Friends of Terwilliger volunteers have spent thousands of hours over the past 23 years removing invasive vegetation in Terwilliger Parkway. Perhaps chief among the bad-news invasives is English or Irish ivy. We all know what it looks like and that it is Bad—but what is it, really?
Hedera helix (English ivy) and its more common, almost look-alike relative hedera Hibernica (Irish ivy) came to the Americas in colonial times. Planted as an ornamental, ivy soon covered walls, houses, and of course trees. For many it was also an easy ground cover. All very well in the British Isles, where many sing its praises.(1) In the Northwest, we just call it “ivy” and consider it a pest.
Firstly, ivy is a trailing and climbing vine. In its “juvenile” form it moves rapidly up towards sunlight and also out along the ground as ground cover. As a vine grows and matures, its long stems become thicker and intertwine with other stems on the same surface. If left undisturbed, the twining vines can become as thick as one’s arm and form layers of vines around a tree. When ivy gets to the top of whatever it is climbing, it “matures.” In the fall, mature ivy leaves begin to change from 3-5 lobed to more heart-shaped unlobed forms. With the mature leaves come greenish white flowers and then blue-purple berries (2). Non-native birds, to a great extent, eat the fruit part of the berries and discard the seed, thus helping to spread the plant over large and diverse areas. Not surprisingly, the highest priority in removing ivy is to eliminate the fruiting ivy seed source before it spreads.
Secondly, ivy has two types of roots. The subterranean roots grow into the soil and work like most plant roots: gathering water and nutrition from the ground. The adventitious or aerial roots attach the vine to the surface it is climbing. In addition, on trees, these roots actually change shape and grow tiny hairs which reach into cracks and anchor the vine as it grows. The ground roots are relatively shallow, thus when the ground is moist, pulling ground ivy can be very satisfying.
As described earlier however, the vines that grow up and have adventitious roots anchoring them to trees and this is where the damage to our parkway trees begins. The tiny roots can sneak into tiny cracks in tree bark or limbs. This opens the way for rot and other diseases. Then the twining, climbing, thickening ivy becomes heavier and heavier with time. The weight and thickness put the trees at risk from wind and ice storms. Just stick your head out your door this winter after an ice storm and listen. Depending on how windy it is, you will hear only a few or many cracks and crashes. Trees and branches covered with ivy give up under the unnatural weight of the extra accumulation of ice. In addition to weighing down, the tree ivy climbs and smothers, while spreading throughout the tree canopy. It not only deprives its host-tree of light and air, but also shuts out the light needed by the under-story shrubs and ground plants. The wild creatures, birds and squirrels, chipmunks and wood mice who rely on these native plants lose their habitat and food. The ground, seemingly green and vibrant, becomes a wasteland: a mono-culture of one species that eliminates all others. Moving along the ground, ivy smothers native plants and tree seedlings. Yet ivy’s wide-spreading roots are relatively shallow and weak. They do little to hold soil as our heavy rains beat down and carry dirt away from our hillsides. In effect, ground ivy contributes to erosion and to loss of habitat for our small wildlife.
Friends of Terwilliger and its allies have pulled and cut ivy off hundreds of trees. As you ride, walk, jog, or bike along the Parkway, notice the bare tree trunks. Notice, too, those still fighting the ivy. If you can, come out to help us—a few hours of cutting and pulling ivy can save a tree.
Picture from Weed Wise, Clackamas
Soil and Water Conservation District website at https://weedwise.conservationdistrict.org/weeds/english-ivy