By Karen Klinger
It is likely most residents have never heard of it, but a noxious, alien weed that is harmful to butterfly populations and bird habitats, can smother native plants and is called a “dog-strangling vine” for its aggressive growth, is spreading like wildfire across Cambridge.
A member of the milkweed family, it is named the black swallow-wort (Vincetoxicum nigrum), and a study by members of an environmental field research class at Lesley University taught by conservation biologist David Morimoto found over 2,600 of the invasive vines in a single square block of the city’s Agassiz neighborhood, near the school’s main campus.
At a recent presentation of their findings, the students said the results prompted them to create a brochure titled “Invasive Species Alert!” which they plan to distribute to promote public awareness of problems posed by the prolific weed.
It isn’t known exactly when the black swallow-wort, a native of the Mediterranean regions of Western Europe, was imported to North America, but the first report of its existence in the United States was in Ipswich in 1854. Ten years later, a collector in Essex County said it was “promising to become naturalized” and by 1867, it reportedly had “escaped” from a botanic garden in Cambridge and started spreading out of control.
Only within the last two decades or so, however, have the numbers of the black swallow-wort, and a cousin named the pale swallow-wort, “exploded,” according to Antonio DiTommaso, a weed science researcher at Cornell University.
The two types of swallow-worts can now be found across much of New England, the Mid-Atlantic States and the Midwest as well as southern Canada. In Europe, the plants are kept in check by certain insects and diseases, but DiTommaso says they have no natural enemies in North America and because they contain strong poisons, deer and other animals will not eat them.
Although it can crowd out native vegetation and alter the habitat of nesting birds, arguably the biggest ecological threat the black swallow-wort poses is to the monarch butterfly, the most recognizable lepidoptera species on the continent. Monarchs reproduce by laying eggs on the leaves of the native common milkweed, which the larvae then feed on. But if by mistake the butterflies deposit eggs on the milkweed’s relative, the swallow-wort, the larvae die.
With this in mind, and knowing the infamous role that Cambridge played in the original spread of the black swallow-wort, the Lesley students set out last fall to count every vine of the weed they could find in a square block area bounded by Massachusetts Avenue and Garfield, Oxford and Sacramento streets. The study site includes the Sacramento Street Community Garden and an adjoining athletic field.
They found the vines, bearing over 1,800 seed pods, on public and commercial property, in residential yards and the community garden and climbing, especially, on chain-link fences. Because the long-lived perennial is amazingly adaptable, able to grow in sun and shade, in soil that is moist or dry, acidic or alkaline, and unaffected by freezing temperatures, the results were not surprising, but still alarming.
The students decided to put together a brochure alerting the public to the environmental threat that exists, literally in some cases, in their own backyards. The aim is to help Cambridge residents and owners of commercial properties learn how to identify and remove the plants. Some of the points the brochure makes:
- The black swallow-wort has a smooth green or brown stem and is often found wrapped around fences or other plants. During the growing season it has dark green, slightly shiny leaves that are oval-shaped and come to a point at the tip.
- The plant produces tiny purple-black flowers with five petals and its fruits are three-to-four inches long, oval in shape and pale green or brown. They often form in opposite pairs.
- It can be very difficult to remove because it can reproduce asexually via underground stems known as rhizomes. Cutting vines will not destroy them unless you are able to dig down and take out all the roots.
- It can be killed with chemicals (such as the main ingredient in the herbicide Roundup). But it is a good idea to consult with a professional before going that route.
With the growing season just beginning, the Lesley students recommend that people who have black swallow-wort on their property take steps to remove it as soon as they can, before the seed pods start maturing in July.
Initially, they intended to distribute the brochure just in the Agassiz neighborhood, but they say they’ll welcome any help they can get, especially from the city of Cambridge, to extend their information to the entire community.