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Invasive Plants:
The problem and what you can do about
it
What are invasive plants?
Bradford pear escapes to roadsides
and woodlands
Photo courtesy Britt Slattery, U.S.
Fish and Wildlife Service, www.forestryimage.org |
Some plant species, often introduced from other continents, can
become so well established that they out-compete native plants.
They may be called invasive species, exotic (or alien) invaders,
invasive non-natives, or noxious weeds. While terminology and definitions
can vary somewhat, the basic problem is the same. Lacking the natural
controls that keep native species in check, invasive plants flourish
at the expense of native plants and threaten the quality of forest,
wetlands, and other natural areas. Once-diverse plant communities
move toward monoculture.
A few well known examples from the plant kingdom include kudzu,
garlic mustard, purple loosestrife, and Canadian thistle. The dramatic
dominance of a species like kudzu is unforgettable, once seen in
the Southern U.S. landscapes that it now dominates. But our urban
and suburban landscapes also include some familiar horticultural
species that are proving to be problematic. A recently hybridized
version of the Bradford pear, developed to correct the tendency
of the popular street tree to crack and split, is now invading natural
areas and roadsides. Butterfly bush and burning bush have traits
that make them attractive to gardeners, but these species have also
proved to be regionally invasive in the U.S., as are a number of
other plants popular in the nursery trade.
Traits of invasive plants
Wooded area overcome by kudzu
Image courtesy U.S. Fish and Wildlife
Service
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Invasive plants have various modes of dominating natural landscapes.
Invasive plants establish and spread quickly, particularly in disturbed
soils. Often, invasive plants excel at vegetative reproduction,
producing quantities of new plants from stems or roots of the parent
plant. Invasives may leaf out early, allowing them to shade out
later-developing natives. Some invasive plants appear harmless,
but produce large quantities of seed or fruit attractive to birds,
which establish colonies of vigorous seedlings at considerable distances.
Invasive plants often are not susceptible to the same pests and
predators as natives.
Some of the very traits that give invasive plants an edge over
native species – rapid growth, attractive fruit, vegetative
reproduction, disease and pest resistance – can be seen as
desirable by homeowners and landscape design professionals. Many
invasives were in fact deliberately imported for agricultural or
landscaping uses, and only later found to be problematic. In other
cases, seeds may have inadvertently been brought into the country.
While in many cases invasive plants are of foreign origin, sometimes
a species that is native to one region of the U.S. may become invasive
in another region.
Costs
Some attempts to calculate the economic costs of invasive plants
have been made. Estimates often focus on the costs associated with
individual species, particularly those that may have impacts on
agriculture. A few efforts have been made to estimate costs more
broadly. Such estimates are difficult, because cost data is not
available for all invasives. Furthermore, most such estimates are
based on the costs of damage and control; assigning a dollar value
to ecological losses is hard to do.
One study, done at Cornell University in 1999, estimated that invasive
species (both plants and animals) cost the U.S. economy $138 billion
annually. This estimate is based on known economic losses and costs
of control efforts, but does not include loss of biodiversity and
other ecological damages. The authors estimate that introduced weeds
result in $27 billion worth of annual losses to agriculture, with
another $1 billion spent annually in weed control on golf courses,
and $500 million on residential lawns and gardens.
According to the National Park Service (NPS), invasive plants infest
some 2.6 million acres on NPS lands, with 196 of 368 National Parks
having serious infestations. NPS estimates that managing these exotic
species cost $80 million between 1996-2000. Another study estimates
that the U.S. Fish and Wildlife Service spends approximately 10
million dollars annually on control of invasive plants.
Prevention
The best deterrent to invasive species is, of course, to prevent
them from becoming established in the first place. However, it is
difficult to predict which introduced plants will become invasive.
The great majority of introduced species are non-invasive. The one
characteristic that scientists have established is that a plant
known to be invasive somewhere in the world is more likely to be
invasive in other locations.
