Pollution-fighting superpowers of a common roadside weed

Plants are famous for their ability to store carbon, but that’s far from their only superpower when it comes to cleaning up the environment. Scientists have increasingly turned towards plants to help detoxify soils in areas rife with chemical contaminants. Some plants can even suck up heavy metals like lead and mercury and transfer them to their leaves, which are easy to harvest and dispose of safely. 

This plant-based cleanup strategy, called phytoremediation, relies on finding plants that can grow quickly and easily and also have the ability to extract specific pollutants from the soil. Patrick Wright and Janet Steven, plant biologists at Christopher Newport University, recently identified horseweed as a potential candidate for the phytoremediation of heavy metals. Wright will present their research findings at the January 2023 meeting of the Society for Integrative and Comparative Biology.

Horseweed is a common sight on roadsides and parking lots, and yet Wright never thought much about it until he was out surveying Superfund sites, which are areas contaminated with hazardous waste. “We just kind of stumbled upon this plant,” he says. 

During the surveys, Wright carried a special handheld device that uses X-rays to measure the presence and concentrations of heavy metals. After pointing the X-ray device at the leaves of various plants, he noticed that the horseweed growing in Superfund sites had extremely high concentrations of heavy metals. 

Based on this observation, Wright suspected that horseweed might have the metal-accumulation ability needed for phytoremediation. To test this ability, he grew horseweed plants in the lab, giving some of them clean water and others water contaminated with high concentrations of either lead, barium, zinc, copper, or chromium. Then he measured the plants’ growth and the amount of each metal that the plants accumulated in their leaves.

Wright found that the horseweed plants were able to efficiently extract and store each of the five heavy metals. The plants grown in zinc-laden water had the most impressive results. “I saw that they accumulate a lot,” says Wright, “almost 1000 times the normal level of zinc.”

And while plants that were watered with heavy metal solutions did grow less over the eight-week experiment than those with unpolluted water, they still grew substantially.

The combination of its rapid growth, stress tolerance, and ability to accumulate a range of heavy metals makes horseweed an ideal candidate for phytoremediation. And Wright also points out that since horseweed is native to most of North and Central America, it can be grown throughout the region without fear of spreading a potentially invasive species. 

Wright hopes that by sharing these findings, he will encourage more research into phytoremediation and more widespread use of this method for cleaning up pollution. “I want to continue pushing for phytoremediation,” he says, “but make sure we’re doing it in a very logical way.”