas and brush control on electric rights-of-way. Chlorsulfuron and metsulfuron prevent susceptible plants from synthesizing certain essential amino acids. They are slow acting, and have considerable preemergence activity. Chlorsulfuron is useful where Canada thistle (Cirsium arvense) is a problem, while metsulfuron is useful for its activity on a wide range of woody species, particularly multiflora rose (Rosa multiflora) and tree-of-heaven (Ailanthus altissima). Chlorsulfuron and metsulfuron are not as selective as the auxin-type herbicides, will inhibit grass growth at labeled rates, and at low rates can actually be used like a plant growth regulator (PGR) to suppress seedheads of many cool-season grasses.
Examples of general purpose mixtures that will provide weed and brush control, and some PGR activity on roadside turf include metsulfuron plus triclopyr or metsulfuron plus dicamba. Where PGR activity is not desired, the combination of dicamba plus triclopyr could be used. Where agencies are relying on spray vehicles with injection capabilities, it is possible to alter the herbicide mixture on-the-fly. An example would be having an injector dedicated to a brush specific ingredient such as metsulfuron, and only activating that injector when brush is in the spray pattern.
Vegetation never stops growing, and keeping it in check with the finite resources available to most public agencies is a daunting task. The key to a successful roadside weed control program is properly identifying your goals — or what you are trying to achieve with the vegetation along your roadways. With your goals in place, you can implement your IPM plan and most effectively use your limited dollars.
Art Gover is supervisor at Pennsylvania State University's Landscape Management Research Center (University Park, Pa.). You can visit the Penn State Roadside Vegetation Management Web site at http://rvm.cas.psu.edu.
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