Invasives threaten National Arboretum
Foreign pests take toll on trees
Foreign plants and insects intent on wiping out native flora have invaded the National Arboretum and the surrounding D.C. area, forcing gardeners and horticulturalists alike into a silent war against destructive pests.
At the arboretum’s New York Avenue NE entrance, green creeping vines — English ivy — hugs the ground, blocking sunlight from reaching the plants beneath. In other spots around the arboretum, the weed can be seen clinging to trees and shrubs.
Elsewhere, woody vines — some with black round berries, others with fragrant white flowers — have grown up thick and formidable. These plants may look harmless at first glance, but they have exacted a toll on the native plant life in the arboretum and the D.C. region.
“There are a ton of invasives we have to fight here,” said Arboretum horticulturalist Joe Meny. “But if you look, you can see the problem pretty much anywhere.”
These plants — often woody vines — have taken over many
areas in which plenty of sunlight is available.
“Historically, you would find vines in these types of places where there was light — where, say, a hurricane had knocked down some trees. Now, particularly in urban areas, lighted areas are everywhere,” said Lea Johnson, an ecology professor at the University of Baltimore.
So much available sunlight speeds up the already-rapid growth of these aggressive vines — Japanese honeysuckle and English ivy among them — and forces the arboretum specialists into a constant fight to slow their growth.
Ferns and azaleas, as well as oaks and dogwoods, are just a few of the species being threatened by the weeds.
But ash trees in the Arboretum and around the country are taking a hit too.
Emerald ash borers originated in Asia, and were first found in Michigan in 2002. The bright-green beetles now are found in 22 states and wreaking havoc on the inner bark of valuable ash trees.
Borer larvae gorge themselves on the channels the trees use to receive water and nutrients, and can kill them within two years of infestation.
This insect menace has become so large that many states have outlawed the interstate transportation of ash wood for fear of spreading the problem even further.
Economics plays a role as well. Consider the ash tree, which often is planted in urban areas like the District and Baltimore because of its beauty.
“It depends on the size of the tree, but a full-grown ash tree can be a great deal of money to remove,” Mr. Meny said.
The vines and weeds that cause so much environmental damage are not cheap to get rid of either.
Japanese honeysuckle, English ivy and Porcelainberry have deep root systems that are almost impossible to remove completely, despite the many herbicides the Arboretum uses against them.
Even so, something must be done quickly as the weeds are choking out the native plants that inhabit D.C. and beyond.
For instance, English ivy can girdle a tree or block its leaves from sunlight, inhibiting photosynthesis. Gardeners sometimes allow the dark-green vine to grow on its own because the ivy is thought to be attractive. However, the weight of years-old ivy can cause a tree to fall in a strong wind or heavy rain.
Similarly, Porcelainberry and Japanese honeysuckle are often bought and planted by those who believe them to be harmless. But these Asian weeds grow along the ground and block light from native plants. Birds spread their seeds, and the invasive process begins anew in another area.
These weeds and insects, like the borer, endanger the arboretum’s plants and trees, requiring persistence by horticulturalists and arborists to stem the invasion. Still, household gardeners should be aware of their role in spreading the problem.
Sandy Kemper, a longtime volunteer at the arboretum, suggests signing up to help, as he did years ago. “I absolutely recommend volunteering. I love it here. If you enjoy plants, Washington, D.C., is the virtual plant mecca,” Mr. Kemper said.
For Ms. Johnson, the battle against invasive species is a must-win effort.
“The Arboretum has done a lot for the horticulture world. It is a great place to get inspiration for what you might want to plant in your own garden, and it is really just beautiful,” she said.
Ms. Johnson said she takes her University of Baltimore students to the Arboretum every year. “It’s truly a great example of a natural habitat right in the city.”
English ivy is an invasive plant that has crept into the National Arboretum and throughout the D.C. region. The plant’s fast-growing green vines block sunlight from reaching native flora beneath it. Nationwide beetle infestations have also killed many native trees.