Combating the “Ivy
Desert”:
The Invasion of Hedera helix (English Ivy) in the
Pacific Northwest United States
Anne Okerman
“O Ivy, growing ivy-like,
You are found in the dense
wood.”
-Robert Graves
(Reichard, 2000).
Introduction
A plant highly regarded for its
decorative value in the landscape, Hedera
helix, commonly known as English Ivy, was introduced to the United States
from Europe in early colonial times (Wyman, 1994; Randall et al., 1996). Since then, Hedera helix has been used extensively in many parts of the United
States as an ornamental landscape plant.
However, in areas where it has been planted as a perennial vine or
groundcover, Hedera helix has invaded
deciduous forests to create what is called an “ivy desert”. An “ivy desert” describes a forested area
that has a limited number of canopy species with entangling H. helix vine wildly climbing up tree trunks and reaching out into the
canopy. In an ivy desert, there is an
absence of understory and ground cover plants as a result of the dense, thick
mat of ivy groundcover (Westbrook, 1998).
Hedera helix has developed into a
problem at a global scale in many areas where it has been introduced. It is a common, naturalized plant in
Australia, New Zealand, Hawaii, Brazil and North America (Laroque, 1998). Within North America, H. helix has been reported as a potential problems in many states
in the U.S. and up into Canada (Westbrook, 1998). It is prevalent in forests in
the Eastern United States (Thomas, 1998) and has also been found to be a
problem from Northern California forests south to the San Francisco
peninsula (see Fig. 1.) (Reichard, 2000). H.
helix is a “serious problem in the coastal Northwest from Portland up into
British Columbia” where it is invading forests and riparian zones especially
those in close proximity to urban areas (Reichard, 2000).
In
the northwestern United States, H. helix grows rapidly in deciduous
forests forming a green carpet across the forest floor and blanketing trees and
other vegetation. The regal stands of the Northwest deciduous forests, composed
of bigleaf maple (Acer macrophyllum),
Oregon ash (Fraxinus latifolia),
black cottonwood (Populus trichocarpa),
Oregon white oak (Quercus garryana), snowberry (Symphoriocarpus alba), and red alder (Alnus rubra), are threatened by the changes evoked on a forest community
by an invasion of H. helix (Hitchcock,
1973). Though the leaf foliage appears
shiny and healthy up close, from a distance, ivy that has taken over a deciduous forest
stand appears unruly and tangled. Hedera helix clearly dominates over all
other understory growth (personal observation, 2000).
While
Hedera helix thrives in moist, open
forests that are predominantly deciduous, it also grows in mixed Pseudotsuga menziesii (Douglas Fir)
ecosystems, riparian zones and wetlands of the Northwest United States
(Laroque, 1998). George Krall (2000), from the Bureau of Environmental
Services, said that H. helix is
likely to grow in rocky, drought tolerant conditions and is becoming an increasing
problem along the northwest coast line.
It has been found to grow anywhere under 3,000 feet altitude.
Hedera helix is still being sold in
nurseries of the northwestern U.S. and landscaping companies continue to use it
in commercial and residential projects.
Ivy is frequently used as a roadside planting by the Oregon Department
of Transportation because it is low-maintenance, provides a uniform groundcover
appearance, and grows in such harsh conditions. The ability of H. helix to
spread adventitiously and its dispersal from birds causes concern when the
plant is used in close proximity to forest or riparian zones.
The
impacts of an “Ivy desert” are a loss of biodiversity and decrease in native
vegetation. Presently, there is little published information on treatment
methods for Hedera helix though it is
increasingly recognized as a threat to restoration efforts and native plant
communities. The Oregon Plan for Salmon
and Watersheds lists H. helix as a
factor for decline of riparian and urban stream habitats (Oregon Plan,
2000). As restoration projects begin in
areas where ivy is likely to invade, an understanding of the species and
knowledge of control methods is important to study in order to combat the “Ivy
desert.”
Biological Control Methods of
Hedera helix
Hedera helix,
a part of the family Araliaceae (Ginseng), is a woody, evergreen climber with
perennial stems. Ivy grows well in
adverse soil conditions, both basic and acidic soils, and it is adaptable to
different levels of light (Reichard, 2000).
Hedera helix is often used as
a landscape plant in the Pacific Northwest United States because of its high
rate of survival, rapid establishment and persistence in temperate to
sub-tropical zones (Wyman, 1994).
