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Public Roads - September/October 2002

War on Weeds

by Bonnie Harper-Lore

Anyone who has swatted mosquitoes buzzing around a patio or pulled weeds in a vegetable garden knows that nature often does not obey human or political boundaries, even those between countries. And nature's weeds find the roadsides along highways particularly attractive.

Invasive plant species, otherwise known as non-native weeds, can be accidentally or intentionally introduced into a landscape. Invasives can have a devastating impact on native ecosystems, out-competing native plants and crops for space, light, water, and nutrients. Scientists blame invasive plants for a variety of environmental and economic ills, including the loss of wildlife habitat, grazing lands, agricultural crops, land values, biodiversity, wetland function, and recreational use.

In May 2002, to help combat the continent-wide problem of invasive plant species, the Federal Highway Administration (FHWA) sponsored an interagency international conference on invasive plants. The "Weeds across Borders" conference in Tucson, AZ, brought together approximately 100 vegetation management professionals from Canada, Mexico, and the United States to share their experiences managing invasive plants along roadsides.

The majority of attendees noted that purple loosestrife is a wetland pest in their country, although few reported it in Mexico at this time.

Highways crisscross and connect all three countries, and the transport of people, plants, animals, and products along the roads serves as a conduit for introducing invasive plants. As Roy Reichenbach, weed and pest program coordinator at the Wyoming Department of Agriculture, says, "Most weeds left to their own devices don't really travel, but once they enter interstate commerce, they can go a long way."

A recent increase in the coordination of Federal and State efforts to control weeds led to interest in what Canada and Mexico are doing to manage the threat of invasive plants. The Weeds across Borders conference provided a forum to learn about our neighbors' activities, as well as to explore common weed problems, applied research, best management practices, and opportunities for international cooperation.

Invasive Plants 101

When invasive plant species—also called exotics or aliens—are accidentally or intentionally introduced into a new environment, they are free from the natural competitors that existed in their places of origin. They can adapt to their new environments quickly and proliferate aggressively—often at the expense of native plants and wildlife in the area.

Documented conduits for accidental introductions include agricultural seed, livestock, packing materials, immigrants, and ship ballasts. Leafy spurge and spotted knapweed are common examples of accidentally introduced weeds.

The spotted knapweed is an invasive plant found throughout the West.


Many intentional introductions occur when horticulturists or farmers import nonnative plants from other countries to solve agricultural problems such as the need for rigorous and hardy pasture grasses (e.g., reed canary grass) or for use as ornamental plants (e.g., Russian olive).

Although many invasive plants travel to their new environments from other countries or continents, some plants that Americans consider "native" are actually invasive. The black locust, Robinia pseudoacacia, for example, is native to the Appalachian Mountains, but it became invasive in other regions of the United States when farmers and other landowners began transplanting the trees for use as windbreaks or to cut for fence posts.

Controlling Weeds in the United States

Most States have noxious weed laws listing the plants that are most troublesome to agriculture, human health, and the environment. State weed lists may contain as few as 4 plants, or as many as 124 species, that State and county weed inspectors monitor or control. Eleven States do not have specific weed laws at all.

Reed canary grass, shown here growing on a raised bank along a highway, is an example of an intentionally introduced invasive plant.

Recently, the Western Governors' Association called for a list that their States could use for targeting improvements for the prevention and control of invasive plants.

Since 1993, 16 Federal agencies have pooled their resources to deal with invasive plants. In 1994, the agencies signed a memorandum of understanding that created the Federal Interagency Committee for the Management of Noxious and Exotic Weeds (FICMNEW) as a mechanism to share information and resources in what the committee refers to as the "war on weeds."

By 1999, scientists at every level of government and land management recognized the costly economic and environmental impacts of uncontrolled invasive plants. A letter signed by 500 scientists prompted then-President Bill Clinton to issue Executive Order 13112 on invasive plants, announced on Earth Day 1999. The executive order, which defined invasive species as "an alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health," directed all Federal agencies to review the issue of invasive plants and attempt new levels of cooperation, communication, and information-sharing.

The order also created the National Invasive Species Council to write a national plan and oversee efforts to control invasive plants and animals. The council published its plan in 2001, and, since then, individual States have begun establishing stakeholder councils to address local concerns. These councils connect with the national council for support and information exchange.

In 2002, when Cornell University reported that the annual economic impact of invasive plants in the United States exceeded $37 billion—including crop and forage losses, property devaluation, and control costs—the National Invasive Species Council began seeking congressional action to support a national war on weeds and funding for research, prevention, control, and public education.

Applied Research along the Borders

The Weeds across Borders conference offered a forum for representatives from Canada, Mexico, and the United States to open a dialogue on invasive plants. During one plenary session, researchers from each country shared the results of applied studies dealing with invasive plants along the U.S. borders.

