It's just past noon when the first of the college students pulls into the small parking lot off Cherry Hill Road in College Park, MD, near a bustling Home Depot and in the shadow of the Capital Beltway.

Marc Imlay is ready for them. He opens his trunk, covered in bumper stickers, and offers the young volunteers four-pronged gardening forks. Wearing a pair of 20-year-old dungarees, two shirts and a sweater, the 73-year-old weed warrior is raring to slay the invasive species that remain in this urban park.

There aren't many left, at least not in the 12 acres of Cherry Hill Park proper that hug the highways in this busy Washington suburb. Imlay and his groups of twice-weekly volunteers have rid the woods of most of the invasive vines that have reigned here for so long, among them periwinkle, English ivy, multiflora rose and Japanese honeysuckle. Now, one can see the forest through the trees. But the park has annexed an additional four acres, and those need attention.

So Imlay - like a super-fit Santa Claus carrying pamphlets on invasive plants instead of a sack of gifts - leads the 20 students from the Alpha Phi Omega service fraternity into a ravine-like area. After a short explanation about what plants need to be taken out, and a demonstration of how to do it, the students are on their way.

One doesn't need a Ph.D. in biology like Imlay has to see the difference between the old acres and the new. The area the students tackled is filled with spindly vines. But in the area they completed, dog walkers pass by on a cleared path just beyond where a babbling stream runs, its clear water rippling across the bottom rocks.

"It shows," Imlay said, "that you can win the battle."

No one is fighting harder than Imlay, who organizes volunteers to remove invasive plants at Cherry Hill and Magruder Woods parks in the Hyattsville area and at Swann Park and Chapman Forest in Charles County. Officially retired for three years, Imlay continues to work part-time as a ranger for the Maryland National Capital Park and Planning Commission. The rest of the time, he volunteers, working almost every weekend in one of his four parks. Species by species, month by month, he and his weed warriors have made headway.

"He has been a tireless crusader on the issue of invasive plants and how they change systems," said Kerrie Kyde, invasive plant ecologist for the Maryland Department of Natural Resources. "He has single-handedly launched probably 30 weed-removal programs. He's amazing. He's the energizer bunny. He's 73, and he's still out there seven days a week."

It has made a major difference not only in clearing the woods of invasive plants that hog nutrients and water, starving natives; but also in making young people more comfortable in nature. Some of the teens involved in Imlay's projects have spent their whole lives inside the Beltway and never ventured into the woods. Imlay makes them feel comfortable, said Colleen Aistis, volunteer coordinator for the city of Hyattsville.

"He is possibly the best teacher you can bring in for the students," Aistis said.

When she started working with Imlay in 2005, Aistis said, Magruder Woods had 60 percent nonnative cover. Now, it's flipped, and it has about 60 percent native cover.

But really, she said, his work is "beyond measure." Water now drains into a natural swamp area instead of collecting in the fields. An old boardwalk nature trail, previously unusable because it was covered with nonnative weeds, is back in action now as a walking path. Recently, a group of interns from the Sierra Club discovered an Eastern red-backed salamander in the stream - a sign of water-quality improvement.

Imlay's work, Aistis said, "gives our residents ownership of our public spaces. It says, 'this is your park, this is where you live.'"

Once, when he was leading a boy's school through the park, Imlay slipped on a wet rock and fell deep into the woods. As they waited for an ambulance, Imlay reached into his pocket and pulled out information on the native river birch. When Aistis finally reached him, Imlay didn't tell her he had a broken hip. Instead, he said, "I think we made real progress today."

That story has made the rounds among Imlay's regular volunteers, and it doesn't surprise any of them.

"If you were looking for Marc Imlay, he would be in the bushes," said David Lai, a senior at the University of Maryland who got to know Imlay through the College Park Scholars Program. Later, he became president of Alpha Phi Omega and helped organize invasive plant removal programs with Imlay.

Of all the service projects the fraternity runs, the invasive plant removals are among the most popular. The group has been coming to Cherry Hill, Magruder Woods and Paint Branch Park for nearly four years, and it brings APO chapters from other schools sometimes, said Barbara Rodriguez, the College Park chapter service vice president of Alpha Phi Omega.

