|fwg is a long-term project of the ottawa field-naturalists' club|
Dog-strangling Vine (a.k.a. Pale Swallowwort)Cynanchum rossicum (= Vincetoxicum rossicum)
This plant is highly invasive. It will thrive in shade, sun and all soil conditions, spread rapidly, reduce or eliminate other plants, and is extremely difficult to control. If you see this plant in your garden, REMOVE IT IMMEDIATELY.
Swallow-wort belongs to the Milkweed family (Asclepiadaceae). It is a perennial, twining vine growing up to 2 m in height, with small pinkish to dark maroon 5-lobed flowers that start to appear in late May to early June. Opposite leaves are ovate, dark green, smooth, and shiny. The seed pods begin appearing in late June and are mature by mid to late July. Each pod produces numerous wind-borne silky-haired seeds.
How to dispose of Dog-strangling Vine
We are often asked what to do with swallowwort plants once you've pulled it up or dug it up. The stems and leaves can be composted, of course, but it's not a good idea to compost either roots or seeds, as either might produce new plants. Questioners are also concerned about putting the bags of plants out with the garbage as she did not want to inadvertently spread them to other locations.
After a bit of research, here's what we found out.
Control experiments at the FWG
Mechanical tilling: In spring 2007, as part of an effort to expand our Butterfly Meadow, we rototilled an area where there was a lot of DSV. We thought the tiller would break up DSV roots, but we weren't sure whether that would prevent them from growing back as root nodes (the critical growing area) might not be damaged.
We rented a hand-operated tiller (our local Home Depot people recommended a machine that would be capable of turning over dense turf and dealing with DSV roots rather than one intended for tilling garden beds). Two volunteers tilled two areas of the Butterfly Meadow. In one of these areas, they were limited on one side by tree roots (we didn't want to risk killing the trees). After the tilling, one volunteer spent an evening looking for DSV root nodes in the tilled areas and removing them.
We planted both areas with native wildflowers and were surprised at the results. Tilling seemed to have done enough damage to prevent regrowth of most existing DSV plants. Of course, there were seeds in the soil and DSV is growing back, but it is meeting a lot of competition from the native plants that are flourishing in this area.
In 2008, a larger area was tilled, using the same sort of hand-operated machine. This time, volunteers spent a huge amount of time sifting the top layer of turned soil using a mesh frame (about 1 cm grid). By fall, the native plants they put in were doing very well and we couldn't see any DSV.
In 2009, Agriculture and Agri-Foods Canada very kindly helped with rototilling of an area next to the birch grove. They tilled in spring and again in the fall. Volunteers also sifted part of the area, but the tilling removed a considerable amount of DSV. We planted this area with a number of wildflowers in 2010.
Mulch: We have tried a number of materials as mulch. Thick leaf cover, landscape cloth or multiple layers of newspapers are all fairly effective when used around the base of trees and shrubs - areas where DSV seems especially dense. Although some plants grow through the mulch, growth is considerably less than in un-mulched areas nearby.
Research elsewhere has shown some success with heavy plastic sheeting placed over mowed or cut plants and left in place for at least 1 year. However, if the plastic is torn allowing light to penetrate, the plants will readily grow.
At the FWG in July 2005, we placed a large tarpaulin over most of the north bank of our Amphibian Pond. This bank is quite steep, so digging out DSV plants was impractical. It has been difficult to keep the tarpaulin anchored securely. We used pieces of heavy coathanger bent into loops and pressed into the ground through the edge of the tarp. These frequently come loose and the tarp has torn in several areas. Monitoring is obviously needed.
In July 2006, although some DSV had grown through holes, no plants could be seen under intact parts of the tarpaulin. And no seedlings were visible, meaning that last year's seeds did not germinate under the tarp.
We took advantage of the mild weather in October and November to pull back one corner of the tarp and start planting shrubs. We put in a number of dogwoods - planted about 2 feet apart.
In spring 2007, we removed the rest of the tarp. Only a few DSV plants were growing under it; we dug these out along with others along the edge of the cleared area and planted the slope with Wild Raisin (Viburnum cassinoides).
In 2006, we also used newspaper as mulch in some areas, especially immediately surrounding trees where there are hundreds of DSV seedlings and little or no other plant species. We place several sections of newspaper up against the tree trunk, then overlap subsequent sections working outward. The idea is to eliminate DSV near and under some trees where the seeds seem to accumulate.
Mowing: Some areas have been mowed twice a season in an attempt to stop seed production and dispersal. We try to time the mowing to occur when the plants are starting to form pods on the theory that they've used a lot of resources at that point and the roots might be somewhat depleted - so cutting the plants will do the most damage. In the last few years, this has unfortunately coincided with bird nesting season and mowing has been delayed as a result. Again, monitoring is required and a second mowing later in the summer may be necessary if the plants grow back sufficiently to start producing seeds.
Cutting: In areas where mowing is not feasible, we tried cutting off the flower heads before they set seed, hoping to reduce the overall impact of seed dispersal and, in several areas, contain the amount of new growth. However, we found the plants re-sprout rapidly below the cut, sending up new flower heads, requiring more cutting in the same areas three or even four times. As with other control methods mentioned, this is impractical for widespread infestations.
In 2006, two of our volunteers bought a scythe and tried it out on DSV. This old-fashioned method of cutting works well - especially when the swallowwort is young and still standing upright. As with all cutting, it's necessary to go back over previously cut areas when the plants grow back, but this method is much quicker than using a weed whipper or clippers (and much kinder to our backs).
This method is especially good in fields where we want to save grasses, vetch, goldenrods, etc. We can cut around these and also cut the DSV low enough to remove its flowers, but save most of the grass.
In 2007, we started cutting DSV much earlier, knowing we'd have to go back over the same fields later. By mid-June, we had cut all major stands of DSV and started recutting some areas. By the end of June the DSV plants were harder to cut as they had begun to twine together.
As of 10 July, many previously uncut plants have formed seed pods. Cutting is much more difficult as the plants tend to wrap around the scythe. All cut plants with pods now have to be picked up for disposal off site. In fields previously cut with a scythe, DSV plants are still upright, are blooming, but have not produced seeds.
Chemicals: Research elsewhere shows that glyphosate might be useful if applied repeatedly and over several growing seasons. However, results are variable and depend on a variety of conditions. Herbicides also kill ALL vegetation in the area of application, leaving the ground open to other invasive species.
Bio-control offers the best hope for managing widespread invasions, and research is ongoing in this area.
Recent discoveries and new information
YELLOWING OF DSV LEAVES: This is the result of some sort of chlorosis (lack of chlorophyll in the leaves). This could be caused by many things including root or stem predators/herbivores, bacterial or viral diseases, lack of nutrients (unlikely if healthy ones are nearby), or toxic levels of something from heavy metals, nitrogen (i.e. too much fertilizer, or dog excrement), petroleum products, herbicide, etc., etc.
It is also entirely possible that it is a genetic problem, just like the lack of pigment production in the flowers or, say, sickle-cell anemia in humans. A spontaneous mutation is a possibility, or it might be a mutation that is present in the population at very low frequencies and rarely found in living plants in any population where the gene may occur.
If there is only one plant in a large population suspect the later. If there are more plants then the herbivore/disease reason is more likely
Sources for the above information include the references listed below and communications with several scientists involved with the study of DSV.
For more information