A European invader has been sneaking onto the New England coast, infiltrating and undermining the natives.

The large gray willow, a dense shrub or small tree that spreads rapidly and closely resembles a native pussy willow, has been flying under the radar for years, colonizing the edges of ponds and crowding out rare plants and animals on Cape Cod and Rhode Island.

U.S. Department of Agriculture maps show the willow has been identified throughout the eastern United States, but it is not on any national or state list of invasive plants.

It could be listed soon, however. It poses a clear threat to native plants on the New England coast and could be equally invasive in wetlands and along streams farther inland, said U.S. Forest Service botanist Thomas Rawinski, who is based in Durham, N.H.

Other countries that have been invaded by the species, also called the European gray willow, paint a grim picture. New Zealand considers it a major “pest plant” that threatens the nation’s remaining wetlands. Australia calls it the worst of the invasive willows and warns it can cross-pollinate with other willows.

This spring, Rawinski checked out reports of a willow that was spreading rapidly in Massachusetts and accurately identified it as Salix cinerea, the large gray willow. He sounded the alarm to land managers around the region and began getting queries and specimens from all over.

Rawinski is most concerned about the threat to coastal plain ponds on Cape Cod and Rhode Island that are host to a complex of rare insects, animals and plants, including the Plymouth gentian, rose coreopsis, hyssop hedge-nettle and slender marsh pink, as well as rare dragonflies and damselflies.

He also has seen it in dune swales – small wetland areas in the troughs of dunes – and near salt ponds and marshes, environments that are fragile ecologically.

“It does look like many of the native willows,” Rawinski said. “That’s one reason the gray willow was undetected for so long. Botanists would go to Cape Cod all the time to see these rare plants and they’d walk right by it and not notice it – including me.”

Rawinski has confirmed the European willow’s presence on Cape Cod and in Rhode Island with other botanists and expects to find it in coastal areas stretching from Maine to Long Island. The Harvard Herbaria in Cambridge, Mass., has a specimen collected in Old Orchard Beach, Maine, in 1967, so the willow has a lengthy head start on efforts to control it.

Tim Simmons, a restoration ecologist for the Massachusetts Natural Heritage and Endangered Species Program, is working with Rawinski. Simmons said he will try killing the willow this fall around a Cape Cod pond.

In addition to rare plants, Simmons is concerned about preserving habitat for the Plymouth redbelly cooter, a federally designated endangered turtle found at only a dozen ponds in Massachusetts and Rhode Island.

“This willow has the potential to take over some of the [turtle’s] nesting habitat on the upper reaches of some of these ponds,” he said. “It’s a real problem for us to get surprised by this in habitats that don’t usually tolerate invasive species.”

Killing the willow won’t be easy, because it spreads both by lightweight seeds that can travel long distances by wind or water and through broken twigs that take root.

Simmons plans to cut down the plants and immediately paint the stumps with herbicides approved for use in wetlands. He will share his findings with land managers around the country.

“We’re going to have to try different herbicides, because there isn’t a lot of literature on this,” he said.

Lisa Gould, senior scientist for the Rhode Island Natural History Survey who is also working with Rawinski, says she wants to know whether it poses a serious threat inland as well.

“When I’ve been out looking myself, it really seems it’s all over the place,” she said.

She and Rawinski plan to send samples from around Rhode Island to George Argus, an international willow expert based in Merrickville, Ontario, who has developed a computer database for willow identification, she said.

“It’s one of those groups of plants you kind of have to spend your life on to be a willow expert, which I will never be,” she said.