English Ivy

The Scourge of English Ivy

by Kathryn C. McGee

Rainier Garden Club

English Ivy (Hedera helix) is an aggressive, invasive, introduced species. It is a problem not only in Rainier, but in Oregon, the U.S., and six other countries. It kills trees, native vegetation, and harbors rodents and trash.

We need to start now to save our trees from this aggressive weed. Look very closely at the trees. Is that green vegetation the tree or the ivy invading it?

The ivy must be cut out from around the base of an infested tree and as high as you can reach up the trunk. Cut it when it is dry so that any small pieces will not root. Carefully dispose of the debris so you don’t spread it to new areas.

This message is endorsed by the Rainier Garden Club, Friends of Fox Creek, Friends of Dibblee Point.

◙ For more information about English Ivy you may contact Kathryn McGee at kcmgroups@q.com ◙

The following information was taken from the “No Ivy League” web site: www.noivyleague.com This is a very informative web site and contains information gathered from the league’s extensive project at Forest Park in Portland. At the bottom of the No Ivy League home page is a slide show on the “Evils of English Ivy,” a must view.

English Ivy (Hedera helix) is an aggressive, invasive, introduced species. It is an alien in this ecosystem and has no natural biological or environmental controls in this or many similar ecosystems. It transforms natural areas into monocultures which do not provide habitat for indigenous wildlife. While it becomes an evergreen ground cover, its landscaping value is otherwise limited and creates undesirable consequences. Its widespread popularity derives primarily from its rapid growth, its suppression of any other plant growth, and its scant requirements in cultivation. These characteristics are major reasons why it is devastating when introduced to land areas populated by native species.

When a juvenile vine comes into contact with a vertical surface and begins ascending it, rootlets will emerge more frequently along the stem, not just at successive nodes. Instead of becoming a true root, the tip of the rootlet thickens and secretes a glue-like substance, attaching the vine to the vertical surface. It is this juvenile process of creeping and developing frequent adhesive rootlets that gives ivy its climbing ability and makes established ivy difficult to remove from surfaces, such as a tree trunk. Once it is climbing on a tree, the shoots grow so as to increase the lamina area of leaves, a behavior that allows ivy to more fully benefit from the additional sunlight.

Due to one or more of these factors a tree will have a noticeable decline in health within only a few years of being infested and eventually will die, either completely collapsing or remaining as a snag. While ivy prefers climbing certain tree species over others (e.g. it will avoid climbing Western Red Cedar if there are plenty of other trees around), it is capable of climbing virtually every kind of tree in the Pacific Northwest. As a result, any infested forest is faced with wholesale collapse of the canopy. Indeed, both on the ground and in the air, once a forest has been infested, all that stands in the way of its long-term destruction are time and human (“de-vine”) intervention.

We ought not to downplay the vegetative spread of English Ivy. Its year-round growth, long juvenile phase, and other competitive advantages allow it to spread extensively and rapidly in this manner. At one site a growth rate of more than a meter per year has been reported. The most active growth occurs in the spring and summer. In places like New England this is from May to September, but in the Willamette Valley it extends from March to October. Ivy climbs deciduous trees more rapidly during the winter as well, due to the additional sunlight afforded to it by the loss of tree foliage.

Also of concern is that, when ivy is not completely removed from an area—when it is given the slightest opportunity to grow back—it usually will. Roots that remain in the ground after a work party has left, for example, can and often do “resprout.” Our own experiments and observations have revealed that the majority of regrowth at removal sites takes place in this manner, from the resprouting of remnant roots.

Cuttings and pulled vines can also reroot at the nodes extremely easily. This rerooting is so effective, in fact, that horticulturalists and commercial growers of ivy would much rather plant from cuttings than from seeds. When trying to establish a large quantity of ivy quickly, they will plant a “nodal cutting”—a section of stem with a node at the bottom and one or two leaves beyond the next node (only 5 inches in total length)—in a sand/peat compost. The best planting seasons are the spring and autumn, the very best period being the “cold frame,” from mid to late autumn.

At first glance ivy’s predisposition for rerooting may cause concern about leaving ivy vines at removal sites. However, we do not see this kind of rerooting when appropriate measures are taken. Direct contact between the node and moist soil is what allows the rerooting to take place. At a removal site, pulled vines are scattered on the ground and the odds that a node would fall into direct contact with moist soil is very low. Therefore, while we ought to be cognizant of the potential for pulled vines to reroot, especially during wet portions of the year, it does not prevent us from leaving pulled vines on site to decay and enrich the soil.

Clearly our containment strategies should reflect upon the ways in which ivy spreads. The observations and research to date point to seed dispersal as the greatest ongoing contributor to the spread of ivy. We would therefore be well-advised to focus as a project and as a community on reducing seed production.

This involves making the removal of mature or maturing ivy a top priority. In forested or other well-shaded areas this means continuing to focus on the removal of ivy from trees, since in those circumstances ivy on trees is in a better position to mature than is ivy on the ground.

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In light of the importance of seed production to the spread and establishment of ivy, the removal of flowers or berries from a mature plant also emerges as a reasonable alternative to the removal of the entire plant. Our own research has shown that if between the beginning of October and the beginning of February ivy’s flowers or berries are cut from the plant, new flowers or berries will not grow and replace them that same flowering season. If the clipping is done within this timeframe each year, the plant will be perpetually unable to produce seeds. We have also observed a reduction in the number of flowers that grow back a year after the clipping takes place, which makes the job of clipping flowers easier year after year.

In some cases clipping flowers or berries may be less or no more practical than killing the whole plant. For example, if there are flowers high up on the trunk of a tree, it would be more practical to girdle the ivy around the base of the tree than to climb up the tree and remove individual flowers. However, for those property owners who would find the removal of flowers or berries to be a more practical or desirable alternative, it would be an equally effective contribution to the community’s efforts to stop the spread of ivy.

Ivy does not stabilize slopes because it has a very shallow, mat-like root system. Vegetative slope stabilization requires a diversity of root masses of different configurations and depths which bring the water down through the soil and form an interconnecting water dispersal system. Ivy's dense ground cover and thick mat-like roots tend to pond the water on the surface of the slope making the top layer heavier and more likely to slide.