Photo by Melody

PlantFiles: Black Swallow-wort, Dog-strangling Vine, Louis' Swallow-wort
Cynanchum louiseae

Family: Asclepiadaceae (ass-kle-pee-ad-AY-see-ee) (Info)
Genus: Cynanchum (sigh-NAN-chum) (Info)
Species: louiseae

Synonym:Cynanchum nigrum
Synonym:Vincetoxicum nigrum

One member has or wants this plant for trade.

Vines and Climbers

4-6 ft. (1.2-1.8 m)

24-36 in. (60-90 cm)

USDA Zone 4a: to -34.4 C (-30 F)
USDA Zone 4b: to -31.6 C (-25 F)
USDA Zone 5a: to -28.8 C (-20 F)
USDA Zone 5b: to -26.1 C (-15 F)
USDA Zone 6a: to -23.3 C (-10 F)
USDA Zone 6b: to -20.5 C (-5 F)
USDA Zone 7a: to -17.7 C (0 F)
USDA Zone 7b: to -14.9 C (5 F)
USDA Zone 8a: to -12.2 C (10 F)
USDA Zone 8b: to -9.4 C (15 F)
USDA Zone 9a: to -6.6 C (20 F)
USDA Zone 9b: to -3.8 C (25 F)

Sun Exposure:
Full Sun
Sun to Partial Shade

Parts of plant are poisonous if ingested

Bloom Color:

Bloom Time:
Late Spring/Early Summer
Blooms repeatedly


Other details:
May be a noxious weed or invasive
Average Water Needs; Water regularly; do not overwater
Self-sows freely; deadhead if you do not want volunteer seedlings next season

Soil pH requirements:
5.6 to 6.0 (acidic)
6.1 to 6.5 (mildly acidic)
6.6 to 7.5 (neutral)

Patent Information:

Propagation Methods:
Unknown - Tell us

Seed Collecting:
Bag seedheads to capture ripening seed
Allow pods to dry on plant; break open to collect seeds
Allow seedheads to dry on plants; remove and collect seeds
Properly cleaned, seed can be successfully stored

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1 positive
No neutrals
10 negatives

Gardeners' Notes:

Negative tutusmith On Sep 21, 2014, tutusmith from Davisburg, MI wrote:

Just found this vine on our property in Davisburg, Michigan. Not happy after seeing how it is climbing up and trying to choke out other plants. Seems it will be very hard to get rid of on our acreage as it takes digging or using roundup which I try to avoid as it is poisoning our soil everywhere. Please do not plant this if you do not have it already.!!! It is a problem for the Great Lakes region and can cause Monarch caterpillars to die after they are laid as the vine is not their food.

Negative coriaceous On Feb 21, 2014, coriaceous from ROSLINDALE, MA wrote:

I've seen this noxious weed replace grass in lawns and meadows. It also grows well in shade.

This is a perennial weed and an invasive plant destructive of natural areas over a wide swath of northeastern and midwestern North America (and California). Its planting, trade, and transport is illegal in my state and three others.

And, yes, it's responsible for significant Monarch butterfly

It's late to emerge from dormancy, often not till June in Z6a. It begins to shed seeds in late July and continues till frost. If you cut the stems or harvest the seedpods, it can produce more that same season.

At least it IS possible to eradicate it from your yard. Here's how:

Just pulling on the stem makes it snap off close to the surface. This does no good. (Nor does weekly mowing.)

It's not difficult to dig out, with the right tool. The fleshy roots radiate from a node about 2 inches below the surface (like tentacles radiating from the head of an octopus). If you can just dig that node out, you're generally good. I use a cast aluminum trowel for this, using a prying motion. This plant won't regenerate from broken roots unless there's a bit of that node tissue attached.

Very old plants may have several nodes along a vertical rhizome, and may require a little deeper digging. But most just have one.

Any nodes you've missed will signal you by sending up a new stem within a week.