Hedera helix can most easily be
distinguished from other vine species by its evergreen leaves that are not
pubescent and lack tendrils (Reichard, 2000). Interestingly, the most commonly
recognized leaf form of Hedera helix is,
in fact, indicative of one of two distinctive life stages of the plant; H. helix has both a juvenile and adult
form (see Fig.2) (Dirr, 1998). The leaf form of the juvenile plant is most
often recognized by its thinly elongated, 3-5 lobed leaves that are typically a
dark, glossy green with whitish veins (Reichard, 2000). During the juvenile or
non-reproductive stage, Hedera helix
is typically a ground cover. The leaves
of the adult or reproductive form are usually a lighter green, thick, ovate to
rhombic in shape and have less prominent whitish veins. During the adult stage, H. helix produces terminal clusters of greenish-white flowers from
September to October pollinated by wasps, bees, and flies along the northwest
coastline of North America (Reichard, 2000; LaRoque, 1998; Tremolieres et al.,
1988). The following spring Hedera helix produces a dark, purple
fleshy drupe fruit (see Fig. 3) (Dirr, 1998).
The
life phases of Hedera helix are an
interesting feature from “an ecological, physiological, and anatomical
perspective” (Laroque, 1998). Drastic
developmental changes occur in the plant during the juvenile and adult
phases. This is believed to be the
result of reduced gibberellic acid caused by the absence of abundant roots,
which triggers the change to its adult or arborescent form (Lee and Richards,
1991; Laroque, 1998).
In
its juvenile form, Hedera helix has
adventitious rootlets located at the leaf nodes on the stem. The rootlets allow
the plant to climb trees, walls, and other vertical structures. The vine attaches to surfaces but does not
penetrate through mortar or tree bark, thus it is not considered to be a
parasite (Elliott, 1995). In its adult
form, on the other hand, H. helix has
erect, woody, non-climbing stems which result in a shrub-form (Reichard,
2000). Botanists have argued that the
thick, woody stem of H. helix allows
it to be classified as a “liana” as opposed to a “vine” (Gentry, 1991; Laroque,
1998). Lianas are primarily found in
tropical forests where they have “evolved their climbing strategies” (Gentry,
1991). Lianas are also represented in
South American temperate forests (Laroque, 1998).
The
growing conditions of lianas are relative to seed dispersal characteristics.
Lianas that grow in dry forest conditions have been found to be primarily
wind-dispersed (Gentry, 1991). The
seeds of Hedera helix, and other
lianas that grow well in moist forest conditions are typically dispersed by
birds and other wildlife (Gentry, 1991). The seeds of Hedera helix are 70% viable though they require scarification of
the hard seed coat for germination; this occurs when bird species eat the
berries, remove the pulp, and pass the seeds through their digestive tracts
(Cleargeau, 1992; Reichard, 2000).
The
berries of H. helix are mildly toxic;
concentrations of the toxins change as the seeds ripen (Barnea et al., 1993). The mild toxicity of the seeds discourages
consumption of too many berries at fruits at once so that few seeds will be
deposited one at a time, thereby resulting in a high dispersal rate (Barnea et
al., 1993). Also, the residual time of H. helix seeds in the stomach of birds
is short enough so that seeds are more viable when they are released
(Cleargeau, 1992; Barnea et al, 1993).
Hedera helix thrives in dark, moist
forest conditions yet also climbs to heights of 270 meters (90 feet) toward the
canopy where greater amounts of light are available (Dirr, 1998). The leaves of the adult form of H. helix are adapted to higher levels of
light than that of the juvenile plants
(Laroque, 1998). The leaves on
the juvenile plant, however, are more adapted to lower light levels
characteristic of the forest floor which allow it to spread and form a dense
vegetative cover that can eventually be 6” thick (Devero, 2000). A high level of adaptability to a range of
light conditions enables H. helix to
more successfully colonize the understories of deciduous woodlands with
fluctuating levels of light, thereby facilitating its invasiveness (Laroque,
1998; Bauer and Thoni, 1988).
Origin and Geographic Location
Hedera helix is native to Europe,
specifically England, Ireland, the Mediterranean region, and northern Europe
west to the Causcaus Mountains (Reichard, 2000). In certain areas of its native zone, Hedera helix is considered to be very weedy as an aggressive vine
that virtually smothers most seedlings, breaks tree branches, and accelerates
the death of trees (Wyman, 1994). In
1939, Queen Elizabeth of England set up “Ivy Squads” devoted to stripping trees
and walls where the vigorous vine has the potential of growing to be six inches
thick (Elliott, 1995). Her goal from
this effort was to protect the walls from destruction and prevent the
strangling of trees as a result of being smothered by the ivy. Conversely,
others argue in the defense of Hedera
helix, insisting that it benefits a forest community by protecting the
woodland floor from frost, supporting trees structurally, and providing a
winter food source for ground foraging birds and mammals, such as starlings (Sturnus vulgaris) and elk (Cervus elaphus) (Fearnley-Whittingstall,
1992). The disagreement of the
character and impact of H. helix has
resulted in considerable debate regarding the destructive nature of ivy in its
native habitat (Elliott, 1995).
