The Science of the City: Winter helps us tell native plants from invasive foes

 By PAUL RUFFINS 

In a few months, this region will explode with solid walls of green leaves that often reach from ground level to the tops of the tallest trees. This biomass helps clean the air, prevent soil erosion and absorb stormwater runoff. Porcelain berry vines produce flowers that feed local pollinators and edible, speckled berries that nourish birds.

 However, all that summer greenery can camouflage a deadly conflict. In early winter, after many species have gone dormant but before it snows, it’s easier to see that many plants are fighting each other for survival. Many local species are losing ground to invaders. Porcelain berry is beautiful, but a menace to native trees and shrubs.

Along Route 1 from Mount Rainier to far beyond the Beltway, several patches of bamboo are growing out of control. Near the corner of Calvert Street, in College Park, is a towering maple that has shed all of its leaves. But most of its trunk and branches remain bright green because it’s under assault from English ivy. This evergreen shades branches from sunlight, competes for nutrients, and holds moisture which can rot tree’s bark.  On the bike trail from the West Hyattsville Metro station to Lake Artemisia, native shrubs are buried beneath dense mats of kudzu, and trees are dragged down by ropey vines of Chinese wisteria.

Some invasive species, like the Burmese python or the snakehead fish, were probably introduced by accident. By contrast, many invasive plants were deliberately sown for their positive attributes. Kudzu is the ultimate example of unintended consequences.

 According to the Georgia Public Policy Foundation, “The ‘Dust Bowl’ 1930s also saw millions of acres of cropland in the South badly eroded and continuing to erode. The newly created federal Soil Conservation Service encouraged farmers to plant kudzu — a perennial legume introduced from Asia in the 1870s as an exotic vine — on their eroded and gullied lands. Some farmers were paid as much as $8 per acre by the Service to do so.”

Kudzu is a good forage for cattle, with a nutritional profile similar to alfalfa. It can grow as much as a foot a day and helps restore the damage done by soil-depleting crops such as cotton, corn and tobacco. During the Great Depression, it seemed like a godsend. Over 2 million acres were planted in the south, one-third of which were planted with government subsidies.

“The problem,” said Andrew Kling, of the University of Maryland Agricultural Extension Service, “is that it grew much faster than the cattle could eat it. Over the last century, kudzu has marched steadily north and overrun hundreds of millions of acres.” 

Since several invasives are already deeply established in this area, do they play any positive role in the local ecosystem? According to Jodi Beder, a member of the master gardener program in Prince George’s County, “Some of these plants might help absorb runoff water or purify the air, but none of them do that nearly as well as healthy trees. College Park, Hyattsville and Mount Rainier are all so concerned about preserving their tree cover that they’ve instituted hefty fines for cutting large trees without a permit. But invasives are killing far more trees than homeowners.”

If damaging trees is the worst sin an invasive can commit in the suburbs, the worst offender is probably English ivy. In his newsletter, “The Invasives in your Woodlot,” Kling explains why the vine is such a threat: “This evergreen vine threatens habitats at all heights. At ground level, its leaves shade out seedlings and herbs, forming acres of monoculture and attracting rodents. In trees, it engulfs branches, shading and slowly killing them. Its weight topples trees in wind, snow, or icy conditions. It serves as a reservoir for bacterial leaf scorch, a serious disease of trees including maples, oaks, and elms. Vines mature in trees, then flower and bear toxic berries which induce birds to vomit them out, ensuring spread.”

A College Park survey determined that the majority of its tree cover is in residential neighborhoods — this pattern probably holds for Hyattsville, Laurel and many other jurisdictions. As a result, there have been several efforts to enlist residents in the battle to uproot English ivy or at least cut it off of tree trunks. Volunteers from Hyattsville’s Green Team teamed up with Hyattsville Aging in Place members to remove English ivy from the property of elderly residents. As part of the Maryland-National Capital Park and Planning Commission’s (M-NCPPC) adopt-a-park program, a group of trained volunteers removed English ivy and other vines from College Park Woods.

Unfortunately, the fight against English ivy is ongoing, and the number of people who volunteer for these formal efforts can be limited because the M-NCPPC and other organizations require that volunteers undergo fingerprinting and background checks. This makes sense for school volunteers but can be a lot to ask of someone who just wants to cut down vines for a day or two. Therefore, the most efficient solution might be to mobilize residents to act in their own self-interests and remove the English ivy on their own property.

  Todd Reitzel, a member of the College Park Tree & Landscaping Board, has been an active proponent of this approach. In 2023, he urged the city to make and distribute door hangers asking residents to remove English ivy from their trees. He also suggested creating a video on how to do it safely and effectively. Simply pulling it off can damage the bark of the tree, and though it isn’t as dangerous as poison ivy, the sap of English ivy can irritate some people’s skin.

Bieber thinks this is a great approach but said, “Unfortunately, I don’t know of any community that has succeeded in convincing large numbers of homeowners to remove English ivy. If people saw thick loops of Chinese wisteria vines hanging down from their trees, they’d probably suspect that something was wrong. But we’ve all seen movies featuring magnificent old mansions covered in ivy. Some people like it because it’s evergreen and think that it’s safe because it’s sold at Home Depot. So we’ve got to do a huge education job to teach people to get rid of it because it’s viciously attacking their trees.”