English Ivy (Invasive Exotic Plants of North Carolina)

English Ivy

Hedera helix

Summary ⁵

Hedera helix (common ivy, English ivy, European ivy, or just ivy) is a species of flowering plant in the family Araliaceae, native to most of Europe and western Asia. A rampant, clinging evergreen vine, it is a familiar sight in gardens, waste spaces, on house walls, tree trunks and in wild areas across its native habitat. It is labeled as an invasive species in a number of areas where it has been introduced.

Ecological threat in the united states ⁶

English ivy is a vigorous growing vine that impacts all levels of disturbed and undisturbed forested areas, growing both as a ground cover and a climbing vine. As the ivy climbs in search of increased light, it engulfs and kills branches by blocking light from reaching the host tree’s leaves. Branch dieback proceeds from the lower to upper branches, often leaving the tree with just a small green “broccoli head.” The host tree eventually succumbs entirely from this insidious and steady weakening. In addition, the added weight of the vines makes infested trees much more susceptible to blow-over during high rain and wind events and heavy snowfalls. Trees heavily draped with ivy can be hazardous if near roads, walkways, homes and other peopled areas. On the ground, English ivy forms dense and extensive monocultures that exclude native plants. English ivy also serves as a reservoir for Bacterial Leaf Scorch (Xylella fastidiosa), a plant pathogen that is harmful to elms, oaks, maples and other native plants.

Impacts and control ⁷

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Impacts: Reports on English ivy's impacts within its North American range are variable. English ivy threatens native plant communities and wildlands in Oregon [106,113], California [17,35], Washington D.C. [169], Kentucky [74], Georgia [45], and Alabama [2]. It is a potential threat in the upper Great Lakes areas [25], Missouri [101], and Tennessee [163]. English ivy is a particularly serious threat to native plant communities in the coastal Pacific Northwest states [106] and was placed on Oregon's list of quarantine species in 2010 [113]. A 1988 publication indicated that English ivy was not widespread in the southeastern United States [19]; however, a more recent review (2007) indicates that English ivy is rapidly invading forests in this area [12]. English ivy impacts may be less in the northeastern United States [33,176] and Canada [20,193].

NatureServe [107] has given English ivy a ranking of medium for its ecological impacts; its impacts to community structure are of greatest concern. The Plant Conservation Alliance [160] considers English ivy a "vigorous" vine that may impact all strata of a forest. In general, English ivy primarily impacts ecological communities by displacing native ground flora, weakening and/or killing host trees and providing opportunity for invasion by other nonnative species [160].

In locations where it is most invasive, English ivy may form near monocultures in the understory [106,160] and suppress growth of ground flora [4,18,24,26,106,146,170]. On Potomac Island in Washington, DC, English ivy suppressed herbs and may have suppressed woody species on upland sites. Because upland sites are not subject to flooding, Thomas [170] speculated that English ivy's impacts may be greater on upland than riparian sites. Thomas further speculated that English ivy's ability to photosynthesize year-round may improve its capacity to suppress the growth of other plants that photosynthesize seasonally [170]. As it spreads, English ivy may eventually displace [140] or inhibit the regeneration of native species [125,190]. Increased shade produced by English ivy may make it difficult for native species to establish in the understory [26]. Because English ivy displaces native plants, wildlife that utilize native plants for forage or cover may also be impacted.

Trees hosting English ivy may be susceptible to windfall during storms [97,125,136,146,160] especially if they are weak [97] or when they are supporting several English ivy stems [136]. Reichard [126] speculated that the additional weight of water or ice on the evergreen leaves of English ivy may increase storm damage to trees. Invasive plant publications suggested that English ivy decreases "vigor" in host trees [99,146], and a study from Oklahoma suggests that English ivy may inhibit development of top and root mass of host trees, particularly maples [141]. Anecdotal information suggests that as English ivy climbs, it covers and kills supporting tree branches by blocking sunlight. The host tree may eventually die from steady weakening [160,170]. American elm trees may be particularly susceptible to weakening by English ivy. In a riparian forest in Washington, DC, 13% of fallen American elm trees had supported English ivy, whereas only 9% of all the other fallen trees species supported English ivy [170].

In a North Carolina riparian forest, English ivy was associated with several other exotic species, and its occurrence was negatively correlated with native species richness (r²= -0.42). Researchers speculate that only the most "aggressive species" were able to coexist with English ivy and that English ivy's presence may promote invasion by other nonnative species because it spreads fast and displaces most native species [183].

Several other ecological impacts of English ivy invasion have been described in the literature, although most have not been well documented. One report from the Pacific Northwest suggested that English ivy may decrease water quality and increase erosion. Researchers have identified English ivy as a host for bacterial leaf scorch (Xylella fastidiosa), a plant pathogen that harms native trees including elms, oaks, and maples [95]. There is some concern that leaf litter from English ivy increases soil nitrogen, which may negatively impact native plant species that grow best in low nutrient conditions (Tremolieres and others 1988 cited in [126]). Based on stream surveys in California, North Dakota, and South Dakota, microinvertebrate frequency was reduced on sites where English ivy occurred in the riparian vegetation compared to sites where it did not occur; however, the difference was not significant [127].

