Unfortunately, there are no correct (or incorrect) answers for these questions. Herbicide applicators must evaluate their turf and factors that may increase turf injury. Several factors turf managers should consider when applying herbicides in summer include:
· Turfgrass species
· Turfgrass stress
· Herbicide chemistry
· Weed species and population
· Past performance of herbicides
Turfgrass species is a major factor in determining tolerance to herbicide applications. The overall sensitivity level of a species should be evaluated before herbicide applications and closely monitored when temperatures are high. For example, bermudagrass and tall fescue are both labeled for treatments with sulfentrazone (Dismiss). Tall fescue is naturally more sensitive than bermudagrass to sulfentrazone and rates must be reduced to account for lower tolerance levels. Turfgrasses that are sensitive to herbicides under good growing conditions may be more susceptible to injury during periods of heat stress and other herbicide chemistries should be considered.
As temperatures exceed 90° F, cool-season grasses become stressed and consumption of carbohydrates exceeds production through photosynthesis. Thus, grasses such as tall fescue with good tolerance to herbicides during active growth may be naturally more susceptible to herbicide injury during periods of physiological stress. Warm-season grasses grow more efficiently than cool-season grasses under these temperatures and generally have minimal stress when water is not lacking. Uninhibited growth of warm-season grasses at higher temperatures may be attributed to better tolerance to herbicide applications in summer relative to cool-season grasses. However, herbicide tolerances for specific species are all dependent on the chemistry of the product applied.
Turf managers must also understand potential effects of temperature on herbicide activity. Many herbicide chemistries, such as synthetic auxins, have greater activity at warm temperatures compared to use during cooler weather. For example, Trimec Classic (2,4-D + dicamba + MCPP) applications often have erratic activity in early spring but perform much more effectively in summer. Sensitive species to these herbicides, such as St. Augustinegrass, have a higher risk of injury due to greater activity under excessive heat. Switching to another chemistry, such as a sulfonylurea herbicide, may be a safer option especially at reduced label rates. Enhanced activity of herbicides during these periods may also allow end-users to reduce rates and applications required to achieve desirable weed control.
Targeted weed species and the benefits of control should also be considered before risking turf injury from herbicide use in hot weather. Warm-season weeds that spread laterally, such as spotted spurge or knotweed, will continue to grow during hot temperatures and out-compete turfgrasses for light, water, and nutrients. If continued growth of weeds may result in loss of the overall turf stand, practitioners should consider applying herbicides. Preventing annual weeds from taking over a turf area during summer may help reduce voids in early fall that may allow winter annual weeds to establish. Thus, turf managers must evaluate the risk of turf injury at the expense of weed control and potential implications in long-term management.
Previous turf injury from herbicide applications during moderate temperatures may indicate risk of greater injury during excessive heat. Similarly, past reports of turf safety under high temperatures may suggest a specific product has potential for use under local conditions. It is recommended for turf managers to record air temperatures, soil temperatures, relative humidity, turf health, and other environmental factors that may influence turf tolerance to herbicides. These records can be referenced to plan future spray programs for mid to late summer in subsequent years.
Other environmental factors influence turf injury from herbicides in summer. While temperature is an important factor, high humidity increases absorption of many herbicides compared to low humidity levels. Herbicide applications in early evening when humidity and temperatures decline may help reduce injury potential compared to midday when these levels are higher. However, subsequent heat and humidity may influence turfgrass translocation and metabolism of herbicides that could also limit tolerance levels after applications.
Unfortunately there are no perfect application programs or predictive models to determine safety of herbicides on labeled turfgrass species. It is recommended to spray a test area and evaluate turf injury before making broadcast treatments during periods of excessive heat. Furthermore, if there is uncertainty over making herbicide applications turf managers should wait and assess the benefits of potential weed control. Applicators may wish to consult local extension agents for further information regarding herbicide applications during summer months.