Feature Story
AN OVERVIEW OF POSTEMERGENCE GROWTH REGULATOR HERBICIDE USE IN TURFGRASS By Jeffrey Derr, Ph.D., Professor of Weed Science, Virginia Tech
A
nnual and perennial broadleaf weeds are management issues for those maintaining turfgrass, be it golf courses, sports turf, residential or commercial lawns, and roadside turf, among other areas, as well as in sod production. A common way that broadleaf weeds are controlled in turfgrass situations is through the use of postemergence herbicides, generally through the use of growth regulator herbicides. Although most of the available chemicals in this group were commercialized years ago, there is one new herbicide to discuss in the growth regulator group. With the availability of a number of active ingredients, most broadleaf weeds can be selectively controlled in warm and cool-season turf. Use of postemergence herbicides allows for spot treatment instead of broadcast applications, especially for perennial weeds that often occur in clumps. A downside to postemergence herbicide use is that it may take a while after application for the weeds to die off. Henbit, for example, is one species that goes out slowly. Also, application of a postemergence herbicide results in dead weed foliage, which can be unsightly. Preemergence herbicides control weeds before they are established, avoiding this issue. Most preemergence herbicides, however, are ineffective against perennial weeds, while there are effective postemergence herbicides for perennial broadleaf weeds.
A. Available active ingredients
There are multiple herbicides in the growth regulator class. They are sold individually or more commonly in combinations with other growth regulator herbicides. The list includes subgroups: phenoxies [2,4-D, 2,4-DP (also referred to as dichlorprop), MCPP (also referred to as mecoprop), and MCPA], benzoic acids (dicamba), pyridines (triclopyr, fluroxypyr), and the newest addition halauxifen in the picolinic acid group. These herbicides all have the same mode of action and act as synthetic auxins. Natural auxins in plants, such as IAA, are involved in root and shoot development. The synthetic auxins promote uncontrolled cell growth,
1 Henbit, a common winter annual weed.
plugging vascular tissue. This explains the wilted appearance of injured broadleaf plants from this class of herbicides. The herbicide 2,4-D was discovered in the 1940s and found to selectively control broadleaf weeds in grass crops. The term 2,4-D refers to the chemical structure, which is 2,4-dichlorophenoxy acetic acid. A closely related structure is 2,4-DP, which is 2,4-dichlorophenoxy propionic acid. Companies, once they identify an active molecule, will test variations to that structure, either to improve control, broaden the spectrum of control, or to improve crop safety. The herbicide 2,4-D effectively controls dandelion, plantains, bittercress, and wild carrot, while relatively ineffective for controlling common chickweed, India mock strawberry, prostrate knotweed, white clover, Virginia buttonweed, yellow woodsorrel, and wild violet. Other herbicides are generally added to 2,4-D to improve control of these species. Adding MCPP to 2,4-D, for example, improves control of common chickweed and white clover compared to 2,4-D alone. These herbicides are systemic, meaning they move throughout the plant after being absorbed. These compounds are both leaf and root absorbed, although leaf uptake is the best target for applicators. Granular formulations of these herbicides can control weeds. Ideally these granular
24 | VIRGINIA TURFGRASS JOURNAL November/December 2019 www.vaturf.org