Invasion by English ivy may have societal impacts as well. Trees susceptible to windfall may create a hazard if near roads, walkways, homes, or other developed areas [160]. Loss of shade trees, increased erosion, decreased water quality, and a loss of forest production due to the invasion of English ivy may be costly for public agencies as well as private land owners [146]. In Mediterranean Italy, English ivy growing on old buildings was detrimental to the preservation of an archaeological site [21].

Control: Control of English ivy has received little attention or research. Past research has focused on establishing new cultivars rather than controlling or eliminating the plant [126]. Complicating matters, English ivy continues to be sold at nurseries for landscaping [32,57,150,159], and the American Ivy Society promotes its use in gardens [165].

Fire: For information on the use of prescribed fire to control this species, see Fire Management Considerations.

Prevention: It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" native communities 91,139 and by monitoring several times each year [69]. Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader [61].

English ivy's escape from cultivation may be slowed or prevented if native species are substituted in landscaping projects. In an attempt to slow English ivy's spread in Oregon, officials have placed English ivy on the list of quarantined species, making it illegal to propagate, transport, purchase, or sell English ivy in that state [113]. It has been suggested that the best way to prevent English ivy invasion is to avoid growing it near forests [25]; however, since its seeds are dispersed by birds (see Seed dispersal), this may not prevent its invasion entirely. One study from the Netherlands suggests that the frequency of English ivy may decrease with increasing size of "woodlot" perimeter [180], so limiting forest fragmentation may reduce English ivy invasion. Thomas [169] suggested that anthropogenic ground disturbance that alters topographic relief may promote invasion by English ivy and other nonnative species and recommended that original topography be restored to sites to preclude or slow English ivy's spread.

Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management [178]. See the Guide to noxious weed prevention practices [178] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Cultural control: See Integrated management.

Physical or mechanical control: Several invasive species publications recommend hand removal to control English ivy. Vines may be cut and then pulled down from trees and off the forest floor [25,125,159,190]. Alternatively, English ivy may be pulled up from its roots; however, this method may disturb soil and promote erosion or compaction of the soil [13]. Soil disturbance may facilitate reinvasion by English ivy and/or the establishment of other invasive plants [25,126]. It may be necessary to follow hand removal with additional types of treatments (see Integrated management). Soll [146] cautions that hand removal of English ivy may be costly. In the Pacific Northwest, 2002 cost estimates ranged from $2,000 to $8,000 per acre when paying minimum wage [146].

Researchers in the United Kingdom suggested early thinning of English ivy to help prevent monocultures from forming [56].

Biological control: There are no biological control agents for English ivy. Because English ivy is an important landscape plant and has strong support from the horticultural community, it is extremely unlikely that one will be developed [146]. A study from Oregon evaluated the use of domestic goat browsing to control English ivy in a mixed-deciduous forest where English ivy formed a near monoculture in the groundlayer vegetation. English ivy's average cover declined significantly (P=0.0002) in plots that were browsed by domestic goats compared to unbrowsed plots. Average cover of English ivy was reduced to 23% on sites browsed for 1 year and to 4% on plots browsed for 2 years [66]. In the Netherlands, English ivy invaded a forest and began to climb trees soon after domestic sheep browsing was discontinued [14].

Chemical control: Information pertaining to the chemical control of English ivy is inconsistent. An invasive species report indicated that at best, chemicals offer incomplete control of English ivy [146]. English ivy may be tolerant of preemergent herbicides (Derr 1993cited in [126]), and its waxy leaves make effective application of postemergent herbicide difficult [190] even when a surfactant is added [126]. Researchers in Portland, Oregon, suggest the under some circumstances, herbicides may provide safe and effective control of English ivy, even during the winter. English ivy's response to chemical control may be influenced by the type of herbicide used, herbicide concentration, and application timing. Herbicide may be most effective when used as a part of an integrated management plan.

Researchers evaluating various chemicals for English ivy control have obtained variable results [13,97,109,161]. For information on using herbicides to control English ivy, see these publications [13,25,109,146,161].

Integrated management: If hand removal is used, follow-up with other types of treatments may improve control. Sprouts from the stumps of cut vines may be treated with herbicide [25,159] or cut repeatedly until sprouting stops [159]. A follow-up planting with native species may help prevent other undesirable plants from becoming established [13,125]. In a Southeastern hardwood forest infested with English ivy, researchers compared the effectiveness of herbicide versus hand-pulling on the establishment of native plants from seed after treatment. In one plot, vines were sprayed with glyphosate (30%) after having their leaves removed. In a 2nd plot, vines were pulled manually from their roots; in a 3rd plot, vines were untreated. Plots were then split; one half was seeded with native seeds, while the other half received no seed additions. Although both treatments reduced English ivy compared to untreated plots, hand-pulling resulted in more native seedlings, increased species richness, and higher species diversity than did spraying. Researchers speculated that ground disturbance from hand-pulling may have facilitated native seedling establishment [13].