Swallowwort

Written by Nexcerpt on May 21st, 2010 in Wildlife & Wetland.

Say hello to a disturbing invasive species: White Swallowwort or Pale Swallowwort. For a detailed technical description, see this excellent abstract on Cynanchum rossicum.

This plant is new to this part of Michigan, and unwelcome. Why? Consider one of its common names: “Dog-Strangling Vine.” The Greek roots of Cynanchum include kynos (“dog”) and anchein (“to choke”). The Greek may refer to effects of ingestion, rather than to physical strangling — but its toxicity is reflected in another appellation: Vincetoxicum (in place of Cynanchum).

If you care about the quality of natural land, please be on the lookout for opportunities to destroy this plant. Understanding the steps I took to recognize the plant may help you identify it, too.

I stumbled upon Pale Swallow-wort for the first time in May, 2009, on a development site in Mattawan, Michigan. I didn’t recognize the plant — neither the leaves nor the flowers. It was only a few inches tall, which didn’t help, but in thousands of field hours in Van Buren County, I’d never seen it before.

It was growing just a few steps outside a steeply banked Amtrak ROW, rather weedy with honeysuckle and other non-native shrubs. The soil at its base there was extremely rich (perhaps a 500-year floodplain) and underlain with clean, iron-rich sand. Virtually any plant would thrive there.

I was fooled (at first by discovering only a single plant) into thinking at first it might be worth preserving. The site had been cleared of timber the previous autumn, and I thought perhaps the increased sunlight had allowed a long suppressed wildflower to flourish.

Also, other parts of the site were fairly conservative (some uncommon native species were doing very well) which made me even more hopeful that something wonderful might have returned.

In the wandering margin between that ancient silty plain, and the upslope of the recently cut red and white oaks, I’d seen a few Cacalia atriplicifolia and Viola pedata (neither particularly common in this area), and bits of Equisetum . In the sandy aftermath of the forestry activities, there were many weeds (vineyards occupied much of the site a century ago), interspersed with hundreds of Tradescantia virginiana, and dozens of Scrophularia lanceolata (which I had never seen within several miles). Violets and sedges of all sorts were common across the site. It was a very well healed old meadow.

Higher up, in what had been mature woods, I had seen Claytonia virginica in vast profusion (carpeting the ground, by the thousands), and a small enclave of Swertia canadensis. That is simply not a common plant; it encouraged me to think the Swallowwort was another rarity.

I made a few observations. (I had no field guide at hand, although this appears in no field guide of mine, including even Weeds of the North Central States from USDA, which I though had most everything!) I broke off a small part of a lower leaf, thinking in the moment that it might be an obscure Asclepias. No milky exudate was obvious; no distinctive odor to the crushed leaf, either.

(In fact, my suspicion was fundamentally correct: Cynanchum belongs to family Apocynaceae and subfamily Asclepiadaceae.)

The leaf petiole on the broader, lower leaves seemed distinctive. (The effect is somewhat visible at the center of the photo, above.) See the flowering stem that points directly rightward? Nearly opposite, a very young leaf sprouts upward. The effect at the base of that leaf — petiole distinctly vertical, parallel to the main stem for the first 0.5 cm — is very distinctive on lower leaves. That is, the fully horizontal lower leaves have a very obvious vertical petiole, almost vase-shaped as it rises slightly ~toward~ the stem (as though clasping, but not snugly), then rolling out to the broad surface of the leaf.

Also, the lower leaves were very dark green — shady pine forest green — and extremely glossy.

Even with all that information… I missed the most important clue! Did you? This invasive was adjacent to a woodlot harvested last fall. Once I realized its invasive nature I reviewed the site carefully, and discovered one other plant, high up on the driest portion of the site.

Reading the landscape a bit, it became evident that the first plant was at the staging area established for the harvest, and the second plant was at the first cutting area. In other words, the heavy equipment brought to the site (to remove the cut trees) contained seeds of the invasive plant. Some rig moved back and forth between those two spots, with its cleats dropping soil from the last, badly infected site it had served.

Many references suggest that a close relative, Cynanchum nigrum, or Black swallowwort, is similarly troublesome. Its flowers are considerably darker. (If this is not yet obscure enough for the botanist in you, the name Cynanchum louiseae also appears synonymous with C. nigrum.)

Alternate names include Vincetoxicum rossicum and Vincetoxicum nigrum.

Thanks to John Legge, Conservation Director for The Nature Conservancy of Michigan, for his rapid help in identifying this unusual and undesirable plant. John himself admitted not having seen Pale Swallowwort in the wild before — but identified it readily nonetheless.

Kill it before it spreads.

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