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The Monday Garden
Swallow-wort: Monarch Menace

Issue No. 191 - October 9, 2005
by Sue Sweeney

An attractive “easy-to-grow” milkweed-family vine, black swallow-wort, was imported to New England from southern Europe in the late 1800’s as a garden ornamental and slowly began to spread west and south, across the egg-laying territory of North America’s awesome monarch butterflies, those mighty, tiny, flying jewels that migrate from as far north as Canada all the way to Mexico.
Picture: This wonderful photo, donated by a reader of The Monday Garden in Derry, New Hampshire, shows one of our lovely monarchs enjoying a local pond as the butterfly slowly makes its way to Mexico. photo: © 2005 Melissa Bolton
Black swallow-wort, Cynanchum louiseae (also known as Cynanchum nigrum, Vincetoxicum nigrum), is a bad, bad invasive plant, and we can say all the nasty things about it that apply to the other invasive perennial vines. It lurks along sunny borders and fences, smothering everything in sight. Fortunately, the vine only grow about 6 feet long, so black swallow-wort can’t pull down trees the way Asiatic bittersweet and porcelain berry do. Otherwise, the vine is nasty enough to justify its alias “Dog-Strangling Vine”. I’m sure a dog going through a fence could easily get tangled in it.
Pictures: black swallow-wort on a fence in Stamford CT is strong enough to bend the fence’s finials. Bedford and North Streets, Stamford CT September 2005; swallow-wort getting established in a yew bush. 4th Street, Stamford CT September 2005
SWALLOW-WORTS: If you happen to be a connoisseur of swallow-worts, there are two kinds in the Cynanchum family that are an invasive threat to North America: the black swallow-wort and its kin, the pale swallow-wort, Cynanchum rossicum. The main difference between the black and pale cousins is the flower color and other flower detail. The pale version’s flowers are light brown to dull red; the black’s are dark purple to almost black. The black swallow-wort petals are only about as long as they are wide and sport tiny hairs. The pale swallow-wort petals are hairless and longer. As a general matter, if you find one Cynanchum swallow-wort, you’ll find the other not far away and they’re equally bad in all ways. As far as I can see, there are no practical value in being able to tell the two apart.
Pictures: pale swallow-wort and black swallow-wort flowers Stamford CT 2005 Note the hairs on the black swallow wort petals (to the right).
However, you should know that there are a bunch of other plants, often tropical or Asian in origin, also called “swallow-worts”; some are milkweeds, some aren’t. It is sometimes said that the name gets attached to plants that bloom when the local swallow birds do something interesting, like return from migration. In any case, the only two swallow-wort vines that are currently menacing North America are the two mentioned in this article (called just “swallow-wort” for simplicity). However, there’s no need to expand the plague by importing others. As you know, milkweeds have a fly-away seed that can travel miles. Therefore, they are very capable of becoming invasive. It’s good to plant milkweed for the monarchs, but, please, stick to our wonderful natives discussed in Issue 167.

RANGE: To date, the two thug swallow-worts have spread to at least 20 states and several Canadian provinces.

ERADICATION: This is a plague that is best stopped before it gets started. You’ll see on the Internet, pleas by local horticultural guardians in the “border” states imploring residents to watch for and eradicate the swallow-worts before they get established. So learn what this plant looks like and keep it from getting started in your neighborhood, if you can. The initial colonists can be hand pulled.

Once swallow-wort is established, as it is where I live, it’s as hard to get rid of as most other invasive perennials. Dig it up and you disturb the seed bank; also any missed root fragments start to grow. Use a herbicide and it’s the proverbial elephant gun problem: you may kill the plant but at what possible risk to kids, and dogs, and frogs? You can stop the spread by removing the pods before they burst. In the open, mow swallow-wort to the ground, 2 or 3 times a year before the pods mature. Along a fence, if there are no valuable near-by plants, dump a couple of feet of mulch on it; repeat annually. See Issue 160 on controlling invasive plants.

Picture: black swallow-wort is all its fall glory, September 2005 Forest and Prospect Streets, Stamford CT
MONARCH THREAT: So what do these two stupidly-imported vines have to do with our beautiful butterflies who are already threatened in many ways by humans’ urbanizing ways?

As readers of The Monday Garden know, a baby monarch’s sole food is milkweed (See issue 167), which contains certain chemicals that monarchs need and most other critters hate. Female monarchs have been found to readily lay their eggs on swallow-wort, as they would on any other milkweed. What momma doesn’t know is that the chemical composition of the swallow-worts isn’t right, and her children will die. According to Internet sources, this discovery was made by Jennifer Dacey, then a student at University of Rhode Island.

Pictures: black and pale swallow-wort leaf samples Stamford CT 2005.
Pictures: black swallow-wort pods -- details, September 2005 Forest and Prospect Streets, Stamford CT
Picture: immature pods of a black swallow-wort, Hoyt Street Alley, Stamford Ct June 2005.
Pictures: black swallow-wort, Bedford and North Streets, pale swallow-wort, Thrid Street, both Stamford Ct July 2005
From The Monday Garden. Copyright © by Sue Sweeney. Reproduced with permission. 

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