By Noah Jenkins
If you sit through all the credits to “Ferris Bueller’s Day Off,” there’s a final scene where Ferris comes out of his room, looks into the camera incredulously, and says, “You’re still here? It’s over! Go home!” With all the effort that’s gone into removing it over the past twenty-plus years, some folks may have the same feeling about English ivy. How can there still be so much of this plant? There are several answers. One is that it is very good at what it does: able to grow in a wide range of soil conditions, light regimes, and moisture levels, it is quick to establish itself, and can grow as much as 10 m (33 ft) annually. This is partly because—excepting periods of severe drought or cold—it never stops growing, which means that our mild Pacific Northwest winters allow it to grow almost year-round. Another answer: it’s been here for well over 100 years, having likely been introduced sometime between 1875 and 1899, and was sold locally from 1912 until February 2010, when the Oregon Department of Agriculture banned the transport, sale, or propagation of the plant in the state. Meanwhile, a number of bird species with a taste for ivy berries—notably the European starling, another invasive species—have carried the seeds far and wide, resulting in the introduction of the plant even to places little touched by human activity. Small wonder, then, that the “kudzu of the northwest” should still hold sway over so much of the landscape here.
There is no “magic bullet” solution for English ivy: blood, sweat and tears—and, occasionally and judiciously, herbicides—appear to be the only tools with which to combat this scourge, as there are no biocontrols on the horizon; even goats will only defoliate the plant, leaving the stems and roots to re-sprout. The closest thing to an easy cure takes advantage of a unique aspect of ivy: in order to produce flowers and fruits, ivy must climb vertically. This is one of the major threats it poses to forests, as the heavy weight of the vines, combined with their smothering of the trees’ leaves to prevent photosynthesis, weakens and eventually fells even mature trees. The good news is that the rootlets that allow it to climb vertical surfaces do not actually penetrate tree bark, and are not capable of removing water or nutrients from the tree. This means that the tree ivy is still reliant on its connection to the ground: cutting it off from the ground will kill the portion that has grown into the tree canopy. The best approach is called “gap-cutting,” and involves using whatever tool works (hands, hand pruners, loppers, saws, machetes…skilled contract crews will even use chain saws on the really thick stuff) to cut the ivy vines in a band around the tree: one cut at waist or chest level, and a second cut close to the ground. This ensures that the cut ivy is not able to knit itself back together, and makes it easier to see that you’ve gotten all of the vines. If you can do nothing else, this will at least protect the trees and prevent the ivy from producing seed. Of course, any remaining ground ivy will commence climbing again in short order, so this would ideally be followed by removing the ivy surrounding the tree. This brings us back to the blood, sweat and tears, aka hand-pulling. Ply friends and neighbors with their favorite vices and make a party out of it, or assign this task to misbehaving children. Be sure to get all the roots, as they will re-sprout otherwise; fortunately, most of ivy’s roots are in the top few inches of soil, and are fairly easy to pull when the soil is moist. Larger stumps of ivy, such as those left after gap-cutting very old vines, may be treated with an appropriate herbicide to kill the roots, as these can go too deep for mechanical removal. (Getting a shovel between a tree and a 6” diameter ivy trunk? Not that fun or effective.) Always read the label before using herbicides. If you just can’t face hand-pulling, and there are no native or ornamental plants mixed with the ivy that you want to preserve, repeated mowing for several years can be an effective approach, as depriving the ivy of its leaves several times each year will eventually exhaust the plant. Since there aren’t any berries on the ground, there’s no need to worry about seed dispersal; be sure that there are no other invasive plants mixed with the ivy that are seed-dispersed, however, as garlic mustard, false brome, lesser celandine, clematis, and other aggressors will happily step into any gaps in the ivy canopy. Once the ivy is under control, you’ll want to re-plant the area (ideally with native plants!) so that these and other weeds don’t show up and undo all your hard work.
References and further information
Urbanizing Flora of Portland, Oregon, 1806-2008. Occasional paper of the Native Plant Society of Oregon, 2009. Christy, John A., A. Kimpo, V. Marttala, P. Gaddis, N. Christy