Hedera helix: Effectiveness of Removal Protocol Adapted to Field Research Data
Forest Park Ivy Removal Program
- English ivy (Hedera helix) is a non-native, invasive plant in Pacific Northwest forests where it can form dense “ivy deserts” that preclude growth of native species. Although ivy can thrive in the dense shade under the forest canopy, it seems to need high light conditions in order to flower and set seed. In the Pacific Northwest forests, ivy usually climbs trees in an attempt to reach light.
- In 1994 the Ivy Removal Project was organized to develop control strategies for invasive Hedera helix in Forest Park, a 5400 acre urban forest preserve within Portland, Oregon. Initial methods for removing ivy from trees involved cutting the vines around the trunk’s lower portion and stripping them away, then pulling ground ivy within a three foot radius around the tree.
- A field research study undertaken by Nancy Broshot and Amanda Durkee into the growth patterns of Hedera helix found that ivy growing within 1.8 meters of a tree was significantly more likely to grow towards the tree than in a random direction or away from the tree (p=0.006). Existing protocol was adapted to these findings by increasing the ground removal zone to a six foot radius around the base of the tree. Because many of our workers are unaccustomed to the metric system, feet were used to clarify the distance.
- The removal protocol was given the moniker “lifesaver” to create a strong visual image of the tree bole being at the center of the six foot ivy free ring. This new protocol was first used in the summer of 1996 with annual monitoring conducted where the modified protocol was employed. Many trees given a “lifesaver” in 1996 and subsequent years still did not have ivy regrowing up the trunk in the summer of 2002.
- The extent to which the tree remains free from regrowth up its trunk appears to depend on the quality of the removal work performed on the ground cover ivy and the extent to which the roots are thoroughly removed within the six foot radius.
- Additional field studies are being conducted to determine the effectiveness of this intervention and to ascertain how long regrowrth up the trunk is inhibited by this control strategy protocol.
- The Problem
- Hedera helix outcompetes all native plants in the Pacific Northwest ecosystems.
- It infests all habitat types but especially is a problem in riparian areas and woodlands.
- It is an escaped ornamental used extensively in landscaping that invades the urban forest and natural areas near housing and commercial development.
- It covers the ground with a thick mat of vines under which lies a shallow but intertwined root system.
- A dimorphic species, it seeks more light than is available under trees or the forest canopy in order to mature and set fruit.
- Its growth on trees creates unfavorable conditions including susceptibility to toppling and blow down.
- Once the trees topple or the canopy collapses, an area becomes vulverable to colonization by other invasive plant species that are not shade tolerant.
- Use of herbicides is problematic in the Park due to high volunteer involvement and public perceptions.
- Initial Removal Protocol
- Ivy is not parasitic, thus once the vines are separated from the root system, the vines dehydrate then ultimately fall from the tree.
- Ground cover ivy at the base of the tree will immediately begin growing back up the tree without some form of intervention.
- A method for removing ivy from trees was developed in the summer of 1994 and implemented at several work sites in the Park.
- The procedure was described as a “lifesaver” to create a visual picture of the desired result.
- Ivy vines growing up trunk of tree were “girdled” by carefully cutting through the vine at two locations near the ground.
- The vines were stripped between the two cuts and ivy pulled away from the base of the tree at least three feet (one meter).
- Monitoring of trees after ivy removal in 1995 revealed that ground ivy was already beginning to grow back up the trees.
- This data indicated that trees needed annual intervention until all ground ivy was removed from the area, thus making the prospect of major impact on the estimated 1000+ acres of the Park problematic.
Getting to ground zero
Left to right: mature fruiting ivy, mature and billowing ivy, mature ivy on trees, mature ivy infestation
Left to right:
Immature ivy on a tree, immature ivy, immature ivy infestation
Implications for Invasive Plant Species Control
Long term control of invasive species that cause a loss of biodiversity requires more than physical and biological intervention in an ecosystem.
Social change including values, beliefs, and behaviors must also occur to have long term benefits from control efforts.
Connecting control activities with community volunteers who are guided by professional resource managers demonstrates the value of systematic inquiry and the beneficial impact of applied research findings.
One especially valuable way to make this connection is through service learning and field research projects by students from either colleges or high schools.
The connection between management activities and inquiry supports increased public knowledge and awareness, which are necessary to influence the social norms associated with desired changes.
That is, the community becomes an important part in solving the problems it has unwittingly created.
Above: Ivy dies off above the ‘girdle’
The Partial Lifesaver
Above: ivy is stripped from the tree but not pulled full 6’ from tree.
- Adapting Research Findings to Removal Protocol
- Research findings indicated the point at which the growth habit is directional versus random to be 1.84 meters but due to the unfamiliarity of most U.S. citizens with the metric system, the measurement was expressed as six feet.
- Starting in the summer of 1996, ground cover around the base of a tree where ivy had been girdled was removed for a full six foot radius around the tree.
- However, volunteers associated with the project’s removal activities would frequently short cut the full six feet of ground removal in order to move on to “save” another tree.
- The actual pattern found at work sites thus included complete “lifesavers,” “partial lifesavers,” and “girdles” only.
- Monitoring data as early as 1997 began demonstrating the differences of ivy re-growth up the tree among the three variations of protocol.
- Full lifesavers with careful, meticulous attention to vine and root removal within the six foot radius clearly continue to have the least re-growth of ivy up the tree by year.
Significant Conclusions and Future Considerations
When carefully performed, the “lifesaver” protocol developed as a result of this field research will keep ivy from growing back up a tree for at least four to seven years.
Although thorough ground removal for a full six feet radius takes an additional hour to hour and a half, the method provides a safety net period of several years before regrowth up the tree needs attention again.
With removal of ivy from trees one of the highest priorities, the adapted lifesaver method supports removal efforts being applied to many additional areas, rather than repetitive intervention at the same work sites.
Future field research and monitoring should determine how long the adaptive lifesaver protocol is effective, as well as investigating treatment amendments such as adding bark dust in the ivy free ring, tipping vines with chemical, and other means of improving or adapting the current method.
Above: Ivy infestation has re-grown
Above: Ivy starts creeping back up the tree
Ivy within an average distance of 1.84 meters from the nearest tree was significantly more likely to be growing towards that tree (p=0.006) as determined using a t-test.
Left: Researchers studying a test plot
Above: Cut the vines at shoulder height and ankle height then strip them carefully off the tree.
The Lifesaver Method
The growth of English ivy was not random, but appeared to grow toward of nearest tree when the ivy vine was at an average distance of 1.84 meters from that tree appeared to grow in a random direction at greater distances. Such growth pattern could be an adaptive strategy to ensure that the ivy will find the nearest tree to grow up and thus complete its life cycle. While the mechanism of such a pattern is not known, it is speculated by the authors that this could be a reaction to light (or lack thereof). A study comparing the growth patterns of English ivy as compared to light intensity is underway, but so far no conclusion has been reached.
Below: left to right: Getting to the root, rolling back the ivy mat, lifesaver in progress, ivy dying on trees, tree recovery.
Volunteers with a trophy taken when girdling ivy on a tree.