At the U.S.-Canadian border, the Canadians want to stop the northward migration of the saltcedar tree, a common invasive tree in the western United States. According to research conducted by Cheryl Pearce at the University of Western Ontario, the common saltcedar, or Tamarix ramosissima, continues to adapt and move northward, now taking root within 100 miles of the Canadian border. Pearce's research found that one source plant in Montana has been linked to 30,000 saltcedar seedlings.

At the U.S.-Mexican border, the U.S. Department of Agriculture is considering introduction of the alien buffelgrass as a pasture grass. But vegetation managers in Mexico are monitoring the spread of this invasive plant, which has proved to be a costly problem on the Mexican side of the Sonoran desert. Alejandro Castellanos, a professor at the University of Sonora in Mexico, has documented the introduction of this grass from Africa and its movement through arid lands in northern Mexico. Originally introduced to improve rangelands, buffelgrass has a propensity to burn easily. Historically, fire is uncommon in the desert because sparsely growing plants cannot carry a fire, but after the introduction of buffelgrass, fires have become more common. Because most desert plant communities cannot tolerate this kind of burn, they consequently decline, resulting in diminished plant diversity and wildlife habitat.

The Wyoming Department of Agriculture uses biocontrols to manage the invasive leafy spurge.


Cornell University's Tony DiTomasso, who researches roadside management issues in Quebec, explained how electing not to manage one particular invasive plant actually caused problems in protecting human health. After Quebec banned the use of herbicides in urban corridors, common ragweed—an invasive plant whose seeds remain viable for 40 years and whose pollen can travel as far as 250 miles in the wind—increased dramatically, creating severe medical problems for individuals with allergies. While eliminating the use of herbicides, vegetation managers actually exacerbated the ragweed problem.

Ultimately, DiTomasso's example illustrates the need for additional research, information-sharing, and consideration of multiple strategies for invasive plant control.

Attendees Share Best Practices

Another session of the Weeds across Borders conference focused on the prevention and control of invasive plants. Representatives from each country described their best practices in preventing the spread of invasives. Geographic information system (GIS) mapping, the use of biocontrols, and weed-free forage and mulch were among the management strategies shared by attendees.

Ira Bickford, maintenance chief with the Utah Department of Transportation (UDOT), reported on UDOT's innovative Roadveg system, which uses GIs to inventory invasive plants. By mapping the location of invasive plants along with other standard transportation-related data, vegetation management professionals can track the spread of invasives or monitor the progress of mitigation strategies. UDOT also included invasive species in its decision-making process mandated by the National Environmental Policy Act. The agency documented potential environmental impacts and listed appropriate best management practices as mitigation commitments for all projects that have the potential to spread or introduce listed weeds.

A few invasive weeds: Kudzu (left), Russian Olive (below center), and Tree of Heaven (right).
Salt Cedar (left), Black Locust (center), and Star Thistle (right).


The State of Wyoming found that biocontrols (natural insect enemies of weeds) are highly effective in controlling certain pesky plants. Roy Reichenbach, weed and pest program coordinator at the Wyoming Department of Agriculture, reported that biocontrols helped the State manage two invasive plants, the musk thistle and leafy spurge.

"Thanks to two insects, Rhinocyllus conicus and Trichosirocalus horridus, we have seen the musk thistle population collapse," Reichenbach said. "We used to have hundreds of thousands of acres of it in Wyoming, but the biocontrols have been so effective we are thinking of removing musk thistle from the designated weed list."

Wyoming, nine other States, and a Canadian province are participating in a certified weed-free forage and mulch program sponsored by the North American Weed Management Association. "Forage," which is vegetation fed to livestock, also is a term used to certify mulches in many western States. The certification process involves a certifier walking the fields of origin and surveying the field boundaries for evidence of noxious plants. Once certified, the forage or mulch can be baled and marked with color twine, or otherwise tagged as certified.

The Wyoming Department of Transportation uses certified weed-free seed and mulch to keep weeds from entering the environment at construction projects. Seed packers use different screens and blowers in the packaging process to remove weed seeds, and laboratory analyses help ensure the purity of the final seed product. Many State departments of transportation (DOTs) now specify weed-free seeds and mulches on construction and reseeding projects.

Montana State agencies, including the Montana Department of Transportation (MDT), also participate in weed-free forage programs. The Montana Department of Agriculture now monitors and certifies forages as "weed-free," while MDT specifies the use of weed-free mulches on all of its highway projects. This interagency cooperation provides farmers with a more pure feed option and promises to help MDT avoid inadvertently introducing weed seeds during construction projects.

In the field, the Canadian, Mexican, and U.S. attendees witnessed invasive plants spreading across the Santa Rita Experimental Range.

