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                                                                                                                                                   Invasive Species in Thickson's Woods

Garlic Mustard

When the first garlic mustard plants appeared in Thickson’s Woods back in the 1970s, local botanist, naturalist and artist George Scott was very excited. George knew every plant and bird in Thickson’s woods, and finding something new was greeted with enthusiasm. Thus it was that, for a number of years, garlic mustard was welcomed. Only when it became obvious how aggressive it was, outcompeting native wildflowers by remaining green all winter, and producing thousands of seeds growing into dense smothering stands, was the alarm sounded. Later we learned that this invader also uses chemical warfare, weakening the ability of native trees and shrubs to survive. Since then we’ve dug up millions of year-old plants and destroyed millions more newly emerged seedlings. Many former patches have been irradicated, but some persist, and there are always undiscovered plants appearing in secret areas, the tiny seeds carried there on the muddy feet of rabbits and raccoons.  




Dog-strangling vine

 It’s mostly due to the hard labour of volunteers, and of summer students, hired three years in a row through the OFAH’s Invasive Species Program, that the meadow isn’t wall-to-wall swallowwort by now. This insidious invader climbs up among shrubs, trees and grasses, hardly noticeable even when it puts out small purplish flowers in clumps along its reaching vines.  Lime-green, pointy seed pods become quite visible by August, when they can be easily picked by hand and put in the garbage—not in compost!—and gotten rid of.  When the pods break open their hundreds of winged seeds go floating off in the wind, spreading dog-strangling vine ever farther.  Dense stands flourish both in sunny areas and shady understories, choking out almost everything else.  Surest way to get rid of it is to dig up the clump of roots at the base of each vine, the corm, knock off the loose soil, then leave them to dry and die in the sun.  Once you recognize DSV you’ll see it everywhere, in yards, gardens, hedges, parks and natural areas.  Look out the GO train window when you’re riding into the city.  Dog-strangling vine is the chief groundcover all along the tracks. 


Sorry, but there’s no hope in sight of even slowing down this dreadful invader from choking all of Corbett Marsh.  We’ve watched it marching down both branches of the watershed year after year, finally filling the wide creek valley west of the causeway and spreading in clumps around the provincially significant coastal wetland, where rails, grebes, black terns and bitterns once flourished. Since we don’t have access to a hundred strapping youths with sharp spades, or the means to spray stands with lethal Round-Up, we’ll have to wait for entomologists to discover some insect that eats “common reed” back home in Eurasia that they can safely release over here.  Meanwhile wouldn’t it be great if some clever entrepreneur started a business making paper

 and toilet tissue out of phragmites over here, to keep more forests intact?



Once a settler’s apple orchard and cow pasture, the meadow was growing up with nannyberry, hawthorn, red-osier dogwood and buckthorn when we purchased it in 2001.  One spring shortly thereafter a volunteer crew moved through, cutting buckthorn trees and placing old gallon tins from a sugar bush over the stumps to keep them from producing suckers.  That at least slowed them down. Our arborist walking through the woods recently pointed out how buckthorn shoots could be pulled by hand, and older ones cut down, the stumps treated with Round-Up to discourage regrowth, and those efforts are ongoing.  Robins, cardinals and mockingbirds eat buckthorn berries in winter—one good thing--and spread the seeds, a bad thing.



Sweet Woodruff

Small, dainty, with fragrant white flowers in late spring, what’s not to like about sweet woodruff?  Only by watching a small patch of this emerald-green ground-cover spread into a wide dense mat with no native understory plants in sight did it come to anyone’s attention as a possible foreign invader.  A tour of the woods in every season since reveals patches of it spreading on many slopes and ridges, a growing concern.  Trial removal by hoeing up the shallow, spreading rhizomes is under way; we’ll keep tabs on the results.






Japanese Knotweed

“Incredibly aggressive” doesn’t cover it.  “Scary” is a better adjective for this semi-woody invader, roots of which are strong enough to break through asphalt and concrete.  It was brought to North America from east Asia as an ornamental, and was that ever a mistake!  Knotweed spreads quickly, into dense thickets, sending rhizomes 10 meters from the parent plant, and shading out native vegetation.  We know of one residential property with knotweed spreading ever closer to Thickson’s Woods, and of two clumps out along the Waterfront Trail, on town property.  Both owners know about them; unfortunately, the Town of Whitby doesn’t have an invasive species plan.