Rodriguez and Lai are regulars with Imlay; he also has his regulars at the Charles County parks. Bruce Kirk, a retired Navy civilian employee, tries to get out a couple weekends a month with Imlay at Swann Park and Chapman Forest. Kirk, who is 67, said he enjoys Imlay's lectures on the plants as well as his energy.

"I can't believe his stamina," Kirk said. "He's older than I am and he never seems to be tired."

Most of these volunteers associate Imlay with invasive plants removal, but his "life's work" came to him in his later decades.

A native of Wheaton, MD, Imlay earned a bachelor of science from the University of Maryland. He also met his wife, Alice, a painter, there when he hired her to teach him how to paint. He went on to earn a master of science in biology at American University, and a Ph.D. in biology at Northwestern University in Evanston, IL.

He spent his early career growing clams and mussels at the National Water Quality Laboratory in Duluth, MN. He then returned to Washington, DC, where he was responsible for listing close to three-dozen species on the endangered list at the U.S. Fish and Wildlife Service. Because that work ruffled some feathers, Imlay said, the agency stationed him in Columbia, MO, for almost a decade, working on mussels. But his absence from Washington did not temper his activism. He became known as the "dam buster" for stopping dams in Wisconsin, Tennessee, Virginia and Missouri. He came back to the watershed in 1985 to work as a wildlife specialist for two naval installations.

In 1990, Imlay went to work for the National Guard, identifying wetlands on Army-owned lands. "The Army said, 'we want to see what happens if we hire a tree hugger,'" Imlay recalled. What happened, he said, was better management and more protection of sensitive habitat. Imlay estimates that his work has led to more than one million acres of military lands permanently protected - either from invasives, timbering, erosion or over-sedimentation.

Imlay's father, Ralph, a geologist known as "Dr. Jurassic," mostly took working vacations. Marc Imlay did as well, but instead of hunting for fossils, his four children searched for rare clams, endangered snails and indigenous mussels. His children would accompany him as he picked up earthworms and measured their biorhythms. Instead of running the air conditioner in the summer, the Imlays would go on a canoeing trip to cool off. Or, they'd go for a walk in the woods.

"We would go to the national parks and go camping and he'd always pull out the geological maps and find the woods where nobody went," said his daughter, Laurel Imlay, who now works for the Sierra Club's Maryland chapter.

In 1994, Imlay took a vacation to Hawaii. But work found him soon enough. He was touring the Waimea Canyon, whose startling colors have earned it the nickname, "Grand Canyon of the Pacific," and met a group of volunteers who were clearing invasive plants. Imlay joined them for lunch. They had lists of plants that needed to be removed, maps of where they were, and a cadre of volunteers ready to work. He was so moved by their success that he considered retiring to Hawaii.

But a battle was awaiting him at home. Developers had planned to turn Chapman Forest, a 2,000-acre woodlands in Charles County, into a planned subdivision with thousands of homes. The developers had won nearly every approval necessary, so the fight appeared futile to some. But Imlay, along with Mattawoman Creek activists Bonnie Bick, Jim Long and Rod Simmons, lobbied the state to save the property. In 1998, then-Gov. Parris N. Glendening bought the land for $28 million. It remains one of the most expensive and controversial land deals in the state's history.

With that skirmish behind him, Imlay turned his attention to English ivy. Taking a page from the Hawaiians, he put together maps across Maryland and Virginia where the invasives were the thickest. He retired from the federal government, taking a job as conservation biologist for the Anacostia Watershed Society. But on weekends and evenings, he spent his own time in the woods.

"The only reason I focused on invasive species is because it was a big hole. It wasn't being taken care of," Imlay said. "People don't spend much time in nature, and they don't know what's going on."

By the time Aistis met him in 2005, Imlay was singularly focused on removing invasive species. Twice a month, his daughter Laurel joins the effort, leading her own groups at Swann Park and Chapman Forest.

Imlay has three other children and eight grandchildren. Twin boys Sherwin and Glen are both doctors; each has a set of twins of their own, plus one more child. Keith, his youngest, is an engineer with two children. Laurel, the oldest, also has two children.

As he walks through a patch of vinca and instructs the students how to remove it, Imlay shows no signs of slowing down. His hip injury put him out of commission for a few months, but he's back. The students may be 50 years younger, but they have to keep up with him, not the other way around.

Asked when he'll stop, Imlay says he won't. "I can't, I'm just not able to."