Turf Talk
Weed Control Lessons Learned in 2018 By Jim Brosnan, Ph.D., Greg Breeden, and José J. Vargas, University of Tennessee
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ach year the turfgrass weed science program at the University of Tennessee conducts hundreds of research trials in the field, greenhouse, and laboratory. Despite having an array of different objectives, all of these trials have a singular focus: to help turfgrass managers with weed management issues. The article below outlines key lessons learned in our program last year that will hopefully be of use to turfgrass managers in the coming season.
Lesson #1 – Explore Weather Data Mother nature certainly threw turfgrass managers across Tennessee many curveballs in 2018. The year began with air temperatures measuring much higher than historical averages leading to spring emergence of warm-season turfgrasses (and weeds) occurring earlier than normal. Air temperatures fell in April, measuring nearly 10 degrees cooler (on average) than April 2017. Warming in 2018 (as measured by growing degree day accumulation) was higher than the previous two seasons and this period of warming continued into the fall. A detailed breakdown of erratic weather across Tennessee in 2018 is available at: https://medium.com/@UTTurfWeeds/several-curveballsfrom-mother-nature-in-2018-d487c9b2015d. Irregular weather patterns will not only affect the severity of weed infestations in warm- and cool-season turfgrasses but the efficacy of several herbicide treatments as well. That said, University of Tennessee Extension is here to help. Should turfgrass managers encounter oddities during the 2019 season, they are highly encouraged to contact the University of Tennessee for a breakdown of climatic conditions that may help explain the situation.
Lesson #2 – Know Your Poa One of the biggest lessons learned last year is that Poa annua, or common bluegrass, populations across the state of Tennessee are very different from one another. Thanks to support from the Golf Course Superintendents Association of America (GCSAA)
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tennessee greentimes Summer 2019
and Tennessee Chapter of GCSAA, our team is currently conducting a survey of Poa annua across the state. Efforts have allowed us to study 72 different populations randomly selected during spring 2018 from golf courses in East, Middle and West Tennessee that had either bermudagrass (Cynodon spp.) or zoysiagrass (Zoysia spp.) as a primary turfgrass species. Things we’ve learned studying these populations include: • Poa annua from West Tennessee takes longer to germinate from seed than that found in Middle or East Tennessee. Moreover, populations from West Tennessee grow less vigorously from seed as well. • Over 64% of the Poa annua collected in this survey has some level of resistance to glyphosate (e.g., Roundup) • Over 58% of the Poa annua collected in this survey has some level of resistance to prodiamine (e.g., Barricade) While we are still continuing to study these populations, implications of this research to turfgrass managers across Tennessee are significant, particularly in regards to herbicide selection. The current recommendation from University of Tennessee Extension is to target Poa annua infestations of warm-season turfgrass with a mixture of pre- and postemergence herbicides in October. This approach is an optimal resistance management strategy in that it facilitates using multiple modes of action for control. Additionally, delaying treatment until October increases the likelihood of turf remaining devoid of Poa annua throughout spring, compared to making a single application of a preemergence herbicide in late August or early September.
Lesson #3 – Soil Moisture Affects Goosegrass Control Postemergence goosegrass (Eleusine indica) control is a challenge every season. In 2018, we learned that the growing environment
