In 1991, scientists used red dye to trace weedkiller in a Washington river choked by milfoil; the study reported a 99% decline in milfoil and up to 1,000% more native plants

By August 1991, an invasive plant, Eurasian watermilfoil, had almost taken over a part of Washington’s Pend Oreille River. It formed such thick mats in the river that it blocked out sunlight and choked out native plants that fish and other wildlife depended on. To tackle this, a team of researchers from the U.S. Army Corps of Engineers tried a unique solution. They used a fluorescent red tracer dye, rhodamine WT, combined with the herbicide in order to see the exact path the chemical took in the water before fading away. According to the 1997 study, published in Regulated Rivers: Research & Management, the trick worked better than expected.

Why one weed caused so much trouble

Eurasian watermilfoil is not native to North America. It may have been introduced into the country many years ago, possibly through the aquarium trade, and is now found in freshwater lakes and rivers across the United States. This invasive species grows in dense underwater colonies and crowds out native vegetation. The USDA plant guide lists the plant as noxious or restricted in 17 states because of the ecological damage it causes. By 1991, the Pend Oreille River had been infested with milfoil for more than a decade. The efforts that had previously been put in place to fight it using other herbicides were not very effective, according to the report. Therefore, rather than conduct another laboratory experiment, researchers turned to a compound called triclopyr, commercially known as Garlon 3A, and ran a real-world field trial.

How the red dye actually worked

The red dye did not trap the weedkiller; it tracked it. Rhodamine WT is a fluorescent dye commonly used for studying river flow dynamics and has been found safe for use in drinking water at low levels. Researchers released the dye alongside the herbicide and measured it. Because the dye’s fading rate closely matched the herbicide’s, the team could estimate how long the plants would be exposed to an effective dose without constantly testing the herbicide itself. It allowed the researchers to determine the precise herbicide dose needed. The study reports that in one of the coves, researchers were able to reduce the dose of the herbicide by 30 percent compared with the maximum allowed rate, while still getting good results.

The numbers behind the headline

The results were tracked for two full years, not just a couple of weeks. After four weeks of treatment, milfoil biomass had declined by 99% in both test plots. After one year, there was still a 72% and 99% decline in the river plot and the cove plot, respectively. By two years, control had eased somewhat in the river plot, where milfoil biomass was 53 percent lower than before treatment, while the cove plot still showed a 76 percent reduction. Meanwhile, native plants took advantage of the extra space and sunlight to recover. The study's own findings show that native plant biomass increased by 500 to 1,000 percent one year after treatment, and remained significantly higher two years later. The number and variety of native plants also more than doubled in the treated areas.

Was the river safe afterward

Safety considerations entered the picture as well. Within two to three days, herbicide concentrations in the treatment zones had fallen below the proposed drinking-water limit of 0.5 milligrams per liter. Near the treatment site, the levels of herbicides increased for a bit, but at 675 meters downstream, they remained below it, and by about 975 meters, they had become completely undetectable. The finding supported the researchers' conclusion that it is reasonable to have a safety buffer of 400 to 800 meters between drinking-water intakes and a treatment site.

Triclopyr's selectivity likely explains why native plants bounced back instead of dying along with the milfoil. According to the National Pesticide Information Center at Oregon State University, triclopyr primarily targets broadleaf plants, leaving grass-like species largely untouched and giving many native aquatic plants space to recover.

A lesson that still holds true

Over 30 years later, this study remains a frequently referenced point of comparison by lake and river managers working on invasive milfoil throughout North America. It suggested that a targeted intervention rather than blanket spraying can help a damaged ecosystem recover. The dye-tracking method has become a go-to tool for studying how chemicals move through flowing water, and not just for weed control. The experiment suggests that careful, well-measured intervention can help ecosystems recover, not just slow their decline.