Herbicides are chemicals that kill plants or inhibit their normal growth. Their means of doing this are varied and theoretically as numerous as the processes essential to plant life. Herbicides are most often and most effectively used together with good cultural practices in a turf weed management program. The choice of the best specific combination varies with agronomic, ecological and economic factors.

Types of Herbicides:

In terms of their effects on plants, herbicides may be described as selective or non-selective. Selective herbicides kill or stunt weeds in turfgrasses with little or no harm to the turfgrass. Non-selective herbicides kill or damage all plants when applied at adequate rates. No herbicides belong rigidly to either group. Non-selective herbicides may, under certain situations, act selectively; if the dosage is excessive, even a selective herbicide may become phytotoxic. Selectivity is, therefore, a property of the type of treatment as well as the chemical, regulated by such factors as time and method of application, chemical formulation, environmental conditions, the stage of turfgrass and weed growth, and particularly the dosage.

Herbicidal Action:

Herbicides kill plants through either contact or systemic action. Contact herbicides are most effective against annual weeds and kill only the plant parts on which the chemical is desposited. Systemic herbicides are absorbed either by roots or foliar parts of a plant and are then translocated within the plant system to tissues that may be remote from the point of application. Although systemic herbicides may be effective against both annual and perennial weeds, they are particularly advantageous against established perennial weeds.

All herbicides can be grouped into classes. Each class has a typical chemical structure and activity. The following classes include herbicides commonly used in lawns.

Phenoxy Acid Type: Examples of this class are: 2,4-D (WEEDONE), mecoprop (MCPP) and 2,4-D plus 2,4-DP (WEEDONE DPC). These herbicides are generally applied as foliar sprays. They are systemic and are selective in that they generally injure or control broadleaf plants without injuring grassy plants. They act as a synthetic growth hormone, mimicking the natural auxins that occur within plants. These herbicides are readily absorbed by the foliage and usually translocate relatively well and, in some cases, all the way down into the root system. Selectivity is usually physiological in nature. General symptoms include growth regulator effects such as twisting and downward curvature (epinasty) of the stems and leaves.

Benzoic Acid Type: Examples are: dicamba (BANVEL, TBA). This class of herbicides is very similar to phenoxy class herbicides. They are applied as foliar sprays. They are systemic, selective and primarily effective in controlling broadleaf weeds. Dicamba is quite mobile in soils and can be taken up by roots of sensitive trees and ornamentals. Selectivity is physiological in nature. Symptoms are very similar to phenoxys.

Dinitroaniline Type: Examples of this class are: pendimethalin (WEEDGRASS), trifluralin (TREFLAN), benefin (BALAN), a combination of trifluralin and benefin (TEAM) and prodiamine (BARRICADE). The herbicides in this class are used for preemergence control of annual grasses in established turf. They are mitotic (cell division) inhibitors and are primarily effective in inhibiting root growth of germinating seeds. Selectivity is physiological or chemical in nature. Some of these herbicides may be lost by volatilization. All of these herbicides need to be watered into the soil for proper activation. Some may persist for several months.

Bipyridylium Type: Diquat (REWARD) is strictly used as a foliar spray. It is a non-selective herbicide. It is a contact herbicide and requires the addition of surfactant for foliar activity. This type of herbicide is adsorbed quickly and tightly to soil colloids. Therefore, diquat has no biological activity in soil. These herbicides are membrane disrupters. They intercept electrons from photosynthesis and transfer the energy to various free radicals which damage cell membranes. Symptoms resulting from these herbicides are quick wilting and desiccation of plants. Activity of these herbicides depends on light.

Substituted Urea Type: Siduron (TUPERSAN), linuron and many others. These herbicides are selective. They are soil-applied and are absorbed through roots and translocate in the xylem. Unlike other substituted urea herbicides, siduron does not affect photosynthesis but is a root growth inhibitor. Symptoms include slow death of young seedling weeds. Siduron is the only herbicide that can be used for selective control of crabgrass and other annual grass weeds in newly seeded turf.

Arsenical Type: MSMA (WEED-E-RAD),WEED-HOE and DSMA are examples of this class. Arsenicals are primarily contact herbicides. They are strictly foliar-applied with a surfactant. Symptoms include yellowing of foliage within 7 days of treatment. They are tightly bound to soil. They are recommended for post-emergence crabgrass and nutsedge control in established turf.

Miscellaneous Types: Glyphosate (ROUNDUP) is a widely used, non-selective herbicide. It is strictly used as a foliar spray and is systemic in nature. It is very effective for perennial weeds because of its translocation characteristics. It inhibits new plant growth by interfering with aromatic amino acid synthesis. Visible symptoms generally occur on annual plants in 2 to 4 days and on perennial plants in 7 to 10 days. Symptoms on the shoots include yellowing and wilting, progressing from the new to the older tissues. Glyphosate rapidly reacts with and is inactivated by most soils. It is commonly used for turf renovation.

Dithiopyr (DIMENSION) belongs to a new class of herbicide known as pyridines. It is a selective herbicide primarily used for preemergence annual grass control in established turfgrass. It can, however, be used for post-emergence control of young (1-3 leaf) grass seedlings. It inhibits cell division and cell growth of meristematic regions (growing points of roots and shoots). Dithiopyr is lost from soil by chemical and microbial degradation.

Glufosinate (FINALE) is a non-selective herbicide which severely limits glutamine synthesis in treated plants. It is strictly a foliar-applied herbicide and can be used for turfgrass renovation. It is absorbed through leaves and stems and degrades into natural compounds such as carbon dioxide, nitrogen and water. Glufosinate has no soil residual activity and therefore sodding or seeding can take place soon after application.

Adapted from the University of Massachusetts Extension, 1999