The National Park Service, which has battled coyotes, prairie dogs, gypsy moths and even litterbugs over the years, is taking on a new foe: Washington-area plant life.

Trees, ferns, weeds and even flowers are threatening historic structures around Washington, officials say. On the dome of the Jefferson Memorial, cottonwood seedlings have taken root; and in the fragile earthworks of the 120-year-old Civil War forts that surround Washington, tree roots are opening gaps and aiding erosion. At Harper's Ferry, the old industrial area of Virginia Island has been covered by trees, plants and ferns.

More severe threats are found along the C&O Canal than in any other single property in the region. There, plant intrusions threaten to harm or destroy canal banks, locks, culverts, aquaducts and lock houses, said Paul Goeldner, the National Park Service's regional chief of historic resource sevices.

A new Interior Department program to study and correct such problems is under way, and this year $226,600 will fund nine projects nationwide, including up to $15,000 to find ways to "combat vegetative destruction to historical structures" in the National Capitol Region of the Park Service.

Locally, Goeldner said, most of the threats from plant life can be corrected with more frequent maintenance, but some require a careful, scientific analysis of ways to preserve both the historic structure and the vegetation.

"The ultimate place we're taking this policy is to determine how to make the vegetative threat less extensive," he said. "We can't indiscriminately love nature and all of its manifestations in this policy."

Throughout the area, 1,100 of the 1,300 historic structures are of sufficient importance and fragility that they need to be preserved exactly as they were originally, Goeldner said. Through a contractor, the National Park Service will try to detemine the extent of the problem throughout the region and ways to correct them.

Other problem areas include tree roots that are upsetting one side of the foundation of the Clara Barton House at Glen Echo; rampant weeds at Antietam Battlefield National Cemetery near Sharpsburg, Md.; and ferns and weeds invading the masonry of Fort Washington Park across the Potomac from Mount Vernon.

The ivy growing into the clay walls and wood frame of the Worthington House at Monocacy National Battlefield in Maryland is considered the most extensive example of "plant damage" to a historic structure in the region.

The house was there as the Civil War raged in nearby cornfields at the time of a July 9, 1864, battle, but because of what officials called misuse by prevous owners and the ivy intrusion, it is falling apart. The battlefield and house are located around a knoll that can be seen from I-270 going south from Frederick, Md., after the trees shed their leaves in autumn. Contributing to its current condition is the fact that at one time it was used as a home for migrant laborers in the area.

"We are close to losing the Worthington House," Goeldner said.

At both Monocacy and the Manassas National Battlefield Park, the vegetation has threatened a unique type of historic structure: the fields of battle. He said descriptions of historical events often are grounded in landmarks such as homes, fields and stands of trees.

Already the changing vegetative patterns at these parks have made it difficult to visualize the battle from descriptions offered by the Park Service. "There are fields where trees or crops once stood," he said, "and that confuses the visitor.

"We have the ultimate goal of achieving the historic vegetative patterns there," Goeldner said. He said that the Park Service may be interested in opening vistas through the forest, though "there are a lot of tree-huggers who would probably object."

"In a place where nature is fragile and man's permanent imprint is hard to erase," Goeldner said, "protection of both is the rational action."