How Herbicide Drift from Farms Is Harming Trees in Midwest

“Once a tree is stressed, there are lots of things [that can] take that tree down,” said Robbie Doerhoff, a forest entomologist with the Missouri Department of Conservation. “It used to be age or storm damage. Now it seems we’re seeing more and more herbicide damage occurring year after year. You see this kind of long, slow decline in some areas, especially among high-quality oaks.”  

A 2024 study commissioned by the Illinois Department of Natural Resources suggested just how widespread the problem has become. Researchers tested leaves, both damaged and undamaged, at 185 woodland, grassland, and wetland sites across the state. They found 41 different chemical compounds, and they found at least one of them at 97 percent of the sites. From mid-May into July, almost all the chemicals were herbicides; later in the summer most were fungicides.

Thomas Benson, a wildlife ecologist at the University of Illinois Urbana-Champaign and one of the authors of the study, said they were surprised by how many chemicals they found and how ubiquitous the contamination seemed. “People weren’t looking,” he said. “Now that we’re looking for it, we’re going to find it.” Benson said he worried not just about the trees but about the “trickle-down” effect on other plants and wildlife, including insects and birds that depend on trees to fuel their spring migration. “There could be far-reaching consequences we don’t know about yet,” he said.

Indeed, there’s a lot that scientists don’t know about herbicide damage to trees. It’s not clear how such damage affects a tree’s growth or its long-term health, or if frequent damage kills it. Many conservationists and scientists believe it does, but they note that it’s difficult to know for sure what finally dooms a tree or the other species that depend on it.

The Illinois study offered a rare snapshot of the geographic breadth of herbicide damage in one state in one year. The Prairie Rivers Network, a nonprofit advocacy group in Illinois, has been tracking herbicide damage to trees and other vegetation over six years. Out of 280 sites visited, broadleaf plants and trees at all but one site showed symptoms of herbicide damage each year, and 90 percent of tissue samples contained detectable levels of herbicides.  

“There was knowledge that this was happening,” said Kim Erndt-Pitcher, director of ecological health at the Prairie Rivers Network. “The fact that it was becoming so widespread, year after year, was very concerning to us.” Perhaps most surprising, she said, was how far from agricultural fields they found signs of herbicide damage: In 48 percent of the cases, researchers found damage more than 1,000 feet from the nearest farm field; six samples showed herbicide damage at more than a mile. 

The Illinois studies are part of a regional effort that’s been growing over the past decade as state forestry officials and conservation groups try to come to grips with a problem that many people were slow to recognize — until they started to see it everywhere.

In Arkansas, ornithologist Dan Scheiman was working as the Plants for Birds program manager at Audubon Delta when herbicides came under fire for damaging nontarget row crops. “No one was looking at native plants,” he said. So he organized Audubon staff and volunteers, starting in 2019, to look for herbicide damage in 20 counties of eastern Arkansas. Over three years, they found it in nearly all of the sites they surveyed.

Although an observer on the ground can’t always tell if a tree is suffering from herbicides, fungicides, or disease, Scheiman said, “My feeling is that if the plants get hit again and again with herbicides, it really weakens them. And they’re susceptible to something else, like a disease or fungus, which ultimately kills them.” 

Herbicides have been damaging nontarget vegetation since the compounds were first applied to row crops in the early 1940s. In the 1950s, the Forest Service linked boxelder blight in the Northern Great Plains to drift from crops sprayed with the widely used herbicide 2,4-D. In the 1960s, the herbicide damaged grapes in central Washington, and 2,4-D drift continues to damage grape vines, fruit trees, and vegetables today.  

Herbicides and other agricultural chemicals can travel beyond the fields in many ways. A breeze can blow droplets away from sprayers or crop-dusting airplanes. Dust from fields has been found to contain pesticides. And many pesticides are volatile: In warm weather they turn into a gas and become part of the air. In the 1990s researchers found several herbicides in the rain falling on Isle Royale National Park, in Lake Superior, demonstrating that contaminants could be atmospherically transported “hundreds of kilometers and deposited by precipitation.” 

