Can a Weed Wiper Control Cottonwood Saplings? Early Results From Nachusa Grasslands
Home > Knowledge Center > Blog > Weed Wiper Brush Control Nachusa Grasslands/
GrassWorks Weed Wipers are commonly used to treat tall weeds in pastures, hayfields, row crops, and roadside areas. A recent field trial at Nachusa Grasslands tested the same selective application method on a different vegetation problem: cottonwood and aspen spreading into restored prairie.
Bill Kleiman, director of Nachusa Grasslands with The Nature Conservancy, documented the trial for the Grassland Restoration Network. His team used a 22-foot GrassWorks Weed Wiper on two areas of woody growth in June 2026.
The strongest early response came from five-foot cottonwood saplings. A taller aspen clone showed less consistent browning and will be tested again after mowing, burning, and regrowth. Kleiman marked treated stems so the team can return next spring and determine whether the visible browning led to lasting control.
Cottonwood and Aspen Can Move Beyond the Reach of Fire
Prescribed fire plays an important role in maintaining tallgrass prairie, but cottonwood and aspen can gain enough height between burns to persist and continue spreading.
Once the woody growth stands above the surrounding prairie, managers face a difficult choice. Treating individual stems by hand takes time across a large restoration area, while broadcast spraying may place herbicide on native vegetation growing around and beneath the brush.
At Nachusa, the height difference created an opportunity to test whether the Weed Wiper could contact the taller growth while passing over shorter plants below.
How the GrassWorks Weed Wiper Was Used
The 22-foot GrassWorks Weed Wiper uses a rotating steel drum covered with a synthetic carpet-like material. As the tractor moves forward, the drum rotates against the direction of travel and transfers herbicide directly onto vegetation above the application height.
An operator-controlled pump adds solution to the drum as needed. The drum must remain wet enough to provide consistent contact without becoming so saturated that excess solution drips onto the vegetation underneath.
Kleiman’s team tested the equipment in two areas with very different plant heights and growth patterns.
Five-Foot Cottonwood Saplings Showed the Strongest Early Response
The first test area contained cottonwood saplings approximately five feet tall. The patch was treated on June 2 and again on June 3 with a mixture of glyphosate, imazapic, and a non-ionic surfactant.
Those details describe the Nachusa test and are not a general mixing recommendation. Herbicide selection, concentration, additives, and application methods must follow the applicable product labels and requirements for the treatment site.
When Kleiman returned 17 days later, most of the treated cottonwoods had browned. The saplings stood clearly above the prairie and were still small enough for the rotating drum to make broad contact with the foliage.
The visible browning shows that the treatment reached the plants, but it does not confirm long-term control. The marked stems will be checked during the next growing season for survival or regrowth.
Taller Aspen Was More Difficult to Treat Evenly
The second test involved an aspen clone approximately ten feet tall in a fen management unit.
The tractor and Weed Wiper were raised to their full operating height to move through the taller growth without forcing the larger stems into the equipment. Some foliage browned after treatment, but the response was less consistent than it was in the cottonwood patch.
The aspen was more established and harder for the drum to contact evenly. Kleiman plans to mow the area during winter, burn it again, and test the Weed Wiper after the aspen produces shorter regrowth.
That follow-up will help show whether the Weed Wiper is more effective on young woody growth that stands above the surrounding vegetation but remains small enough for consistent drum contact.
What the Cottonwood and Aspen Results Suggest
The trial does not yet establish long-term control of either species. It does show a clear difference between the two treatment areas.
The five-foot cottonwood saplings were shorter, more flexible, and treated on two consecutive days. The ten-foot aspen was taller, more established, and harder to contact evenly. Because several conditions differed, the stronger cottonwood response cannot be attributed to the second treatment alone.
The early results suggest that timing may matter as much as plant height. A Weed Wiper may fit best after woody plants rise above desirable vegetation but before they develop large, rigid stems or dense mature growth.
Drum Saturation Remains an Important Variabl
Kleiman also noted that the solution remained thinner than he expected on the drum. A commenter on the original field report pointed out that the surfactant used in the trial improves wetting but does not function as a thickener.
Better wetting helps the solution spread across plant surfaces, while viscosity affects how the liquid remains on the drum. Future passes may help the Nachusa team refine pump timing and drum saturation while staying within herbicide-label and site requirements.
A New Application for the GrassWorks Weed Wiper
GrassWorks Weed Wipers are designed to contact taller target vegetation while shorter grasses, legumes, and crops pass beneath the drum. At Nachusa, that same height-based approach was tested on young woody plants growing above native prairie vegetation.
The cottonwood saplings showed the clearest early response. The taller aspen showed where consistent contact became more difficult and will be tested again after the growth is cut back.
For prairie restoration teams, conservation groups, and land managers, the opportunity may be to treat woody encroachment while the plants are still tall enough to reach but small enough for the rotating drum to contact effectively.
Read Bill Kleiman’s original report on the Grassland Restoration Network blog for the full field notes and photographs. (Photos courtesy of Bill Kleiman, Nachusa Grasslands. Used with permission.)
To see the GrassWorks equipment used in the trial, compare GrassWorks Weed Wiper models.
Managing woody encroachment on restored prairie, conservation acreage, grassland, or rangeland? Talk with the GrassWorks team about your target plants, acreage, terrain, and equipment, or call (888) 809-4737.