Importance of landscape architects
Landscape architects are in a position to help control the spread
of invasive species by choosing with care the plants they use in
their designs and recommend to their clients. To do this, landscape
architects will need to be knowledgeable about invasive plants of
the region or regions in which they work. ASLA’s Board of
Directors adopted a policy on invasive species in 2003. The policy
encourages landscape architects “to use responsible design
practices that sustain the local, regional ecosystem without introducing
non-native invasive plant species.” For specific actions that
ASLA encourages its members to undertake, see
http://www.asla.org/membersonly/pdf/governance/Invasive_Species.pdf
Species Identification and Information
Resources – National
• NatureServe – This conservation
organization and its associated natural heritage centers provide
authoritative data on threatened and endangered species in North
America. Recently, the network has developed detailed assessments
of the impact ranking (I-rank) of over 450 invasive species in the
U.S. Data can be requested by location, by I-rank, or by species
name. Select the “impact rankings of invasive non-native plants”
link for instructions on conducting searches. Users can access information
in the NatureServe database at: http://www.natureserve.org/explorer/
• National Invasive Species Information Center (NISIC)
– This information center is a part of USDA’s National
Agricultural Library. It provides a variety of information including
laws, definitions, fact sheets, management plans, species profiles,
and a plant id feature. http://www.invasivespeciesinfo.gov/
• Calphotos – University of California at Berkeley
– photos database of over 120,000 photos and images, including
plants, animals, and landscapes. May be searched by state/location,
scientific or common name. Thumbnail sized images may be used for
educational purposes with proper citation.
http://calphotos.berkeley.edu/
• U.S. Geological Survey – Non-indigenous Aquatic
Species – Database searches may be conducted by species
name, state, or USGS Hydrologic Unit Code.
http://nas.er.usgs.gov/
• Plant Conservation Alliance Alien Plant Working
Group – The web-based project of this working group,
Weeds Gone Wild, provides a national list of plants infesting natural
areas. The list can be searched by state, scientific name, presence
in National Parks, and presence in mid-Atlantic Region. Detailed
fact sheets, with photos, are provided for some plants.
http://www.nps.gov/plants/alien/index.htm
• USDA-NRCS Plants Database – This
comprehensive source provides standardized information about plants
of the U.S. and its territories. Includes links to noxious weed
lists of the Federal and state governments.
http://plants.usda.gov/
Resources – Regional/State
NEW ENGLAND
• Invasive Plants of New England (IPANE) – this database
includes descriptions, photos, and distribution information for
invasive species in New England.
http://nbii-nin.ciesin.columbia.edu/ipane/
• New England Wildflower Society – Resources and links
include an online Guide to the Identification of Invasive Species,
species lists for New England states, and alternatives to invasive
speices
http://www.newfs.org/conserve/invasive.htm
MID-ATLANTIC
• Plant Conservation Alliance Alien Plant Working Group (see
description under National)
http://www.nps.gov/plants/alien/list/midatlantic.htm
SOUTHEAST
• Nonnative invasive plants of southern forests: a field
guide for identification and control – by James H. Miller.
Online book provides identifying photos and descriptions for 33
species invading southern forests.
http://www.srs.fs.usda.gov/pubs/gtr/gtr_srs062/
MIDWEST
• Exotic Plants Information Center – The Great Lakes
Indian Fish and Wildlife Commission operates this website which
provides data on exotic plants in Michigan, Wisconsin, and Minnesota.
Provides distribution maps, an extensive list of internet resources,
and a slide library with photographs of exotic species which may
be downloaded for educational purposes. Additionally, the site includes
an extensive list of internet resources. http://www.glifwc.org/invasives/
WEST
• Western Aquatic Plant Management Society – Links
to descriptions of problem aquatic plants of the western U.S.
http://www.wapms.org/plants/
• Invaders Database System – A database of exotic species
and noxious weeds in 5 states of the northwestern U.S.
http://invader.dbs.umt.edu/
Landscaping Practices
• Brooklyn Botanic Garden – An article by BBG staff
member Janet Marinelli provides a good overview of the problem and
some tips on avoiding invasives, plus links to relevant websites.
http://www.bbg.org/gar2/topics/sustainable/2000sp_invasive.html
• US Dept. of Transportation, Federal Highway Administration.
Online book - Roadside Use of Native Plants. Section 1 includes
essays on roadside restoration and management. Section 2 provides
links to plant lists by state (native plants for landscape use,
threatened and endangered species), and other resources.
http://www.fhwa.dot.gov/environment/rdsduse/
• ASLA San Diego Chapter – Invasive Ornamental Plant
Guide – includes a section on “what can I do about invasive
plants” as well as species information specific to San Diego
County.
http://www.asla-sandiego.org/content/plantguide.html#G
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