Is it Black Swallow-wort or Pale Swallow-wort?

Vincetoxicum rossicum
Pale Swallow-wort L Black Swallow-wort C & R

I am assembling a talk on Michigan Invasive Species and needed photographs of Black Swallow-wort. I asked several friends for locations. I was surprised to discover that the first three stations I visited were Pale Swallow-wort.

Black Swallow-wort (Vincetoxicum nigrum or Cynanchum louiseae) and Pale Swallow-wort (Vincetoxicum rossicum or Cynanchum rossicum) are two similar invasive vines. Black Swallow-wort is also called Louise’s Swallow-wort or Black Dog-strangling Vine. Other names for Pale Swallow-wort include: Dog-strangling Vine or European Swallow-wort.

Vincetoxicum rossicum
Pale Swallow-wort flowers L Black Swallow-wort flowers R

Most people attempt to distinguish the two species based on flower color. However, it is difficult, without some field experience, to separate these two species using the flower color. Dr. Anton Reznicek on the Michigan Flora website writes, “The corollas in both our species are dark purplish, but they run toward black in V. nigrum [Black Swallow-wort] and tend to be somewhat paler in V. rossicum [Pale Swallow-wort], which also has longer corolla lobes (at least twice as long as broad) and only half as many chromosomes.” In Black Swallow-wort the corollas (petals) are about as long as they are wide and are covered with fine hairs. In Pale Swallow-wort the corollas are about twice as long as wide and are hairless. Peduncles (flower cluster stalks) are shorter in Black Swallow-wort than Pale Swallow-wort.

Vincetoxicum rossicum
Pale Swallow-wort flowers L Black Swallow-wort flowers R

My wife commented that the tips of the flower buds on Black Swallow-wort are rounded and on Pale Swallow-wort they are bluntly pointed. I do not know if this is constant, but it held true for our limited sample.

Swallow-worts are in the family Apocynaceae which includes the Milkweeds and Dogbanes.

Vincetoxicum rossicum
Pale Swallow-wort seedpods

Swallow-worts are poisonous to Monarch larva. If a Monarch butterfly lays her eggs on the similar Swallow-wort instead of Milkweed (Asclepius spp.) then the larva die. An additional problem is that Swallow-worts are toxic to livestock and deer. In northern Oakland Co. Pale Swallow-wort is moving into pastures and causing problems for livestock.

Black Swallow-wort has been in Michigan since the 1880s. Its distribution is poorly known and should be reported wherever it is seen. Collected specimens are from the southern four tiers of counties and then at the tip of the Lower Peninsula. It must be found at other sites in northern Michigan. There are reports from the Traverse City Area. Pale Swallow-wort was first reported in 1968 and we have records from four counties in the southern three tiers. As the Michigan Flora website says, “It should be stamped out wherever observed.” Reports of both Swallow-worts should be made through the Midwest Invasive Species Information Network (MISIN). They have an online reporting tool and also a downloadable smart phone app.

I thank John DeLisle of Natural Community Services, Donna Kuchapski, and Debbie Jackson for helping me locate Swallow-wort stations.

 
Copyright 2019 by Donald Drife

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Pale Swallow-wort: Another Invasive to Watch Out For

Vincetoxicum rossicum

Pale Swallow-wort

Pale Swallow-wort (Vincetoxicum rossicum) also called European Swallow-wort or Dog-strangling Vine is an invasive species moving into southern Michigan. Dr. Reznicek writes, “East of Michigan, this species has become a terrible invasive; it should be stamped out wherever observed.”  It was first recorded from Michigan in 1968 from Berrien County.

Vincetoxicum rossicum

Pale Swallow-wort showing leaves and seedpods

It is a vine in the Dogbane Family (Apocynaceae) that includes our Milkweed species. When not flowering it resembles Bittersweet (Celastrus spp.) but can easily be distinguished by its opposite leaves (Bittersweet has alternate leaves). Flowers are five petaled with the petals longer than wide. Normally they are pale pink but burgundy or dark brown flowers occur. It has narrow, pointed pods resembling true Milkweeds (Asclepius spp.).

Vincetoxicum rossicum

Pale Swallow-wort showing growth habit with opposite leaves and a closeup of flowers

Black Swallow-wort (Vincetoxicum nigrum) is another invasive similar to Pale Swallow-wort. Its petals are hairy on top and about as long as wide. It was first recorded from Michigan in the 1880s.

This aggressive plant will take over a field by choking out competing vegetation. It will out compete our native Milkweeds. This plant also impacts Monarch butterflies because it is toxic to Monarch larva. Monarchs will lay eggs on this species but they don’t survive to adulthood. Pale Swallow-wort appears to be allelopathic meaning that it alters the soil chemistry so seed from other species can’t germinate.

I’d like to thank John DeLisle from Natural Community Services for providing location information.
Copyright 2015 by Donald Drife

Webpage Michigan Nature Guy
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