Usually, not even a scorching, bone-dry Texas summer can kill a mesquite.

The native tree is known for its skill at surviving drought and 100-degree heat. So when groves of them started dying along Interstate 35W in southern Denton recently, people noticed.

DRC/Barron Ludlum

Dead trees line Interstate 35W near Pilot Knob in Denton, on land owned by Fort Worth-based development company Hillwood. The landowner said mesquite trees are being killed off to make way for cattle.

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“I had no idea what it was, but I just thought that something must have happened,” said Floyd Rosenkranz of Krum, a truck driver who spotted the dead trees while traveling between FM2449 and Robson Ranch Road recently. “A lot of people, the first thing they think is gas wells. There’s a couple of gas wells right there, but the trees right around them are green, so I don’t think that’s it.”

“I just thought it was odd,” he said.

He wasn’t alone.

Kathleen Wazny, who lives in the area, started asking questions after noticing that dozens of once-green mesquites and other trees near the highway had turned brown. Some of the dead or ailing trees are near the southern edge of Pilot Knob, a 900-foot-high summit once used as a signal post and lookout station by Caddo Indians and Spanish and American pioneers.

All of the trees are inside Hunter Ranch, a planned 3,000-acre housing development on both sides of Interstate 35W between FM2449 and Robson Ranch/Crawford Road. The project, which has also gone by the name Inspiration, has been on hold in light of the housing market slump and the loss of an out-of-state developer.

The landowner, Fort Worth-based development company Hillwood, had told residents of the neighboring Robson Ranch retirement community that it planned to spray herbicide to kill mesquite saplings before bringing cattle to Hunter Ranch, Wazny said. Records show Hillwood notified the city in April 2009 that it planned to clear trees to use the property for agriculture.

Wazny said she watched trees disappear from the land over the past year but grew alarmed late last month when entire stands started withering after being sprayed by a crop duster. She worried the chemicals could affect the health of area residents and that the dead wood could pose a fire threat.

“If somebody flipped a cigarette off of [Interstate] 35 and the trees caught fire, that would be a major problem,” she said.

Dave Pelletier, a Hillwood spokesman, said the company was removing “old, ugly scrub” to bring cattle to Hunter Ranch, but he was unaware of any herbicide use. Company officials were unavailable to comment because of the extended holiday weekend, he said.

Although mesquites provide food for wildlife and shade for livestock, ranchers often consider them a nuisance because of their thorns and tendency to take over rangeland. Common control methods include bulldozing, chemical spraying and prescribed burns.

The city’s tree code offers varying protections for mesquites and other species depending on the size and type of the development, among other factors. In this case, much of Hunter Ranch is agricultural and therefore exempt from the code, said E.J. Cochrum, urban forester for the city of Denton.

“They are using an herbicide to suppress the mesquite out there,” Cochrum said. “I don’t have any jurisdiction over herbicide use.”

Many herbicides can only be used by an applicator licensed through the Texas Department of Agriculture, and users are supposed to follow the directions provided on the products, said Bryan Black, a department spokesman.

The department, which investigates claims of improper herbicide use, had not received complaints about the Denton site, Black said. The public can file a complaint by calling 1-800-835-5832.

Wazny said she planned to call the state this week after hitting a “brick wall” at City Hall. City officials looked into her complaints and determined there was nothing they could do, Wazny said, although she believes Hillwood is responsible for preserving trees on Pilot Knob under an agreement with the city.

Ken Banks, the city’s environmental services director, said a field assessment found that trees near the base of some hills at Hunter Ranch, including Pilot Knob, were “not doing well.” Still, Hillwood appears to be working within its agreement with the city to preserve a certain percentage of trees on each hill — land known in city code as upland habitat, Banks said.

“They are certainly required to preserve trees on all of the hilled areas there, but that doesn’t mean 100 percent preservation,” he said.

LOWELL BROWN can be reached at 940-566-6882. His e-mail address is lmbrown@dentonrc.com.