Herbicides are pesticides used to control unwanted vegetation (weeds). Weeds along rights-of-way include those that are a safety hazard, a nuisance, or are unsightly to the traveling public. Right-of-way weeds also include those plants that impede the use and maintenance of rights-of-way, cause injury to workers, interrupt flow of electricity or communications, are declared a noxious weed under state laws, crowd out desired native plants, damage structures and ballast, or reduce crop yield or injure livestock.

Control methods applied to rights-of-way must be part of a sound weed management program that is sensitive to the environment. Herbicide application is one of the methods available to rights-of-way managers. This chapter discusses herbicides in general use for rights-of-way vegetation control. It is not expected to be all-inclusive.

The effective use of a herbicide in weed management depends on knowledge of the characteristics of its active ingredient. We can classify herbicides by: 1) preemergent (root-absorbed) vs. postemergent (foliage-absorbed), 2) contact vs. translocated (systemic), 3) selective vs. nonselective, 4) persistent vs. nonpersistent, and 5) by their modes of action. Effective weed control can be accomplished by combining the characteristics of individual herbicides (assuming no incompatibility or label restrictions exist) when integrated with weed biology information.

Root-absorbed herbicides enter the plant through the roots. They are generally most effective when applied before the weeds emerge from the soil; thus the term preemergent. Foliage-absorbed herbicides primarily enter the plant through the leaves which means the weeds have already emerged above the soil when spraying occurs; thus the term postemergent. Some herbicides are absorbed by both foliage and roots (Table 1). Herbicide formulation, method of application, and adjuvants can influence which part of the plant absorbs the herbicide. Examples of foliage-absorbed herbicides include 2,4-D, diquat, fosamine (Krenite), glyphosate, and triclopyr (Garlon). Root-absorbed herbicides include bromacil (Hyvar), diuron, oryzalin (Surflan), and tebuthiuron (Spike). Herbicides that can be absorbed by either the leaves or roots include clopyralid (Transline), hexazinone (Velpar), imazapyr (Arsenal), picloram (Tordon), and sulfometuron (Oust).

Selective herbicides control only certain types of plants. When applied to mixed vegetation some plant types or species will not be affected or will show minimal signs of injury. Nonselective herbicides generally control most plants and are used where complete control is desired. Selectivity may occur because of true physiological selectivity. For example, grasses are naturally tolerant to the herbicide 2,4-D whereas dandelions and ragweeds are susceptible. However, rate, timing, method of application, and plant characteristics also can determine selectivity (Table 2). Nonselective herbicides include bromacil (Hyvar), glyphosate, sulfosate (Touchdown) and tebuthiuron (Spike). Some herbicides such as diuron, hexazinone (Velpar), imazapyr (Arsenal), and sulfometuron (Oust) have selective uses in other situations such as crop production and forestry, but are considered to be nonselective herbicides as they are used for rights-of-way vegetation management (Table 1).

Table 1

Active Ingredient	Select an Active Ingredient 2,4-D bromacil chlorsulfuron clopyralid dicamba diquat diuron fenoxaprop fluazifop fluroxypyr fosamine glufosinate glyphosate hexazinone imazapyr metsulfuron MSMA norflurazon oryzalin pendimethalin picloram prodiamine prometon sethoxydim sulfometuron sulfosate sulfosulfuron tebuthiuron triclopyr
Trade Name

Pre-emergent Post-emergent Selective Non-selective Xylem translocated Phloem translocated Persistent Non-persistent Limited mobility

Table 2: Selectivity of herbicides commonly used on rights-of-way.

Herbicide	Plants Not Controlled
2,4-D	grasses
chlorsulfuron (Telar)	perennial and many annual grasses
clopyralid (Transline)	grasses, many broadleaves
dicamba	grasses
fosamine (Krenite)	non-woody plants
metsulfuron (Escort)	grasses
oryzalin (Surflan)	most broadleaves
pendimethalin (Pendulum)	many broadleaves
picloram (Tordon)	grasses
prodiamine (Endurance)	most broadleaves
sulfosulfuron (Outrider)	bermudagrass and bahiagrass
triclopyr (Garlon)	grasses

We generally think of a contact herbicide as one that kills only the green portion of the plant that it touches. The activity of these compounds is so rapid that the herbicide moves (translocates) very little in the plant. Consequently, good coverage is necessary. A contact herbicide is usually nonselective on annual species since they do not resprout. However, contact herbicides cause only temporary suppression of perennial weeds because they will resprout from the underground root system. Translocated (systemic) herbicides move throughout the plant, whether they are taken in by the foliage and translocate downward to the roots or enter the roots and translocate upward to the leaves (Table 1). Foliage applied herbicides that translocate are useful for controlling perennial weeds because the herbicide will move to and destroy the growing points in the roots, leaves, and shoots. Nearly all herbicides used for rights-of-way vegetation control translocate within the plants, including 2,4-D, bromacil (Hyvar), dicamba, diuron, glyphosate, imazapyr (Arsenal), picloram (Tordon), sulfometuron (Oust), and triclopyr (Garlon). The few contact herbicides used on rights-of-way include diquat (Reward), glufosinate (Finale), and MSMA.

Persistent herbicides remain active in the environment for an extended period of time (Table 1). The ability of the herbicide molecule to persist depends on its ability to resist microbial and chemical degradation in the soil. Herbicides vary greatly in their resistance to breakdown. Nonpersistent herbicides are short-lived in the environment. Soil microorganisms easily break them down, or they become so tightly bound to soil particles that they are not available for plant uptake.

Persistence is an important characteristic when long-term (residual) weed control is desired for total vegetation control around a guide rail, median barrier, substation, or rail yard. Persistence allows the control of shallow-rooted annual weeds among deep-rooted established perennial plantings. Persistence is undesirable when it hinders or delays desirable plant growth as in re-seeding or plant release programs. Usually persistence increases with cold temperatures, dry, compacted or clay-textured soils, and high use rates. Conversely, warm temperatures, frequent rainfall, low use rates, sandy soils, and high organic soils reduce persistence. Herbicides generally considered to be persistent include bromacil (Hyvar), diuron, imazapyr (Arsenal), picloram (Tordon), sulfometuron (Oust), and tebuthiuron (Spike). Some nonpersistent herbicides include 2,4-D, fosamine (Krenite), glufosinate (Finale), glyphosate, and triclopyr (Garlon).