H. helix is abundant in the alluvial
temperate forests of Europe. In eastern
France, H. helix favors moist,
calcareous, eutrophic soils. In the Rhine forest, ivy is most adaptable to
highly fragmented or successional forests that have been disturbed by an
increase in light (LaRoque, 1998, Tremolieres, 1988; Schnitzler, 1995). Hedera helix has also been found to be
invasive in floodplain areas where a flood disturbance has occurred (Thomas,
1998). Though it is most commonly seen
growing along forest edges in these areas, seed dispersal is not limited to
edge habitat. Hedera helix, in its juvenile stage, will grow along a forest floor
until a gap in the canopy stimulates its growth up host trees (Laroque, 1998;
Schnitzler, 1995).
Many
of the species in the genus Hedera
are widely distributed and adaptable.
The species H. helix is
noted to have over 400 variants or cultivars each numbered and accounted for by
the American Ivy Society (AIS) and each with slightly different ecological
tolerances (Laroque, 1998; American Ivy Society, 1999). The adaptability of this species or
“ecological plasticity” results in its popularity as an ornamental among
landscapers and horticulturists as it is adept at filling so many rolls and
versatile enough to find a niche in almost any space [or] climate
(Wellingham-Jones, 1985). Though Hedera helix will grow in variable light
conditions, it prefers shade, damp soils, and a moist, cool environment
(Morisawa, 1999). H. helix is limited by extreme moisture due to high water tables
(Schnitzer, 1995; Thomas, 1998). Ivy
also requires temperate to subtropical climate where it is able to winter over,
accommodated by the climate and conditions of the Northwest United States
(Reichard, 2000).
Ecological Impacts
A
dominance of Hedera helix
significantly changes the structure of a forest community. It outcompetes many native plant communities
of grasses, herbs, and trees, reduces animal feeding habitats, and creates
general competition for light, nutrients, and soil (Morisawa, 1999) (Randall et. al., 1996).
In an “ivy desert,”
previous generations of ivy that have died out on the forest floor create a
thick blanket of vegetative mass that prevent the regeneration of understory
trees, shrubs, and perennial groundcovers.
This has an effect at the local scale for ground flora, tree sprouts,
and other vegetation (Devero, 2000; Reichard, 2000). By suppressing the
regeneration of a diverse forest community, the long-term persistence of a
forest is also jeopardized (Reichard, 2000).
As an evergreen vine, Hedera helix has an advantage of being able to photosynthesize during the winter months in temperate to sub-tropical climates while deciduous trees are dormant. The increased light that is available to H. helix by the absence of deciduous leaves allow it to grow more rapidly up the trunk of the host tree (Thomas, 1980). The evergreen leaves of the plant also inhibit the leaves of the deciduous tree thereby suppressing the growth of the host tree (Reichard, 2000). The increased openness of the tree crown further stimulates the growth of the vine (Reichard, 2000; Thomas, 1980). As H. helix grows up a host tree to reach the canopy, the density of the vine as well as the weight of water and ice on the leaves increases the weight of trees. The added weight increases the susceptibilty of tree branches to snap and break during moderate to high wind storms (Devero, 2000).
Devero (2000), the manager of Tryon Creek State Park in Portland, Oregon, stated that Hedera helix provides minimal habitat value where it is dominant in a forest. Hedera helix provides little coverage for wildlife because the foliage of the canopy vines and on the ground is so dense. H. helix also provides very little to almost no subsurface habitat. Devero also speculated that H. helix provides little nutrition for the birds and wildlife that consume its berries (2000).
Other major concerns, voiced by M.G. Devero as well as George Krall of the Bureau of Environmental Services in Portland, Oregon, are that an increase in exotics are establishing within ivy deserts (2000). Exotics such as Clematis vitalba, another worrisome invasive, are able to compete with Hedera helix (Krall, 2000).
Methods of Control
The adaptability of Hedera helix to different light and soil conditions as well as its
long season of active growth, rapid growth rate, and ability to root along the
stem enables the H. helix to become an invasive and undesirable plant in
deciduous forest ecosystems (Derr, 1993).
