05/15/2024
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Be Careful Using Preemergent Herbicides on Lawns
Greg Grant, Tyler Morning Telegraph
May 12, 2024
We have had numerous yellowing St. Augustine turfgrass samples brought to the Smith County Master Gardener Help Desk that showed signs of clubbed rooting. Rather than a normal tapered root the roots are short and swollen on the ends, much like a club in their shape. This condition was traced back to misuse of an herbicide in the dinotroaniline (DNA) herbicide family. Examples include prodiamine (Barricade), benefin (Balan), pendimethalin (Pendulum, Pre-M, Weedgrass Control), trifluralin (in Snapshot), and oryzalin (Surflan). Dithiopyr (Dimension) is another ingredient that is from a different herbicide family but works in a similar way. These herbicides inhibit cell division in the plants. When a w**d seed emerges and grows through the herbicide zone at the soil surface they absorb the product resulting in death of susceptible w**d species.
A preemergence herbicide is an herbicide that is designed to control w**ds by interfering with seedling germination and emergence. They are commonly referred to in the lawn care industry as "w**d preventers," and essentially form a protective barrier on the soil during critical seasons when w**ds are most actively germinating. Conversely, postemergence herbicides will control established w**ds that have already germinated and emerged. Some herbicides have both pre and postemergence activity.
Preemergence herbicides can also affect cell division in all turfgrasses here including bermudagrass, centipede, St. Augustine, and zoysia where emerging roots on new stolons can be affected and exhibit a “clubbed root” appearance. We see the damage most commonly on St. Augustine and centipede lawns. It can also cause looping of St. Augustine stolons where they do not root into the soil treated with herbicide. As a result, the turfgrass does not fill in as rapidly which can be especially problematic when the turf is trying to recover from stresses such as drought, traffic, insect injury, or diseases like Take All Root Rot. When preemergence herbicide products are applied at excessive rates or frequencies, the clubbed rooting problem is much more pronounced. Turfgrasses will typically outgrow the effects of these products in time, but during periods of drought, insects, disease or other stresses may be severely affected. It is important to note that pre-emergence herbicides such as the ones listed above can be an important tool in managing w**ds in lawns, and these products are labeled for use in these areas. However, the potential for “club rooting” to occur on the desirable turfgrasses should be one of many factors in your decision on whether or not to apply these products. The use of pre-emergent herbicides should be limited to those in the professional lawn care service or who have spent adequate time studying how they work and reading and following label specifications.
Download A Homeowner's Guide to Herbicide Selection for Warm-Season Turfgrass Lawns or Preemergence Herbicides for the Home Lawn-A Quick Guide for Homeowners from aggieturf.tamu.edu or the AgriLife Bookstore (agrilifelearn.tamu.edu) for a more thorough overview of turfgrass w**d management.
Greg Grant is the Smith County horticulturist and Master Gardener coordinator for the Texas A&M AgriLife Extension Service. He is the author of Texas Fruit and Vegetable Gardening, Texas Home Landscaping, Heirloom Gardening in the South, and The Rose Rustlers. You can read his “Greg’s Ramblings” blog at arborgate.com, read his “In Greg’s Garden” in each issue of Texas Gardener magazine (texasgardener.com), or follow him on Facebook at “Greg Grant Gardens.” More science-based lawn and gardening information from the Texas A&M AgriLife Extension Service can be found at aggieturf.tamu.edu and aggie-horticulture.tamu.edu.
Image caption: Clubbed roots of St. Augustinegrass caused by a preemergent herbicide.