Information Exchange

In the course of the discussions, conference attendees provided updates on current projects and offered recommendations on strategies for sharing information. Shaffeek Ali, head of pest risk management with Alberta Agriculture, Food, and Rural Development in Alberta, Canada, noted that his country, like the United States, historically tended to be more reactive in dealing with invasive plants. He recommended that vegetation managers focus more on preventative and proactive measures. In the interest of sharing information, he told attendees that the invasive plant, scentless chamomile, is a widespread problem in Alberta and advised that measures be taken to prevent this weed from becoming established in the United States. Canada already has developed containment programs to control the weed.

"By alerting each other to certain weed species that have become problems in certain areas, and sharing control strategies," Ali said, "we can avoid recreating the wheel and save resources as well. Alberta, for example, undertook an eradication program for diffuse and spotted knapweed to avoid the large infestations that are present now in Montana and British Columbia."

Richard D. Lee, an extension weed scientist at New Mexico State University, pointed to rapid response as a highly effective tactic for controlling invasives. "If, while working in the field, traveling down a road, or even walking to a favorite camping site, you recognize an invasive weed, then pull it up, bag it, and dispose of the culprit, and notify the landowner of your find," he said. "Taking a few minutes and eliminating a single plant could prove to be much more effective than attending a dozen weed meetings."

Bruce Eilerts of the Arizona Department of Transportation (ADOT) reported to attendees on one of the first international roadside vegetation agreements on the control of invasive plants. Vegetation managers from ADOT, following in the footsteps of highway design and construction engineers when they go across the border, will begin demonstrating management methods to their colleagues in Sonora, Mexico. With this precedent established, other border States could consider similar information exchange partnerships.

John Randall, director of The Nature Conservancy's wildland invasive species team, discussed the results of a recent workshop in St. Louis, MO, aimed at exploring new ways to stop the introduction and spread of unwanted plant species. Botanical garden representatives, nursery professionals, landscape architects, garden clubbers, and government experts put their heads together to produce the St. Louis Declaration on Invasive Plant Species, a document listing findings from the workshop.

Be Alert for These Roadside Invasive Top 10

1. Kudzu, Pueraria lobata—spreads from Washington, DC, to Portland, OR.

2. Purple loosestrife, Lythrum salicaria—invades wetlands in all 50 States.

3. Leafy spurge, Euphorbia esula—compromises grazing and Western State economies.

4. Spotted knapweed, Centaurea maculosa—moves throughout the West and eastward.

5. Star thistle, Centaurea solstitialis—degrades natural areas and rangeland.

6. Giant phragmites, Phragmites australis—reduces diversity in wetlands.

7. Reed canary grass, Phalaris arundinacea—infests wetlands across the country.

8. Russian olive, Elaeagnus angustifolia—dominates lowland forests and pasture meadows.

9. Tree of heaven, Ailanthus altissima—appears from coast to coast.

10. Black locust, Robinia pseudoacacia—decreases biodiversity in deciduous forests.


The declaration contains a set of draft voluntary professional codes of conduct one produced by each professional or interest group at the workshopdesigned to curb the use and distribution of invasive plant species through self-governance and self-regulation. The American Nursery and Landscape Association, the American Society of Landscape Architects, and the Garden Club of America already have endorsed the codes of conduct designed for their groups.

According to Ron Hiebert, research scientist with the U.S. National Park Service, 100 highways go across the U.S.-Canadian border and 20 cross the U.S.-Mexican border. In order to monitor these borders effectively, Hiebert recommended that agencies agree on a risk assessment model to determine priorities for which weeds to watch. Hiebert discussed how he and colleagues at The Nature Conservancy are collaborating to design such a model.

Continental Cooperation

Following a day-long field trip to the Mexican border and group discussions at weed-infested stops along the way, the conference concluded with a review of existing international frameworks to support further cooperation. Each framework offers a piece of the puzzle that can help control invasive species in the Western Hemisphere and beyond.

The Global Invasive Species Programme (GISP) serves as an umbrella organization, facilitating the operation of the Global Invasive Species Information Network. Currently GISP is focused on aquatic nuisance species, and it will be 3 years before GISP begins work on invasive terrestrial plant species.

One of GISP's 10 strategic responses to the problem is to promote information-sharing. Already, 120 major sources of information on invasives are available electronically. Among GISP's projects to improve the exchange of information are linking regional and national databases, developing an early warning system to share alerts of new threats, and establishing a database of case studies detailing successful control methods.

The Comisión Nacional para el Conocimiento y Uso de la Biodiver-sidad (CONABIO) is an organization dedicated to building a national database for all biological resources in Mexico ( The group uses the data to understand biogeographical patterns and centers of biodiversity and to make more informed conservation choices. So far, CONABIO has developed a GIS database for invasive species in Mexico and most of South America (14 countries), enabling it to predict future hot spots and distribute warnings about invasives based on a plant's biological characteristics and the biogeography of regions south of the equator. CONABIO also is contributing to the development of an information hub for all of North America.