In California, Oregon, and Washington, efforts are being made at this
time to list it as a noxious weed, though it has not yet happened (Krall,
2000). H. helix is recognized as a serious threat to forest longevity and
to beginning restoration projects (Laroque, 1998). At this time, restorationists consider site preparation,
maintenance, and treating adjacent H.
helix infestations as necessary factors to prevent an “ivy desert.” Methods are being tested at this time to
control Hedera helix and some have
proven effective. However, there is not
yet a guaranteed approach (Krall, 2000).
Herbicide application
The
waxy cuticle of H. helix allows the
plant to have a high resistance to herbicide uptake, thereby, creating great
complication during attempts to treat the plant (Morisawa, 1999; Derr,
1993). Studies prove that H. helix is tolerant of preemergence
herbicides (Derr, 1993). Multiple
applications of postemergence herbicides have proven to be more effective
though success varies according to the age or maturity level of the plant
(Derr, 1993). Herbicides absorption is greater in newer shoots compared to older,
more mature leaves (Derr, 1993).
Round-up (glyphosate) applications of 3.0 kg/ha (2.7lb/Acre) on younger
plants proved most effective when applied during spring months (Derr, 1993;
Reichard, 2000). Application of
Round-up (glyphosate) on mature plants retarded growth up to 60% though proved
ineffective to completely destroy H. helix even with a higher
concentration, a second application, or use of a non-ionic surfactant
(Reichard, 2000; Derr, 1993)). Weedar
64 (2,4-d) applied at a rate of 1.1kg/ha (1lb/A) did control H. helix when applied twice (Derr,
1993). In some of the treatment plots,
however, ivy was reestablished after two years from advances of adjacent
populations that were untreated (Reichard, 2000).
George Krall of the Bureau of Environmental Services in Portland, Oregon (2000) also mentioned the combination technique of using Scythe (pelargonic acid) with Round-up (glyphosate). Scythe is a non-selective herbicide that effectively burns through the leaf cuticle, killing active leaf tissue; it is appropriately named after the Grim Reaper’s tool (Thomson, 1997; Gilman, 2000). It is assumed that once the pelargonic acid has been applied, Round-up (glyphosate) will be able to penetrate through the leaf cuticle more successfully and then be absorbed by the plant through transpiration (Krall, 2000). This method is being used in riparian zones, wetlands, and upland forests throughout Portland.
There is speculation that the method of combining
pelargonic acid and glyphosate may not be very effective. Round-up (glyphosate), a systemic herbicide,
needs active tissues to enable transportation to the roots of a plant for it to
be effective (Gilman, 2000). Applying
Scythe (pelargonic acid) will destroy leaf tissue resulting in a “reduced effect”
of the Round-up (glyphosate). Gilman
(2000) suggested researching an alternative method of using Round-up
(glyphosate) with a controlled droplet application or electrostatic sprayer. This treatment system would charge the ions
of Round-up (glyphosate) so that herbicides will more successfully adhere to
stomates on the underside of the plant, avoiding its waxy cuticle and resulting
in increased uptake (Gilman,2000).
Studies have examined this method and determined that it allows for a
more accurate application of herbicides and increased effectiveness (Gebhardt,
1984).
Physical Removal
Persistent
cutting of H. helix is a method that
is being used in many parks and nature areas within the Northwest United States
(Morisawa, 1999). Cutting with a pruners and then pulling the plants from trees
and the forest floor may be the most effective technique (Reichard, 2000). Tryon Creek State Park in Portland, Oregon
has an official Ivy Removal Day on a monthly basis; volunteers visit the park
and cut H. helix from the infested
areas (Devero, pers. comm., 2000). In
these areas, it is most effective to separate the climbing ivy from its roots
by cutting a 3-foot swatch around the host tree (Devero, 2000). H.
helix vines begin to die after 2 weeks during drier summer months and
within a month during the early spring or early fall (Devero, 2000). If vines are too thick to cut, one can strip
back the bark, notch the exposed section, and apply a diluted herbicide such as
Round-up (glyphosate) (Morisawa, 1999). Other programs in state parks include “Adopt-a-Plot” where
volunteers visit the park and remove ivy in a specific place and then routinely
visit for two years to check for new shoots (Devero, 2000).
Other physical removal methods include using an
edger/trimmer (manufacture’s name: weed eater) to cut the woody stems of H. helix, exposing the inner bark. An application of an herbicide such as
Round-up (glyphosate) or Garlon (2,4-d) on cut stems and leaves can then
effectively penetrate into the plant (Reichard, 2000). In one case where a
string trimmer was used in combination with herbicide application, the
treatment successfully killed the plants though the area was invaded soon after
by adjacent populations of H. helix (Reichard,
2000).
