The next morning we returned with a shovel and garbage bag, intending to remove the few plants and their roots. While Richard was digging up the plants, I wandered around the area and discovered that the vine was everywhere! It was growing in smaller patches as well as thick mats that covered the tops and all around several nearby rock piles.
Our idea of digging out a few plants by the roots was out the window. Our new goal was to pull out or cut down all the vines with seed pods as soon as possible, before they could ripen and split open later in the summer. Each pod carries dozens of seeds attached to silky threads that blow away and settle down on any spot with a bit of bare earth.
The most densely affected area is over 800 square metres, located toward the edge of the pasture and close to a 30-year-old red pine plantation above the field. A major objective is to contain the DSV in the pasture and prevent its spread into the pine plantation. So far, we have only found one very small and isolated patch of DSV that had reached the edge of the pine plantation.
Over the next three weeks, we worked hard to pull or cut down well over 10,000 plants with pods, sealing them into industrial black plastic bags and leaving them to bake in the sun over the summer. We estimate that each bag held about 500 stems with pods. We had to ignore the many thousands of younger plants that had not yet produced seed pods, but we kept a watchful eye on all these patches as pods kept forming into the fall.
Searching online for advice, we found the website of Ontario's Invading Species Awareness Program (http://www.invadingspecies.com) and its helpful series of guides to best management practices for a wide range of invading species. Drawing on its recommendations, we laid out tarps over most of the intensely affected area where we had pulled up plants, in an effort to kill their extensive root systems. Before tarping, we trimmed down other bushes and saplings that interfered with the tarps. Before leaving the field for the winter, we placed heavy rocks and the plastic bags of DSV cuttings along the edges of the tarps. These will stay in place for two or three years to ensure the DSV roots and stray seed pods are destroyed.
Unfortunately, the entire area will be heavily damaged by the tarping and the plant toxins emitted by all parts of the DSV plants. We will need to rehabilitate the soil and reintroduce native plant species in all heavily affected areas.
This isn’t all there is to the story. As we walked from rock pile to rock pile and through all corners of the field, we found many single vines or smaller clusters that had somehow managed to move in with the goldenrod, raspberries, tall grass and other wildflowers. These DSV patches eventually petered out, leaving about two-thirds of the 25-acre field unaffected. We pulled out all of these vanguard vines if they had pods, leaving the roots and smaller vines behind, knowing we would be back next spring and summer to pull or dig them out again.
We don’t know where the vine came from or how it got into our field. No vines have appeared in the nearby fields or forested areas and only one or two grow along the shady road that runs beside the affected field. We will keep looking and asking family and neighbours to keep an eye out.
Nor do we have a clear idea of how long the vines have been spreading in the field. Each year the vines die back to the ground and start out again in the spring, getting denser over the years as the roots send out ever more stems. Finally, they crowd out the native competition and destroy the diversity and bounty of the ecosystem. Given the density and spread of this invasion, we assume it’s been a number of years.
The DSV is a foreign relative of our native milkweed (native in Ukraine and east central Europe), but unlike our milkweed, it does not attract or nourish most insects or animals. One exception is the Monarch butterfly that can mistakenly lay its eggs on DSV and thus deprive its larvae of a critical food source, since DSV cannot sustain the caterpillars.
What have we learned in the process?
First, it’s important to survey the whole area and figure out the size and locations of the densest infestation — those places where there might be dozens of vines in a square metre. Don’t panic and start pulling and cutting, like we did for a day or two. The densest areas are where you will want to mow and tarp.
Second, make a map of all the affected areas and a plan to monitor the site in different seasons each year. We learned that our site is large and well established. As human interveners, we see our job as stopping the spread of the vines and shrinking the areas of monoculture. In future years, we hope to pull and dig out all of the isolated clumps, but aren’t sure we will ever be able to eradicate the vine completely. When the flowers have formed this coming June, we will need to pull out the vines around the tarped areas and visit the whole field fairly often, through June and early July.
Third, tackle the isolated outposts first, especially if they already have seed pods. Work from the edges into the densest area. Keep checking for more vines throughout the area and especially where you’ve already looked. We continually missed plants in areas we had already searched more than once. Search the areas where native vegetation is the least dense, such as rock piles, or any place where the soil has been exposed, for example, where bears turn over rocks and leave exposed soil. Bears might also be seed carriers, so check carefully in the wild raspberry patches in the late summer.
Fourth, be sure to flag every DSV location, even a single plant, with bright marking tape so you can find all outposts later and next year.
Fifth, pull and remove all vines with flowers and seed pods before you cut or mow an area. Any flowers or pods left on the ground by trimming, can still mature and release their seeds. This is the most challenging part of the whole experience. The vines are tricky things, hiding in plain sight among the milkweed, goldenrod, grasses, and other small bushes and saplings. And don’t forget to bring your friends with their fresh set of eyes. They all found some stragglers that we’d missed.
Sixth, try to feel a little respect for the resilience of the vines, grudging though it might be. It helps keep one’s spirits up while awaiting the arrival of a biological attacker — like the small eastern European bee that coevolved with the DSV and is now being field tested near Ottawa before it can be released in the Ottawa area. There is also a controlled herbicide that will do the job but you need to have a licence, or licensed operator, to handle it. We don’t have a crop at risk from DSV and don’t want to add a chemical toxin to the soil, especially over such a wide area.
Seventh, plan ahead for disposal of the bagged plants. The Ontario Invasive Plants Council has advised that landowners dispose of the well-baked plants in their garbage bags at the local landfill. The Council is currently completing research on how well this disposal method works. We contacted our township office to find out whether the township could handle the secure disposal of such an invasive plant. After considering our request, the township decided it would not accept invasive species for disposal.
Erring on the side of caution makes good sense for the township but leaves many landowners having to figure it out for themselves until better means are assured. Under no circumstance should any invasive plant material be composted or disposed of as garden waste, whether living or presumed dead, in your own or the township composting facility. You cannot assume that all seeds will be dead inside their pods by the end of summer. Further, the Ontario Invasive Species Act explicitly states that it is illegal to import, deposit, release, breed or grow, buy, trade or sell prohibited invasive species: namely dog-strangling vine (two species), Japanese knotweed or phragmites (European common reed). These four legally prohibited invasive species are already in the province.
With this sobering caution front of mind, our new disposal strategy is to build a sturdy fire pit in an open area close to the DSV site and burn all the waste from 2017, along with the new plants we pull and cut down this year. We will let you know how well this works out and what we learn in year two!