Todd Schwarzrock had his lawn treated for weeds in May. Weeks later the grass around his Maple Grove home looked good. But some of his trees didn't.

Needles on a 35-foot white spruce on the corner of his lot apparently died. The lower branches on a Colorado blue spruce turned brown. A couple of smaller evergreens showed damage, too.

Schwarzrock's lawn had been sprayed with Imprelis, a new, supposedly environmentally friendly herbicide from Dupont. Lauded as a biggest-in-decades advancement, it showed effectiveness against weeds such as creeping charlie. Schwarzrock, chief financial officer of Rainbow Tree Care, had Imprelis applied by the firm's lawn care unit.

Evergreens around the Twin Cities area are now showing twisting and distorted branches, needle browning and drooping that may be linked to the weedkiller, said Mark Stennes, a pathologist with S&S Tree and Horticultural in South St. Paul.

"I've been called out to one site, and other arborists have seen damage all over the west metro," he said. "I think some people don't even realize what's happening."

Imprelis was not available to homeowners through garden stores. It was used by lawn-care companies that heard Dupont's pitch that the herbicide was effective on tough weeds but "easy on the environment," a trade journal ad claimed.

"I don't know how it got through testing," Schwarzrock said. "We'll make it right with our customers no matter what Dupont does, but we hope Dupont stands behind their product."

Suit filed, probe underway

A class-action suit was filed Monday against Dupont by an Indiana golf course management firm and a Pennsylvania homeowner. The golf course operator alleges hundreds of thousands of dollars in tree damage; the homeowner claims her trees died.

Imprelis is still on the market. Dupont is investigating and has cautioned customers not to apply the chemical where Norway spruce and white pine are planted.

"We have an investigation team and are contacting our customers and working with them to really understand the circumstances and whether Imprelis may have contributed to the symptoms," said Dupont spokesperson Kate Childress.

Dupont advertised it as safe for the environment but effective against difficult weeds such as creeping charlie and violets. Schwarzrock saved an ad from a trade magazine that called Imprelis the most scientifically advanced turf herbicide of the last 40 years. Dupont said it could be applied in rain or on sunny days and had low toxicity rates to mammals.

But in the spring, possible evergreen damage was reported on the East Coast. By May those reports had spread to the Midwest.

Evergreen damage locally

Experts from the Minnesota Department of Agriculture and the University of Minnesota began visiting sites where Imprelis had been used and trees were in trouble. The department set up a Web page telling people who thought they had damage to contact their lawn service and Dupont.

John Lloyd, director of research and science for Rainbow Tree Care, said he has visited sites where Black Hills spruce (a variety of white spruce), Norway spruce, Colorado blue spruce and eastern white pine show damage. Deciduous trees such as honey locust and gingko also showed signs of possible herbicide damage, he said.

Kathy Zuzek, U of M extension educator, has seen similar evergreen damage. At one site, leaves were distorted on a dogwood and amur maple as well as on black-eyed Susans in a garden, though Zuzek said all those plants looked vigorous enough to survive.

Lloyd and Zuzek said they probably won't know what to recommend until Dupont finishes its investigation or more is known. One puzzler is that while other broadleaf weedkillers often carry precautions for use near broadleaf trees and shrubs, Imprelis appears to be most damaging to conifers.

"Nobody knows why this is happening," Zuzek said. "Everyone was very excited about it and Dupont marketed it very strongly. I feel for the landscape managers who took what they heard and believed it and now are dealing with big townhouse associations and others and are trying to explain the damage."

Watering worsen damage?

When the East Coast problems surfaced, Lloyd said, his firm wrote to clients that had Imprelis applied to their grass to ask them to report any symptoms. The firm is taking pictures and documenting damage, he said. "If there's a class-action settlement we're ready to go."

But he and Zuzek aren't sure what to tell people who are worried about their trees. Aggressive watering may actually flush the chemical toward roots, he said. Zuzek said examination of some trees with browning needles showed that the buds that had next year's growth were alive, while others were "absolutely killed."

"Nobody knows what the prognosis is because this is a new herbicide," Zuzek said. "In terms of survivability, we're taking a wait-and-see attitude."

Mary Jane Smetanka • 612-673-7380