"As the world becomes a smaller and smaller place, and we move things from place to place more, we increase the number of species that are moving across what would otherwise be insurmountable geographic boundaries," such as oceans, Kyde said.
Not all the invaders are worth government eradication efforts. For example, dandelions, originally from Europe, are a nuisance to homeowners who want putting-green smooth lawn - but they don't threaten other plants or animals.
On the other hand, a few exotics are monstrous. Among the worst: The dreaded Heracleum mantegazzianum, or giant hogweed. This fleshy-leafed shrub grows up to 15 feet tall and has sap so acidic that touching it can cause blisters and swelling, and contact with the eye can cause permanent blindness, according to the state wildlife agency.
The state is trying some creative tactics to fight rogue plants. It's not just using weed whackers - which can make the problem worse by scattering the alien seeds.
In Howard County, officials over the past two years have been releasing a variety of Asian weevil, Rhinoncomimus latipes, to gobble up the rampant Devil's Tear Thumb, Kyde said.
It's fighting fire with fire. Exotic species are being used to attack exotic species. The state doesn't believe the introduced insects will multiply out of control because just about all they eat is Devil's Tear Thumb, said Kyde.
Brooks, the park ranger, assembled a handful of local plant enthusiasts at a picnic table yesterday morning beside a small lake in Patapsco Valley State Park.
He held up the spiny stalk of a kudzu vine that he had ripped from a roadside area near the park's entrance. He explained that this Asian plant has crept steadily into Maryland in recent years, overwhelming trees and blocking their light.
Then he led the group on a hike along a shady path beside the Patapsco River. He pointed out the long rows of pointed leaves that mark the exotic Tree of Heaven, or ailanthus, whose dense roots proliferate underground to spread offspring.
"It creates great shade," he said, holding up a branch. "But its roots are so strong they can break up concrete and driveways."
Beside the path, he pointed out the spade-shaped leaves of a garlic mustard plant. This Eurasian shrub was imported as a spice for soups and stews during the 1800s, and some people consider it tasty boiled like spinach.
"I've had it on salad," Brooks said.
But ecologically, it's poison. The garlic mustard suppresses the growth of fungi that other plants need to break down rotting plant matter and make nutrients available so they can grow.
Helen Orem, a teacher from Halethorpe who took the tour, said that more people should be educated about exotic plants. "I think people should know the damage that non-native plants can do to our local plants," she said. "I didn't realize that they could actually kill our native trees."
tom.pelton@baltsun.com
