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Issue No. 0191 - October 9, 2005
by Sue Sweeney
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| 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. |
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| 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 |
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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. |
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| 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 |
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| 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. |
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| Pictures: pale swallow-wort and black
swallow-wort flowers Stamford CT 2005 Note the hairs on the
black swallow wort petals (to the right). |
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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. |
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| Picture: black swallow-wort is all its
fall glory, September 2005 Forest and Prospect Streets,
Stamford CT |
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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. |
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| Pictures: black and pale swallow-wort
leaf samples Stamford CT 2005. |
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| Pictures: black swallow-wort pods --
details, September 2005 Forest and Prospect Streets,
Stamford CT |
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| Picture: immature pods of a black
swallow-wort, Hoyt Street Alley, Stamford Ct June 2005.
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| Pictures: black swallow-wort, Bedford and
North Streets, pale swallow-wort, Thrid Street, both
Stamford Ct July 2005 |
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