Where you can't dig out the node---this plant often seeds into cracks in pavement and masonry---you can paint it with concentrated generic glyphosate herbicide. Do this when topgrowth is mature but before it can go to seed. (I do this around July 1 here in Z6a.) Buy the generic 41% concentrate and dilute it only to 25% (yes, that's right, 25% is the minimum for the job) by adding one part water to two parts concentrate. Cover all available green surface, using a paintbrush. And you may sometimes need to do this a second time, the next year.

One thing that's in your favor: this plant doesn't leave lots of dormant seeds in the soil. Unlike many persistent weeds, which can be controlled but not eradicated, you can actually eradicate this from your yard.

If your neighbors have this too---a good bet---don't despair. More than 99% of the seeds land within 10' of the parent plant. You'll need to monitor your property in June and July for the occasional pioneer invader, but you won't get masses of seedlings from seeds drifting in on the wind. The important thing is to keep any plants on your property (and especially your fences) from going to seed.

Negative RCCWMA On Jan 21, 2014, RCCWMA from Little Canada, MN wrote:

This plant is a noxious weed in the State of Minnesota (and others). Do not plant it. It is also a plant that is detrimental to the monarch butterfly population. Butterflies are confused by this plant and lay eggs on it, but the caterpillars die. If you have this weed on your property, please get rid of it.

Please always check to see whether that new and interesting plant is invasive before planting. This is easy to do online. Just type in the common or scientific name and the word "invasive". Thank you!

Negative Jadwin59 On Jun 4, 2013, Jadwin59 from Rochester, NY (Zone 6a) wrote:


*Please don't cultivate this plant.* It's invasive, very difficult to eradicate, and it kills butterflies that mistake it for the more beneficial milkweed. It will choke out everything else in your garden and spread, through its floating seedpods, all around your neighborhood.

Negative Dean48089 On May 19, 2013, Dean48089 from Warren, MI (Zone 6b) wrote:

I originally found this plant growing in a sidewalk crack. The glossy leaves and black flowers looked interesting, so I snatched a seed pod and raised some seedlings. At the time I was gardening in sandy loam with a lot of shade, where this plant stayed small and grew 2-3 feet up the fence. My new house, where I still live, has heavy clay soil and lots of sun -- where the black swallowwort quickly turned into a monster. Besides filling the air with billions of seeds, which germinate immediately as soon as they land, it also spreads via underground stems. The waxy leaves repel all herbicides and any little bit of root left in the ground will quickly turn into a new clump.

A local specialty nursery used to sell this plant because it is apparently the food source for a particular butterfly and the nursery's owner was really into butterflies (they no longer sell it). Unfortunately, when this plant was brought over from Europe they forgot to bring along the butterfly that eats it. the foliage, when bruised or crushed, smells horrible -- worse than Tree-Of-Heaven (Ailanthus). This plant should be the poster child for noxious weeds.

Negative theoldhorse On Jul 30, 2012, theoldhorse from Hull, MA wrote:

This is an absolutely TERRIBLE INVASIVE WEED. It will grow quickly up and engulf adjoining plants in your garden, entwining its single central stem and climbing other plant stems and foliage and jumping from plant to plant.
As it gets larger it produces many pepper-like seed pods that are heavy enough to weigh down other plants. The very tiny near-black precursor flowers are virtually unnoticeable having no aesthetic value whatsoever. It will spread from the garden to the lawn where mowing will not eliminate it.

It is extremely invasive and practically impossible to eliminate. Its central stem is attached to a spider weblike root system made up of hundreds of radiating and branching white roots. Even the smallest of plants cannot be pulled because the roots concretely anchor it to the soil and the stem breaks off. Days later it re-grows. It MUST BE dug out, and then it is not likely that all of its tiny fragile rootlets will be removed. Spraying it with Roundup will temporarily brown up the leaves but not kill the root system. I've had some success killing it but cutting the stem just above the ground and spraying the herbicide on the cut end of the stem, a method recommended by a nurseryman. Short of this time consuming and frustrating process, the garden must be constantly inspected for signs of the plant, and pulling it before it entangles itself with your desirable plants and produces seed pods.

Beware if it ever gets into your garden, or a neighbor's!!!

