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A New Advantage for Introduced Weeds
by Bob Hartzler

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December 19, 2000 -  The majority of our weed species are non-native plants that have been introduced to North America (either purposely or by accident).  Examples of introduced weeds include velvetleaf, giant foxtail, woolly cupgrass, common lambsquarter, crabgrass, field bindweed, Canada and musk thistle, multiflora rose, quackgrass, and many more.  In many instances, the introduced plant does not demonstrate weedy tendencies in its native land, but rather simply coexists peacefully with other native plants.  Only when the plant is taken from its home and released in a new area does it invade habitats where it is not desired.  The traditional explanation for the aggressive behavior of introduced plants is that the new region lacks natural enemies that regulate its population in the plant's native home.   Much of the effort in biological control of weeds involves searching for insects or pathogens that keep the plant in check in its native home.  While natural enemies are undoubtedly important in many situations, recent research with diffuse knapweed (Centaurea diffusa) has revealed a new mechanism that may provide it and other introduced species another advantage that allows them to successfully compete in their new homes.

Diffuse knapweed  is a native of Eurasia that has rapidly spread across rangeland in the western United States.  Researchers at the University of Montana recently reported a completely new mechanism that may explain the overwhelming success of knapweed in the U.S. (Callaway, R.M. and E.T. Aschehoung.   2000.  Invasive plants versus their new and old neighbors:  A mechanism for exotic invasion.  Science 290: 521-523.).  The researchers found that knapweed was much more competitive with grass species originating in the U.S. compared to grasses that originated in Eurasia.  Further studies found that the knapweed exuded chemicals from its roots that reduced the ability of the U.S. grasses to absorb nutrients from the soil, whereas the Eurasion grasses were largely unaffected by the exuded chemicals.  Apparently the grass species from Eurasia have mechanisms that protect them from the chemicals exuded by knapweed.

These findings indicate that plants that evolve together may develop protective mechanisms that prevent one species from having an unfair advantage over another.  When a foreign plant is introduced into a new region, the native plants would be unprepared for the offensive weapons carried by this new plant species.  The lack of protective mechanisms by the native plants therefore allows the new plant to rapidly spread across its new home and develop into a serious weed problem.  This is just another example of the complex interactions that occur between plants competing amongst each either.  It's also reassuring to know that humans aren't the only species to compete in an arms race.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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