The scale of herbicide damage is just one consequence of our ongoing battle with weeds. Herbicides became popular in the U.S. after World War II. Their use increased as farms grew in size and as farmers adopted conservation practices, like cover cropping and no-till farming, that reduce soil erosion. Instead of disking, plowing, and cultivating to control weeds, they simply sprayed chemicals. The amount of herbicides, pesticides, and fungicides used in the U.S. more than tripled between 1960 and 1981.

Philip Marshall, Indiana’s forest health specialist, first saw damaged trees in 1983, when herbicide use was rising. “We had some damage in the northern parts of the state, on white oak, then red oak and maples,” he said. Some years later, he conducted an experiment. He tied pollen bags around the branches of some trees to test if herbicides were the cause of leaf damage. “Any leaf inside was fully opened,” he said. “Everything outside was tattered.” 

The appearance in the 1990s of Roundup Ready crops, which were genetically modified to withstand applications of Roundup, only tightened farmers’ embrace of herbicides. Now, instead of spraying their fields before planting and applying multiple formulations, they sprayed only the broad-spectrum Roundup, which contained the herbicide glyphosate, atop their growing crops.

For a while, Marshall said, herbicide damage seemed to disappear. Glyphosate was less volatile; it tended to stay put. But the heavy use of a single herbicide soon yielded glyphosate-resistant weeds. And as glyphosate lost its effectiveness, agrichemical companies developed new GMO crops that would tolerate other, older herbicides — including 2,4-D and dicamba. 

These compounds worked where glyphosate no longer did, but they were also more volatile, experts say. Soon, Marshall and his colleagues began seeing tree damage once again. That was around 2010, Marshall said, and the damage hasn’t stopped yet.

One of the frustrations for forestry experts is how little they know about the consequences of herbicide damage. It’s not clear how stunted and deformed leaves affect a tree’s growth or its long-term health, or if recurring damage can kill it. 

“It appears that the amount of herbicide damage from indirect sources [via the wind or the rain] on a single tree during a single growing season is never enough to outright kill a tree,” Doerhoff wrote in an email. “However, herbicide injury is just another stressor on a long list of stressors that trees are facing. I think the herbicide is weakening them and making them more susceptible to attack by insects and diseases.”

And then there are the potential broader ecological consequences. Studies have documented the damage that herbicides inflict on flowering plants that are important to pollinators. But what happens when trees thin, weaken, or even die? Oaks are of special concern to forest health biologists not just because they seem especially susceptible to herbicides but because they’re considered a “foundational” tree of major ecological importance in Midwestern and Eastern forests. They offer food and habitat for many birds, insects, and mammals. The mycorrhizae and other fungi associated with their roots release nutrients into the soil, where they support the growth of other plants.

Doerhoff said she’s noticed fewer insects around herbicide-damaged trees. “If we’re affecting the plants and the insects, the base of the food chain, with these herbicides,” she said, “we’re going to be in a world of hurt in the next few years, I think.”

Concerned scientists and conservation groups are beginning to grapple with these fundamental questions. In one new study, forestry biologists in several states plan to take coring samples of trees known to be hit frequently by herbicides to study their growth rings. In the Seventy-Six Conservation Area, in southeastern Missouri, state workers are monitoring 20 frequently damaged white oaks, visiting them at least once a year in an effort to see how they fare over time.

In the meantime, climate change is almost certainly making the problem worse. It’s bringing longer periods of drought to the Midwest, stressing and weakening trees. In many places, but not all, it’s hastening the arrival of spring, so that trees leaf out earlier than they once did, making them more vulnerable when farmers are entering their fields to kill weeds or cover crops, a period they call “burn down.”

It’s that time now in the Midwest, and at the Beadles Barrens Nature Preserve. Oaks on the Barrens are just beginning to leaf out; elsewhere, sweetgums and elms are well on their way. It’s been a wet spring in Illinois, and farmers are just starting to head into their fields. 

Beadles lives about a mile from the preserve and stops to take a close look at it every few days. He learned the trees and other plants there as a child, when his grandmother took him out to hunt for medicinal plants. With at least 235 different plant species, the Barrens has a biological richness that belies its name. It’s a tiny remnant of native vegetation that has all but disappeared from the state. 

Now, Beadles worries for the preserve’s future — the uncertainty of how others might care for it, the growing threat of invasive species, and now, it seems, the annual toll from drifting herbicides. “I’m sure I’ll lose a few more trees this year,” he said.