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Backcountry Road Maintenance and Weed Management

Plant Biology and the Spread of Weeds

Disturbed Ground

Most noxious weeds are early successional species that prefer highly disturbed sites, such as areas along rivers and streams, trails, trailheads, roadsides, building sites, wildlife bedding grounds, overgrazed areas, and campgrounds (Baker 1986, Sheley and Petroff 1999). Parendes and Jones (2000) found that the presence of exotic plant species was highly correlated with sunlit soil and frequent, severe disturbances, such as those resulting from road traffic and from road maintenance activities such as grading. In Glacier National Park, exotic plant species showed a continuous distribution along road and trail corridors in the majority of study transects (Tyser and Worley 1992). Chicoine (1984) found that spotted knapweed was readily disseminated along transportation corridors. Managing knapweed required preventing roadside infestations from spreading.


Road construction and maintenance activities mix soil layers, increasing soil microbial activity. Weeds exploit these newly available nutrients efficiently (Best and others 1980, Belcher and Wilson 1989). This may be one reason that the density of weedy plants increases as intensity of disturbance increases (Jensen 1995).

Transport of Seed From Infested to Uninfested Areas

A study in Kakadu National Park in Australia found that weed seed was transported into the park on tourist vehicles and was more likely to be transported by four-wheel-drive vehicles that had been driven off road (Lonsdale and Lane 1994). A study in California found that native plant cover and the number of species were greatest in sites farther than 0.6 mile from roads and least in sites 30 feet or less from roads. Conversely, noxious and invasive plant cover was greatest closer to roads (Gelbard and Harrison 2003). Vehicle undercarriages can trap and transport weed seed (Sheley and Petroff 1999). It is reasonable to assume that maintenance equipment used for work that disturbs the ground will transport weed seed more readily than recreational vehicles.

Soil "Banking" of Seed

Plant biology is an important factor in identifying the road maintenance activities that help weeds become established and spread. Seed of most perennial noxious weeds such as leafy spurge (figure 1), oxeye daisy, rush skeletonweed (figure 2), and spotted knapweed (figure 3) can remain viable longer than 5 years (Liao and others 2000, Davis and others 1993, Wicks and Dersheid 1964, Toole and Brown 1946). For example, 82 percent of buried oxeye daisy seed was viable after 6 years, and 1 percent of the seeds were still viable after 39 years (Toole and Brown 1946). Studies of Scotch broom (figure 4) have shown that 0.6 percent of seed in dry storage were still viable after 81 years, and that Scotch broom seed may remain viable in soil for as long as 100 years (Turner 1934). Viability of yellow star thistle seed declines rapidly, but 20 to 40 percent are viable after 1 year and 10 percent can remain viable for longer than 10 years (Callihan, and others 1989).

Photo of a yellow flowering  leafy spurge plant, a noxious weed.
Figure 1—Leafy spurge aggressively displaces native
vegetation not only by usurping available water and
nutrients, but by releasing toxins that prevent
other plants from growing.
Photo by William M. Ciesla. Image 3943076
courtesy of Forestry Images

Photo of a yellow flowering rush skeletonweed plant growing in dry grasslands.
Figure 2—Rush skeletonweed plants have invaded
this dry grassland, replacing beneficial forage
species grazed by livestock and wildlife.
—Photo by Gary L. Piper, Washington State University.
Image 0022088 courtesy of Forestry Images

Photo of a spotted knapweed plant, a purple flowering noxious weed.
Figure 3—Spotted knapweed has a purple flower that
superficially resembles the garden flower, bachelor's
button, and several native aster wildflowers.
Photo by Jim Story, Montana State University.
Image 0886049 courtesy of Forestry Images

Photo of a hillside alongside a road  in Oregon becoming infected with scotch broom plants.
Figure 4—The ancestors of the scotch broom plants that infect this
hillside in Oregon were brought to the United States as an
ornamental garden plant.
—Photo by Eric Coombs, Oregon Department of Agriculture.
Image 0023001 courtesy of Forestry Images

Hard seed coats and burial in soil may extend seed viability in some species. For example, spotted knapweed seed has a hard outer coating that protects it from being degraded in soil, while burial under as little as a ½ inch of soil "banks" the seed and prevents it from sprouting. Davis and others (1993) found that 29 percent of spotted knapweed seed was still dormant 8 years after burial, and 90 percent of the dormant seed was still viable. Because spotted knapweed produces an average of 1,000 seeds per plant per season (Schirman 1984, Story and Anderson 1978, Watson and Renney 1974), the amount of seed stored in soil can be extremely high. After seed production was halted for more than 7 years, spotted knapweed seed in formerly weed-infested soil ranged between 129,500 and 170,000 seeds per acre (Davis and others 1993). A similar study of creeping buttercup (figure 5) indicated that 51 percent of buried seed was still viable after 20 years in undisturbed soil (Lewis 1973). Long-term survival of the seed was attributed to their hard outer coat.

Photo of a yellow flowering creeping buttercup plant growing in gravelly soil.
Figure 5—This attractive little creeping buttercup has established
itself in gravelly soil similar to a roadway shoulder.
—Photo by Ilkka Korpela, University of Helsinki, Finland.

The incredibly large quantities of seed produced by weed species can produce extremely high plant densities. Scotch broom was found to produce over 16,000 seeds per square meter in a study in Australia (Smith and Harlen 1991). Yellow star thistle (figure 6) has been recorded at densities of 2 to 3 million plants per acre (Callihan and others 1989) with possible seed production of 24,000 achenes (one-seeded fruits) per square yard (Maddox, Mayfield, and Porits 1985).

Photo of the yellow star thistle which has spines on the seed heads that give this plant its name.
Figure 6—The spines on the seed heads of this yellow star thistle show
how it received its common name.
—Photo from USDA Animal and Plant Health Inspection Service,
Oxford, NC, archives. Image 1148062 courtesy of Forestry Images

Vegetative Fragments

Vegetative propagules such as plant root fragments, stolons (runners), and stem fragments can spread weed infestations. Species such as rush skeletonweed, leafy spurge, purple loosestrife (figure 7), kudzu (figure 8), and all varieties of hawkweed can be transported vegetatively. Plant parts moved about during road maintenance can spread weed infestations nearly as effectively as seed.

Photo of purple loosestrife plants.
Figure 7—Purple loosestrife has an attractive flower, but it can
takeover an area, creating a loosestrife monoculture and
completely replacing native species.
—Photo by Bernd Blossey, Cornell University.
Image0002038 courtesy of Forestry Images

Photo of trees overgrown with kudzu, a plant that mimics the foliage of the trees it kills.
Figure 8—This established kudzu infestation mimics the foliage on the
trees it killed and whose trunks it now uses for support.
—Photo by James H. Miller, USDA Forest Service.
Image 0016123 courtesy of Forestry

Shading the Soil

Establishing and maintaining competitive, desirable plants along roadsides and other areas vulnerable to weed colonization helps prevent or slow establishment, growth, and reproduction of noxious weeds (Sheley and Petroff 1999). A study in western Montana determined site factors influencing weed establishment and spread on roadsides. Results showed that shading of the roadway by tree and shrub overstory was a primary factor limiting spotted knapweed establishment on roadsides in forest habitats (Losensky 1989).