A perennial problem

Dallisgrass is one of the most difficult weeds to control in Tennessee, according to the University of Tennessee Extension. To find out the best practices for getting rid of the perennial pest, we talked to Dr. Jim Brosnan, associate professor of turf and ornamental weed science at the University of Tennessee.

What are the dallisgrass pressures looking like this year?

Here in Tennessee, these weeds are continual problems. Dallisgrass is arguably the most difficult weed to selectively control in fine turf in our state and kyllinga problems have become more common the past few seasons.

What are the optimal growing conditions for dallisgrass?

Dallisgrass seems to grow in all conditions throughout Tennessee. We see it thrive in both irrigated and non-irrigated turf, and areas receiving regular mowing/fertilizer applications as well as areas mowed sparingly and not receiving fertilizer. Again, it is arguably the most troublesome turf weed in the state.

What should superintendents look for to spot dallisgrass problems?

Dallisgrass is fairly easy to identify in a lawn situation. Its bunch-type growth and coarse, textured leaves differentiate it from desirable lawn grasses. Often it can be confused with smooth and large crabgrass when mowed at similar heights.

Besides having a different growth habit, dallisgrass has a prominent membranous ligule and ridge along the mid-rib of the lower leaf surface. Additionally, when left un-mowed, dallisgrass will form a distinctive seedhead different from either smooth or large crabgrass.

What products work well to control dallisgrass?

There are minimal options to selectively control dallisgrass in either warm- or cool-season turf. In warm-season turf, many of the ALS inhibitors can suppress this species when applied with calibrated equipment at the correct timing. This would include herbicides such as combination of thiencarbazone, foramsulfuron and halosulfuron, trifloxysulfuron, foramsulfuron and flazasulfuron. None of these products will provide complete control with a single application though. Multiple applications are required to control dallisgrass over several seasons regardless of product.

In cool-season turf, fluazifop is probably the best option for selective control based on the research we’ve done here at UT. However, it is limited to only fall and spring applications in tall fescue to limit the threat of desirable turf injury. In other cool-season lawns, such as those containing Kentucky bluegrass and perennial ryegrass, we don’t have many options.

Is there anything technicians should know about treating turfgrass?

Timing is critical, especially for a perennial species like dallisgrass that over-winters from thick underground rhizomes. We’ve found that dallisgrass in most susceptible to herbicides when it is beginning the process of transitioning to dormancy (even when there is no visual sign that such a transition is happing).

Our cooling degree day trigger of initiating applications once the average air temperature falls below 72 degrees F has proven to work quite well here. Additionally, it is important to recognize that sequential applications are going to be required over multiple years to eradicate plants from a lawn. Frequently, we see situations were individuals need to make a decision about whether or not the resources and effort behind a selective control program is warranted. Basically, a decision needs to be made whether or not there is enough desirable turf to save. In many situations, the dallisgrass pressure can be so great that a complete renovation may be the best option.

What’s the latest research on dallisgrass?

My recent Ph.D. student, Matthew Elmore, devoted a good portion of his graduate research to controlling dallisgrass. Matt evaluated how dallisgrass susceptibility to various herbicides varied throughout a growing season. This allowed him to identify optimal windows of opportunity for controlling dallisgrass using atmospheric data.

He worked with growing degree day accumulation in spring (i.e., a measure of the continued increase in average air temperature) and cooling degree day accumulation in fall (i.e., a measure of the continued decreases in average air temperature) to provide end users with target timings to start programs for dallisgrass control.

Thanks to his work we now can advise turf managers that the best time to initiate programs for dallisgrass control in Tennessee is in the fall when the average daily air temperature falls below 72 degrees F for at least three days. In spring, we know that the best timing to apply treatments is approximately when 270 to 360 growing degree days have been accumulated since the first of the year.