Negative iloveperennials On Feb 1, 2012, iloveperennials from Rochester, NY wrote:

This is probably THE WORST WEED I have on my acre...and that's saying alot because I also have poison ivy and bittersweet vine. It grows about 2-3 feet tall, twining around itself so you can't walk through it, hence one of its many nicknames 'dog strangle weed'. It makes winged seeds that the wind blows around. Roots are hard to get out IF the ground is dry. I'm trying to eradicate it by pulling it up by hand when the ground is moist but we're talking 1 plant/3 inches, BEFORE it goes to seed. You there in Seattle, who is growing it on purpose - HUGE MISTAKE! imho.

Negative LOVIE2 On Jun 30, 2010, LOVIE2 from Boston, MA (Zone 6a) wrote:

I hate this vine! It's taken over my yard and is trying to work it's way to my gardens. Round up for poison ivy works wonders!

Negative gregr18 On Jun 19, 2010, gregr18 from Bridgewater, MA (Zone 6b) wrote:

Extremely aggressive and extremely foul-smelling vine that can grow anywhere. It can strangle trees and shrubs with ease if given the chance. It smells like a dirty rabbit hutch or guinea pig cage. The smell gives me a headache. Although the dark green foliage can be attractive, I recommend killing this vine. Cut the stems close to the ground and dip the freshly wounded main stems in RoundUp concentrate.

Negative Malus2006 On Feb 22, 2008, Malus2006 from Coon Rapids, MN (Zone 4a) wrote:

This is consider an invasive species in North America and acts as an attractive for Monarch butterflies to lay their eggs on it. The larvaes have a high death rate as the plants don't have the chemicals that the larvaes need for defense and also larvaes starves to death because swallow wort don't have the right kind of nutrients for them. This vine is currently the most common around New England, most of the Great Lakes area west to Wisconsin.

Positive MontanaVineMan On Apr 21, 2006, MontanaVineMan from Helena, MT (Zone 5a) wrote:

This is a very interesting vine, I must say. So far, I have had a positive experience with it. How I acquired the plant is actually fairly funky. When I lived in Seattle, I was the landscape manager for a large and well known nursery. One day, the crew came back with this plant stuffed into a pot. They saved it as none of us had ever seen it before.

It had been growing in the middle of a patch of low growing Juniper! Obviously a very tough plant! I saved the plant for about a year or so in our company grow lot, in just a 5 gallon black plastic generic pot. I eventually moved back to my native state of Montana and brought this plant with me among countless others. I thought it would for sure die in our cold Winter climate as I had left it outside as an experiment. Well, sprouted right back in Spring, even though it looked like it had croaked. Funny, because in Seattle it stayed evergreen all year. Now that I have planted it in a bed near my home in Montana, it behaves as a deciduous perennial vine and has totally tripled in size now that it is out of its pot and has lots of room for its roots.

I find it to be a beautiful vine, with its tiny star shaped black-mauve flowers that really stink if you smell them, although the hover-flies, regular flies and some bees really love this plant! I've seen other VERY odd insects on it as well. It is definitely an insect attracting plant. The glossy leaves are also quite attractive. Mine is now well established and it produces milk-weed like pods in fall. They are full of cottony seeds and lately I'm noticing baby Black Swallow-worts in some odd places around my house and property. I hope I didn't create a weed. Doh! I've done it before...........but only by accident. Anyway, if you're looking for a vine that will grow fairly tall given support; grows fast and has very attractive leaves and tiny clusters of beautiful blacky-mauve coloured flowers, this vine might be a good choice. At least in an area where it doesn't have the potential to become weedy and crowd out any natives. Sorry this was so long!

Other names I have learned for this vine are: Vincetoxicum Nigrum, Cynanchum and "Dog Strangling Vine"........which I just can't seem to picture in my mind nor do I want to. - Cheers - MontanaVineMan


This plant has been said to grow in the following regions:

Boston, Massachusetts
Bridgewater, Massachusetts
Hull, Massachusetts
Waltham, Massachusetts
Davisburg, Michigan
Warren, Michigan
Helena, Montana
Farmington, New Hampshire
New Milford, New Jersey
Rochester, New York (2 reports)
Seattle, Washington

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