We asked experts from pesticide manufacturers: 2015 was the hottest year on record, continuing a trend of warmer weather. Assuming the warm weather continues, what impact will it have on weed control on golf courses? Could more Northern golf courses begin to see more Southern weeds?
NASA reported that more than 97 percent of publishing scientists attribute global climate change to human activity, noting that carbon dioxide in the atmosphere has been steadily increasing. Studies show that weed control significantly decreases as carbon dioxide levels increase, potentially dictating the use of higher rates of active ingredients to control weeds. Couple this with continued concern about resistance, and weed control becomes difficult.
Warm and dry weather is never a good combination for a superintendent’s weed control efforts. Over the years, some weeds that were typically found in Southern regions have slowly encroached into some Northern states. For example, yellow nutsedge, historically found in more Southern regions, is now common in other parts of the country. Therefore, it is not unreasonable to believe that if warmer temperatures continue to push higher into the mid-Atlantic and parts of the Northeast that weeds typical to the Southern states will follow.
With warm-season turf, heat will help produce dense turf that will choke out weeds. With coolseason turf, it is important to make sure that a solid herbicide program is in place as prolonged heat is likely to thin the turf, giving weeds a chance to emerge. Regardless, weeds do not significantly evolve after just one year of warmer weather. Changes in herbicideuse patterns, cultural practices or resistance issues are much greater contributors. Warm and wet conditions will promote Poa annua to become well established and will also break down preemergent herbicides quicker, so superintendents should be mindful not to stretch applications.
With the trending warmer weather, I believe we see a few things happening with regard to weed populations. Milder winters allow perennial tropical/Southern weeds like bermudagrass to persist, when in the past the cold would keep plants from surviving in Northern climates. This means weeds that have not been a problem in the past may now be a new target for superintendents. With the warmer summer temperatures, it means Southern weeds will have a longer growing season in Northern climates, helping them to persist by increasing weed density and ultimately providing greater weed seed dispersion. A good example of this would be goosegrass, which normally has a very short growing season as we move northward, limiting its spread.
According to my dusty college weed control handbook, every weed species has a temperature range that is suitable for survival and growth. So it stands to reason that if the trend of warmer weather continues, then weed patterns will continue to evolve. If that’s the case, then weed control — both cultural and chemical — will also need to change. You can pick a day as far into the future as you want and superintendents will be managing some kind of weed infestation. As always, they’ll adapt because of increased knowledge, improved cultural practices and enhanced herbicide technology.
Weeds (and other turf pests) naturally evolve to fill open niches and gradually expand from their current distribution over time. This evolutionary process can be sped up with human help by moving soil and plant parts intentionally or unintentionally, such as planting species where they are only marginally adapted or weakening adapted species with extreme management. A warming climate will also help move traditionally Southern problems northward. A good example is goosegrass, which was traditionally a problem in the transition zone and farther South. Now it’s becoming common in the Mid- Atlantic and farther North. Warming temperatures may also affect our ability to apply herbicides, which are usually safer to desired cool-season turf at cooler temperatures.
Weeds are extremely efficient at adapting to change. As climate conditions change, there may be weeds commonly found in the South that adapt and thrive in traditionally cooler Northern climates. Turf managers and chemical manufacturers will have to pay careful attention to this trend as chemical options for control may be very limited in these nontraditional environments. Limited chemical options may lead to overuse and little chemical rotation may speed up future resistant populations, creating even more difficulty for control.
Weeds are tremendously opportunistic, and changes in weather dictate which weeds are most problematic. With warm and mild winters, the less coldhardy Southern weeds are moving farther North. Additionally, warmer weather extends the growing season and provides additional time for weed escapes. This improves not only the survival of annual weeds that are dependent on yearly seed production, but provides perennials with additional time to establish energy reserves in tubers, stolens and rhizomes. Often, this warm-weather trend has associated extremes in rainfall. The heavy rainfall shortens the residual control provided by preemergent herbicides, thus leading to more breakthroughs of late-emerging summer annual weeds as well.