Every year many turf managers make preemergence (PRE) herbicides an integral part of weed management programs. By in large, PRE herbicides are used to control summer annual weeds such as crabgrass, goosegrass in addition to small-seeded broadleaf weeds. Numerous options are available for use in warm- and cool-season turf at variable price points. Regardless of product selected, turf managers should consider four things to improve efficacy of PRE herbicide programs in 2015.
Check for potential winter injury
Many turfgrass managers lost areas of warm or cool-season turfgrasses following the extreme winter conditions of 2013-2014. Losses occurred from winter desiccation, exposure to lethal temperatures, or a combination thereof. Characteristics of affected sites included: turf with shallow root systems; turf subjected to winter traffic; turf in areas of heavy shade (particularly during morning hours); as well as poorly drained areas of turf.
In many cases the extent of winter damage was not fully apparent until early summer, several weeks after PRE herbicides are commonly applied. This was problematic in that many PRE herbicides have soil residual activity that can compromise rooting from stolon growth or prevent establishment of new seed altogether. It is highly recommended that turf managers check sites for potential winter injury before applying a PRE herbicide in 2015. The basic process of evaluating potential winter injury is simple: remove a core of turf from the field, place it in a south facing window, and keep it watered. If new leaf growth initiates, that is a sign that winter conditions have not harmed the turf and PRE herbicides can be used. If no new leaf growth forms, that should be a signal that winter conditions may have harmed the turf to the extent that re-establishment could be required; thus, PRE herbicide use in these areas should be avoided.
Many think of PRE herbicides as tools only used for grassy weed control. Exploring product labels will reveal that many PRE herbicides are labeled for control of dozens of small seeded broadleaf weeds, as well as sedge and kyllinga species. For example, dithiopyr (e.g., Dimension) and pendimethalin (e.g., Pendulum AquaCap) labels claim control of more than 25 different broadleaf weed species. Moreover, prodiamine + sulfentrazone (e.g., Echelon) and pendimethalin + dimethenamid (e.g., FreeHand) labels claim PRE control of numerous broadleaf weeds as well as yellow nutsedge and green kyllinga. To maximize the efficacy of PRE herbicide programs in 2015, turf managers can review the diversity of weeds across their fields and select a product that helps manage as many weeds as possible.
Manage turf to reduce weed pressure
Another step that can be implemented to improve the efficacy of PRE herbicide programs in 2015 is to manage turf with an eye on reducing weed pressure during the summer season. Annual weeds such as crabgrass germinate from seed present in the uppermost layers of soil. Like all plants, these seeds require sunlight for germination. Practices to maximize turf cover during the season will minimize the amount of sunlight reaching the soil surface ultimately lowering crabgrass pressure.
Practices to maximize turf cover include everything from the selection of traffic tolerant cultivars, management of summer diseases that can reduce turf cover, to changes as simple as increasing mowing height. Recent research has shown that increasing turf mowing height during the summer can improve the efficacy of several PRE herbicides including dithiopyr, oxadiazon (e.g., Ronstar), pendimethalin, prodiamine (e.g., Barricade), and prodiamine + sulfentrazone. Increasing mowing height will also improve the ability of turf to compete against weeds for essential water, nutrient, and light resources. Research has shown that increases in mowing height as small as 1/64 of an inch can increase photosynthesis as much as 13%. Thus, increasing mowing height not only improves PRE herbicide efficacy but also helps turf produce carbohydrates needed for growth and vigor during fall sports.
Acknowledge that resistance is real
Perhaps the biggest step field managers can take in maximizing PRE herbicide efficacy in 2015 is to acknowledge that herbicide resistance, particularly to PRE chemistry, is a very real phenomenon that can compromise weed management programs. Biotypes of annual bluegrass and goosegrass with resistance to commonly used PRE herbicides are being identified, with increasing frequency, throughout the transition zone southward. In most cases, these biotypes have evolved following repeated use of the same PRE herbicides over consecutive years without rotation to different herbicides that make use of variable mechanisms of action to control weeds. Many mistakenly assume that herbicides with different trade or active ingredient names work differently—this is not the case. For example, prodiamine, dithiopyr, and pendimethalin all control weeds in the same manner (inhibiting cell division). Therefore, if resistance evolves following exclusive use of one of these products then weeds will be resistant to the others as well.
Failure to rotate herbicides with different mechanisms of action or implement diverse weed management strategies (other than spraying) can lead to severe consequences should herbicide resistance manifest. Case studies of facilities suffering from herbicide resistance have shown that the cost of weed control can increase three-fold once resistance is apparent. For example, the cost of annual weed management at a facility with resistance to dinitroaniline herbicides (e.g., prodiamine, pendimethalin, etc.) was $143 per acre compared to ~$50 per acre before resistance reached a critical level. In addition to economics, resistance can drastically reduce the number of herbicide options available for effective weed management regardless of price. This is concerning given that no new herbicidal mechanisms of action have been introduced into the agricultural marketplace since the late 1980s. Fewer tools available for weed management coupled with an increased implementation cost is troubling considering that the presence of weeds on athletic fields can directly compromise athlete safety.
PRE herbicides can be a highly effective tool for managing weeds on athletic fields. Always refer to the product label for specific information on proper use, tank-mixing compatibility and turfgrass tolerance. Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the University of Tennessee’s Institute of Agriculture. For more information on turfgrass weed control, visit the University of Tennessee’s turfgrass weed science website at www.tennesseeturfgrassweeds.org.
Dr. Jim Brosnan is the head of the turfgrass weed science research and
extension program at the University of Tennessee. Greg Breeden is a weed science extension assistant at the University of