Revolutionizing Weed Control in Turf

Revolutionizing Weed Control in Turf: Thermal Energy and Organic Alternatives to Synthetic Herbicides

By Juan R. Romero, Navdeep Godara, Daewon Koo, John Peppers Graduate Students, Virginia Tech and Shawn D. Askew, Ph.D.

Researchers, professionals, and homeowners have long struggled with the battle against annual bluegrass (Poa annua) in turfgrass systems. Known for its shallow root system, heat is one of its weaknesses. Annual bluegrass is a resilient weed that also has a tendency to develop resistance to traditional herbicides, making it one of the most troublesome turfgrass weeds in the United States. As herbicide resistance grows and environmental concerns intensify, the search for effective, sustainable, and selective weed control methods becomes a necessity.

Here at Virginia Tech, we have been exploring innovative approaches to this challenge, developing alternatives that range from lasers and cryogenic liquids to thermal energy. During the winters of 2023 and 2024, we conducted field studies in our research facility located in Blacksburg, Virginia. Our goals were to assess the potential of thermal treatments to control annual bluegrass in dormant warm-season turfgrass systems. We also wanted to compare them against organic and synthetic chemicals.

Figure 1. Radiant heat treatments were applied with a 1/4-inch metal plate that was heated to 700 °F. The device was placed at 1.5 and 3/4 of an inch above the turf canopy.

Why Thermal and Organic Treatments?

Thermal weed control has been around for a while. However, the primary constraint to its widespread implementation has been its lack of selectivity. By targeting annual bluegrass in dormant warm-season turf, we can circumvent this problem, making thermal and organic treatments an option for some practitioners. The principle behind thermal treatments involves transferring heat to the target plant. By transferring heat to plant tissue, thermal methods can damage or kill weeds without relying on chemical compounds. This makes them especially appealing in light of regulatory pressures on synthetic herbicide use.

Acetic acid is the compound found in kitchen vinegar. However, at higher concentrations, 20 to 40%, it becomes a herbicide. Organic chemicals, such as acetic acid, have proven effective at controlling weeds in dormant turf. Our studies have shown that three applications of acetic acid during the winter season can indeed control annual bluegrass similarly to synthetic chemicals. Innovative Field Studies: Radiant Heat and Hot Water

Our team evaluated several treatments, including radiant heat, hot water, acetic acid, and glyphosate, to determine their effectiveness in reducing annual bluegrass density and cover. Thermal treatments were applied either thrice monthly or six times biweekly, acetic acid was applied thrice monthly, and glyphosate was applied once.

Radiant heat applications involved heating a metal plate to 700°F and placing it 1.5 inch above the turf canopy for 15 to 25 seconds (Figure 1). The study found that biweekly radiant heat treatments reduced annual bluegrass density by up to 87% at 70 days after the initial treatment (DAIT), equivalent to acetic acid and glyphosate and outperforming all other treatments (Figure 2). However, at 117 DAIT or two months after the last treatment, annual bluegrass had recovered from radiant heat treatment, but not from acidic acid or glyphosate (Figure 2). Hot water treatments were less effective, likely due to energy dissipation during application. This finding underscores the importance of heat transfer and suggests that engineering solutions could lead to better heat transfer and associated weed control.

Figure 2. Density reduction in the number of annual bluegrass plants compared to the non-treated plots. Higher numbers indicate better weed control at 70 and 117 days after initial application. Different letters above the bars indicate statistical differences.

Understanding the Dynamics of Heat Application

Since radiant heat biweekly performed at a commercially acceptable level for at least one month after the last treatment, we decided to conduct another study focused only on that alternative. We examined how exposure duration and distance from the heat source influenced surface temperatures and weed control. Results showed that reducing the distance to the heat source from 1.5 inches to ¾ of an inch significantly increased surface temperatures, reaching up to 381°F, and improved weed control.

These findings highlight the potential for refining thermal treatment techniques to enhance efficacy. Adjustments in engineering, such as optimizing the heat source’s distance exposure time and insulation, could pave the way for more effective and economical thermal weed control devices.

Implications and Future Directions

With increasing regulatory pressure on synthetic herbicides and the rise of herbicide-resistant weed populations, the need for alternative solutions is more pressing than ever. Thermal treatments, particularly radiant heat, show promise as a viable option for managing annual bluegrass during the winter dormancy of warmseason turf.

However, challenges persist with organic treatments. While effective, they remain costly for large-scale applications, with acetic acid treatments in the current study costing over $10,000 per acre. In contrast, using a device towed by a turf tractor to deliver radiant heat with propane could reduce costs to below $100 per acre. Advancements in machine vision technology promise to overcome the non-selective nature of many organic weed control methods by allowing precise targeting of weeds alone, potentially lowering costs further. Future research will aim to refine the engineering of radiant heat devices and explore supplementary technologies like cryogenics and lasers. These innovations will harness machine vision to expand the application of organic treatments into previously unfeasible areas.