Another more drastic method has been to use a
blow-torch to repeatedly blast the plant with a hot flame. By repeatedly exposing the plant to high
heat, this method is intended to exhaust the H. helix of its energy so that it is unable to multiply or produce
berries for reproduction (Reichard, 2000).
For the physical removal strategies, care must be taken to not disturb
the soil which might encourage invasion of other exotics. Immediately after H. helix removal, native plants should
be planted to replace vegetation and for soil stabilization (Reichard, 2000).
Biological Control
Presently, biological control of ivy has not been
attempted. A fungus (Phoma hedericola) has been damaging a
population of H. helix in Italy and
may be a potential candidate as an introduced biological control (Franco et
al., 1992). Extensive research is
necessary, however, before considering this as an optional treatment
method. It has also been speculated
that it will take considerable persuasion to be able to introduce a biological
control for H. helix (Krall,
2000). Some argue that introducing a
biological control is extremely unlikely due to the importance of Hedera helix as a landscape plant and
support it receives from the American Ivy Society (Reichard, 2000).
Discussion of Control Methods
The
success of the control methods for Hedera
helix may be dependent upon
the size and scale of the restoration project.
For small scale projects, such as a backyard or small forest, herbicide applications
and physical removal strategies, described previously, can be most effective
with constant monitoring and repetitive application. Large scale projects might also be treated with these methods
although they are incredibly time and labor intensive and not always a
“satisfactory treatment” (Krall, 2000).
For chemical, physical, and biocontrol methods, there are many factors
to consider when selecting a proper treatment to make it the least harmful to
other species and most efficient in terms of time and energy.
As
stated previously, Hedera helix invasions
are more likely to occur in close proximity to highly developed areas,
therefore, close to human habitation.
Considering the effect of herbicide application is necessary so that it does
not have the potential of threatening human health as well as other species
through groundwater or airborne movement. Physical removal strategies are most
successful with repetitious treatments to prevent H. helix from encroaching from other areas. Physical removal methods may be most
successful when used on small scale restoration sites or when there is a large
group of committed people to frequently visit and manage the area. A biological control method may be an answer
to dealing with the widespread invasion of Hedera
helix. Before considering the
introduction of a biological control method, however, it is important to
consider the long-term consequence of introducing a non-native species through
research.
Prevention in
Restoration Areas
As
restoration projects are considered in regions where Hedera helix is present, methods for controlling this invasive
species need to be considered and further explored. The efforts to protect
restoration areas from Hedera helix include
eliminating the existing plants, reducing the seedbank, and controlling
adjacent populations of ivy (Krall, 2000).
Constant monitoring is necessary and may require physical cutting and
pulling of new shoots of H. helix in
addition to applying herbicides (Krall, 2000).
Considering a biological control may be a method to address the larger
landscape.
In
the case of Hedera helix (English
ivy), community education and involvement is necessary. Action at the local government level would
enable establishment of restrictions for general use of this plant. Hedera
helix continues to be sold in nurseries in the Northwest and landscaping
companies continue to use H. helix in
projects. The ability of H. helix to spread adventitiously and
its dispersal from birds causes concern when the plant is used in close
proximity to forest or riparian zones.
As urban areas grow and the landscape is fragmented, there is an
increased opportunity for Hedera helix
to invade susceptible ecosystems expanding the “Ivy desert”.
Invasive
plants, such as Hedera helix (English
Ivy), are encouraged by human activity when they are used in residential and
roadside planting (Laroque, 1998). The
human use of English ivy as an ornamental plant has a great potential to alter
natural ecosystems (Laroque, 1998).
Awareness can be increased through educational programs of H. helix’s long-term effects that
threaten native plant communities, increase other exotic plant populations, and
decrease habitat for bird, invertebrate, and wildlife populations.
Those working in Portland, Oregon speculate that
because of its “ecological plasticity,” the invasion of H. helix has the potential to increase its severity in the future,
comparable to Pueraria lobata (Kudzu)
in the southern United States (Krall, 2000; Kimpo, 2000). There is a great need to recognize Hedera helix as a species that has
serious potential for altering ecosystems for the long-term in the Northwestern
United States. Natural ecosystems as
well as ornamental landscapes are amenities for humans and a balance must be
achieved for both (Laroque, 1998). The
repercussions of introduced species that invade indigenous ecosystems is a
serious problem that is often overlooked. The global issue of invasive species
is a critical and underestimated problem that has the potential to
significantly change all remaining “wild” areas, thereby insisting that action
begins to restore ecosystems and control exotic species.
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