I came home from a week’s vacation to find that my garden had virtually “exploded” while I was gone. The heavy rains of June followed by the July heat wave turned my garden into a lush jungle of towering lilies, heliopsis, and hydrangeas. The explosion of flowers was coupled with an explosion of weeds, including the invasive black swallow-wort vine that I battle every year.
Also known as Louis’ swallow-wort, Cynanchum nigrum is a member of the milkweed family imported to the U.S. from its native Europe as an ornamental vine. It was first noted in Ipswich, MA in 1864, “escaping from the botanic garden where it is a weed and promising to become naturalized.” Since then, black swallow-wort has become an invasive nuisance in the Northeast and Midwest, crowding out native plants in fields and forests. It is also a deadly host for the monarch butterfly, which lays its eggs on the vine, but its larvae do not survive feeding on this plant. Conservancy groups throughout the country are trying to educate the public about the threat of this plant to the environment.
The vines emerge in the spring, and can literally grow a foot overnight in wet weather. They quickly wind around your precious peonies, pulling the heavy blooms down to the ground, sprawl over entire shrubs, or grow right through the center of a rare epimedium that you bought from a hybridizer for a tidy sum. These vines thrive everywhere, from sun or shade, rich humus or heavy clay, alkaline or acidic soil.
Black swallow-wort sports glossy oval-shaped leaves with pointed tips, 3-4 inches long, that occur in pairs along the stem.
The small, dark purple flowers are star-shaped and borne in clusters.
The flowers are self-pollinating and quickly produce a bounty of seed pods that dangle from the stems. The seeds are equipped with their own downy parachutes that aid in wind dispersal, which begins in late July and continues through fall. The plants also spread through underground rhizomes that form large clumps.
So how do we control this thug in the garden? As with all invasive species, early detection and removal is the best approach. There are no biological controls for black swallow-wort available in the U.S. Mowing will not eradicate the plants, but will at least prevent them from forming seeds. Once seeds have formed, the plants should be carefully cut down and bagged so that the seeds don’t disperse. The vines can also be dug out, but care must be taken to remove the complete root crown, which is difficult to do. Even a tiny piece of white root left behind will soon sprout into a new plant. Dug plants should be burned or bagged and disposed of in the trash.
Although I dislike using herbicides in the garden, chemical control is recommended as the most effective means for large, established infestations. Glyphosate (Roundup) is effective if sprayed on when the plants are in flower (prior to that, the plants do not have enough leaf surface area to deliver a killing dose to the roots.) If the black swallow-wort is twining around desirable plants, the glyphosate can be applied with a sponge or brush. Another alternative to spraying is to cut and dispose of the stems and apply a 100% solution of the herbicide concentrate directly to the cut stem surfaces with a small brush.
Unfortunately, once black swallow-wort appears in your garden, you will be battling it for several years. So don’t wait – if you see even the tiniest sprout of this tenacious vine, pull it immediately! Or you may return from vacation later this summer to find your favorite plants “swallowed up” by this insidious creeper!
Photos courtesy of Marie Brown