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 Tuesday Invasive Species Group blog How the Tuesday morning crew is battling DSV and other invasive species  The silent invader that's not from Marsby Barry Cottam A silent invader is craftily taking over our gardens and green spaces. Innocent enough in first appearances, it quickly spreads until, before we know it, it is dominant and impossible to resist. No, we're not talking about aliens from Mars, though these aliens are as green as the Martians are reputed to be. This is the ignored and sneaky Dog-Strangling Vine, also known as Pale Swallow-wort. Take a look at the accompanying photos. Have you seen this plant? Now that you know what it looks like, you'll be seeing it everywhere. One of hundreds of imported species, DSV arrived in eastern North America from the Ukraine and Russia over a century ago, a fact reflected in its scientific names Vincetoxicum rossicum and Cynanchum rossicum. First establishing itself in New York State in the 1880s, it has spread into southern and eastern Ontario. Earliest observations in Ottawa date from about 1907, but it was relatively unknown until about 15-20 years ago, when for reasons not well understood its growth exploded exponentially.
While it is establishing itself, DSV is pleasant enough in appearance, with opposing deep green gently lobed leaves on a nodding stem, with clusters of small yet pretty flowers coming in May. It's easily missed in a field or hedgerow. What makes this vine such a danger, and why should we care? Most imported species have few or no natural enemies in the host country and so, where conditions suit, are able to spread unchecked. As with other invasives, researchers are working on biological control through an insect natural enemy. This creature would itself be another imported species, so it will take years yet to determine if it is both effective and safe to release in North America. DSV has other advantages as an invader. It is highly prolific, able to produce up to 2000 seeds per square meter. Furthermore, the seeds are often polyembryonic, so instead of producing a single plant, they could produce 2, 3 or more. While it thrives in sunlight, it is not fussy about where it grows. For example, it can take over the understory of a wood lot, with single-stem plants that do not flower or produce seed pods until, say, a dead tree falls or is cut, creating a sunlit clearing that the full vine rapidly takes over. DSV can even, so early research suggests, modify soil composition in its favour. Its growth habits are also invasive; a twining vine, it climbs up into anything it reaches, trees, shrubs, and nearby plants, including other DSV vines. The result can be thick mats of almost impenetrable plant matter, perhaps the true source of the name Dog-Strangling Vine. By this stage, other gentler, less prolific - and often more desirable - plants have long disappeared. DSV is related to common milkweed and, like the latter, disperses its seeds on the wind after its dried-out pods have opened up and released them. Cutting or pulling DSV loaded with pods can actually facilitates this dispersal, for the pods of the cut plants dry out and release their seeds more quickly. Milkweed is the primary food plant of the Monarch butterfly, which lays its eggs on the plant so that the hatching larvae have a ready food source. Unfortunately, DSV is close enough to milkweed to fool the Monarchs, but the larvae find it incompatible and slowly starve. Thus, DSV is a threat to our most iconic butterfly - already under siege without this additional danger - as well as our gardens and green spaces. What can be done about DSV? Where it is well established it is extremely difficult to eradicate. Early detection and continuing vigilance are key to not having it take over in the first place. Where it has moved in, various means can be used to control it. Small patches can be dug up, by hand or by tiller, and roots sifted out. Note those roots! Numerous buds surround the crown, out of sight just below ground, waiting to spring up as new vines. DSV can be cut down - scything works - where it occurs in larger patches; cut over areas can be covered with tarps or mulched with newspaper and wet leaves. Tarps need to be in place for at least a couple of years, however, and, given the sensitivity of DSV to light, should be opaque rather than translucent. DSV will send stems for several feet under a tarp so it can pop out at the edges, so again vigilance is key. Old pool liners work well, as do boat covers, often discarded at the end of winter - you can enquire at your local marina. In general, spraying is not allowed in many parts of Ontario and in any event, has not proven to be particularly effective. A final point: DSV must be disposed of appropriately, so that the plant is destroyed. This is especially the case for roots and pods; once plants are cut, pods will dry out and release their seeds even more quickly than those on uncut plants. Heavy duty garbage bags destined for the landfill are required - don't even think about composting! In the end, then, it seems the only enemy DSV has is us. If we act. To learn more about this plant, visit the web - there are many resources, from detailed scientific studies to fact sheets put out by municipalities, conservation groups and invasive species organizations. The Ontario Invasive Plant Council, formed in 2007, hosts a website that provides information on many invasive species, including DSV, with guidelines for their management. Information is also available on the website of the Fletcher Wildlife Garden, which has been struggling with this invasive for almost a decade. Given that preventing the spread of DSV is so much easier than eradicating it, hopefully as awareness increases, prevention will too. Its our land and each of us must take steps to preserve it. Barry Cottam is a volunteer and a member of the management committee at the Fletcher Wildlife Garden. He got to know DSV first hand during the 2010 season and it was hate at first sight.  |
