A dog-strangling tangle

Let's say you were walking through White Pine, Hemlock, and mixed hardwoods with a diverse forest floor flora, on shallow soil over limestone, just north of Spencerville, Grenville County, Ontario. Nothing could be further from your mind than Cynanchum, Vincetoxicum, Dog-strangling Vine, Black, White, or Pale Swallow-wort, the many-named bane of Toronto-area oldfields. But as you proceed along, past the Canada Ginger, and Meadowsweet & Sarsaparilla, Trillium & Bloodroot, Large-leaved Aster, and Bracken under the moderately dense August canopy, you begin to notice a few unfamiliar low plants with smooth-edged flat dark green leaves.

At first there are only scattered plants, but as you proceed they become more frequent, and you notice the little dark oval leaves of what seem to be many seedlings of the same species among the twisted stems of the larger ones. After a while they coalesce into a blanket on both sides of the track, and this comes and goes patchily for a while, until it closes into a dense boot-strangling thicket, knee- to waist-high, only broken where slabs of exposed bedrock are more than two metres wide. It seems to be amazingly shade-tolerant, covering the forest floor even under the low canopies of White Cedar, where nothing else would grow.

There's no interruption in the blanket when the woods open out onto an oldfield. The stand that completely fills the ground between the trees is yellowed by pale pods and some yellow leaves, 90 cm tall with an acrid milkweed smell, but no milky sap when you cut a stem or leaf.

It's a dense stand of narrow vertical stems not tangled among themselves except at the tips - but they tangle quickly where you walk through them. Each stem bears opposite pairs of spaced-out smooth green leaves: like long-pointed Lilac leaves low on the stem, they narrow to a parallel-sided shape towards the top. The twisting uppermost 10-15 cm is bunched with spiky long-pointed green pods 5-7 cm long and only 5-7 mm wide. They look like the climbing spines of a tropical liana, especially where they're tangled up under a White Pine or over a bush. Inside they're like a miniature milkweed pods, with green seeds and moist maturing fluff. On the ground is the grey straw of last year's stems and the bare rock.

You cross Goodin Road to a thinned sugar bush, still cabled together for the spring tubing. There's a tousled green carpet between the trees, generally almost a metre high, its surface marbled or flecked with pale pods, and climbing another 30-50 cm up onto saplings - especially under small White Pines, Pinus strobus, scrambling up into the thin dead lower whorls of branches. Only a few stumps project above the sea of green.

This, you think, must be an invasive alien. And indeed it is: Vincetoxicum rossicum or Pale Swallow-wort, more dynamically known as Dog-strangling Vine. This and a similar European, Vincetoxicum nigrum, Black Swallow-wort, have colonized North America. They're members of the Milkweed family Asclepiadaceae, and used to be grouped with native North American species in the genus Cynanchum, but they're now generally segregated under the frightening-sounding name Vincetoxicum. V. rossicum is often strikingly successful in areas of shallow soil over limestone bedrock.

The almost hypnotic regularity of the perfect leaves shows clearly that Vincetoxicum has no Insect herbivores here, but it has so many in Europe that it's never a pest and is sometimes rare. This means that research into biological control will very likely find the appropriate Insect to munch through the glossy stands, as Chrysolina Beetles munched through the Saint John's-wort (Hypericum perforatum) in the 1970's and Galerucella Beetles are now munching through the Purple Loosestrife (Lythrum salicaria).

That day, if it comes, will be at least a decade in the future, and you don't want your woods strangled, even for a decade. "When it first appears, it's very deceiving. Unless you know what you are looking for, you wouldn't even notice them. It's just a plant here, a plant there, no big deal. The next year each of those single plants becomes a little group... and still it's no big deal. ...In the next couple of years those small groups have become small patches 6 to 8 feet in diameter... you watch helplessly as the patches merge together until it is so thick it completely smothers everything.... Nothing else grows, not even grass." (footnote 1)

The first part of control is learning to recognize the plants and eliminating them before they spread. The August growth-stage in which you've walked through the Spencerville stand is your last chance to cut and destroy the tops before the pods open and the seeds blow away to initiate plants that will grow into downwind colonies. Then you've got to dig out the pioneers before they can spread. The experience of those who have tried to control Vincetoxicum is that "Roundup kills everything except the Swallow-wort roots, which sprout again in a week," and that digging up the root crowns of the mature plants - labour intensive and painful - is the only way to really get them out.

Then you have to watch for tiny seedlings, since seeds may remain viable in the soil for more than two years. Mulching the site may help. Slow maturation delays population growth but also complicates control, since four-year old seedlings may be only about 10 cm high, with only 5-6 leaf nodes. Reproduction usually starts when the plants have 9-10 nodes. So you may think you've eliminated a patch, only to have tiny seedlings lurking about, putting energy into their roots and waiting until you congratulate yourself on the success of your control efforts. (footnote 2)