Established in 1993, the North American Weed Management Association (NAWMA) is a network of public and private professional weed managers who are involved in implementing any phase of a local, State, or Federal noxious weed law ( NAWMA meets the needs of on-the-ground vegetation managers, offering educational opportunities, regulatory direction, professional development, and environmental awareness geared toward preserving and protecting the continent's natural resources from exotic, invasive, and noxious weeds. NAWMA's current goals include establishing and maintaining standards for North American weed-free forage (mulch), developing international mapping and data standards for invasive plants, and defining a certification program for invasive plant managers.

The World Conservation Union (IUCN), founded in 1948, brings together States, government agencies, and diverse nongovernmental organizations from nearly 140 countries into a global partnership to protect biodiversity and manage habitats and natural resources ( IUCN believes that improved education within the international community is essential to stopping the spread of alien species, and the organization recently finalized a set of guidelines for preventing biodiversity loss due to invasive species. Another IUCN concern is the buying, selling, and shipping of plants globally through e-commerce.

Since its creation in 1951, The Nature Conservancy's (TNC) mission has been to preserve the plants, animals, and natural communities that represent the diversity of life on Earth by protecting the lands and waters they need to survive ( Because invasive species are among the gravest threats to TNC's mission, eradicating non-native weeds is now one of the organization's five goals for the Western Hemisphere.

"We need to have a system to better understand where invasives are and where they are coming from," says Ann Bartuska, executive director for the invasive species initiative at The Nature Conservancy. "When we see new invasives, we need to be able to identify them quickly and mobilize a rapid response tocontrol them before they become established."

The Canadian thistle threatens croplands nationally.

Established in 1952, the North American Plant Protection Organization (NAPPO) is a partnership of Canada, Mexico, and the United States to protect North American plant resources from the entry, establishment, and spread of regulated plant pests, while facilitating intra/interregional trade ( Because key stakeholders include a much broader audience than environmentalists, policy decisions take into account economic and trade issues in addition to environmental issues.

Tying It All Together

After representatives from each country and various agencies reported on the status of their weed problems, highlighted ongoing research, and presented best management practices, the attendees had established a common ground for discussion using a broad North American focus. At the close of the conference, the attendees had the opportunity to use a new keypad technology for completing an opinion survey to learn more about each other.

Attendees learned, for example, that 53 percent of their peers had traveled more than 1,600 kilometers (1,000 miles) to attend the conference, demonstrating truly continent-wide interest in the subject matter. The majority—73 percent of attendees—had come from various regions and agencies in the United States, not surprising given the home-court advantage. Canadian and Mexican attendees were equally divided in the remaining 27 percent. The make-up of the participants included 47 percent scientists and 22 percent policy-makers.

Major findings revealed through the opinion survey include:

  1. Most attendees (92 percent) agree that they do not have adequate funding for invasive plant prevention and control.
  2. Attendees strongly support a North American information exchange.
  3. Attendees believe that an international agreement would be helpful in establishing uniform goals and making funding a higher priority.
  4. Workers at all levels in Canada, Mexico, and the United States have access to the Web, indicating that the Internet is a logical means for sharing information.
  5. Attendees agree that early detection is the highest priority for controlling invasive species.
  6. State, Federal, and university representatives prefer biocontrols to other available vegetation management tools.
  7. Attendees believe that road-building and housing developments surpass overgrazing as the most common activities that lead to plant invasions.

The attendee responses to question 7 about road-building's contribution to spreading invasive plants highlights the importance of raising awareness of invasives within the transportation industry and confirms the value of continent-wide cooperation, especially along interstates.

"Even though invasive species represent a huge problem, it is one that we can do something about," says The Nature Conservancy's Ann Bartuska. "You can see the results of your success right in front of you, whether it's watching native prairie plants come back inTexas after Chinese tallow was eradicatedorseeing hikers clean off their boots to prevent spreading seeds on their next hike."

As State DOTs explore ways to implement Executive Order 13112 by reviewing internal policies, vegetation management practices, and construction processes, FHWA will continue fostering cooperation with the Nation's neighbors to the north and south. In the war on weeds, your neighbors are your allies, and information is your best artillery.

Bonnie Harper-Lore heads FHWA's Vegetation Management Program. She serves as a technical resource for all State DOTs pursuing an ecological approach to roadside vegetation management. As a founding member of the FICMNEW, Harper-Lore collaborates with 16 Federal agencies to coordinate efforts to prevent and control invasive plants, restore native plants, and educate the public. Harper-Lore co-edited "Greener Roadsides," FHWA's quarterly newsletter sharing information about roadside vegetation management. She also co-edits Roadside Use of Native Plants. Harper-Lore conceived the idea to host the "Weeds across Borders" conference and was one of the primary conference organizers.