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The roots of noxious weeds

Bob Berwyn

COLORADO SPRINGS ” In the bad old days, the post office featured most-wanted pictures of horse thieves and cattle rustlers. But this is the new West, and images of scruffy outlaws have been replaced by posters of a new public menace ” invasive weeds that threaten the ecological well-being of the land.

But how serious is that threat, and what is the best way to address it?

Those questions were raised Monday at the State of the Rockies conference by Anna Sher, research director at the Denver Botanical Gardens. Sher, also a professor at the University of Denver, has developed models for predicting the growth of the invasive weeds and how well restorations efforts will work.



For years, Sher has focused on the environmental threat posed by run-amok plant life, with a special eye toward a water-hogging weed called tamarisk, which has been called a huge threat in the Rocky Mountains.

In Eagle County, various groups have been removing the tamarisk from riverbanks but, Sher said, simply removing the unwanted weeds without addressing the underlying ecological reasons for the “invasion” is unlikely to resolve the problem.



“Restoration of functioning ecosystems should be the goal,” Sher said. “This is why creating state weed lists is still an important strategy, but we must also consider the … lists as indicators of larger problems that may need to be addressed.”

Sher said invasive plants and noxious weeds didn’t just arrive from outer space and start growing in random places. In many, if not all cases, human activities like development, road-building, stream channelization and water diversions led directly to the profusion of unwanted plants, she said.

The tamarisk, which was intentionally imported into this country as a way to stabilize stream banks, is a prime example, Sher said.



After studying tamarisk, also known as salt cedar, in its native Middle East habitat, Sher has come to the conclusion that dam-building, stream-channelization and changes in the natural flood regime have directly contributed to the spread of tamarisk in the Colorado River drainage and the rest of the country.

Sher advocated managing tamarisk with water management regimes that mimic natural cycles, as well as with fire.

“If we manage with flooding, the natives will win out eventually,” she said. Without that flood cycle, there is no “safe place” for native plants to gain a toehold, she explained.

And fire should also be cautiously used. Timing is everything, she said, explaining that, if the fires burn at a time when it’s advantageous to native seeds, it can help promote the growth of cottonwoods and willows.

In both cases, the research suggests that the invasive tamarisks are not highly competitive as seedlings. Given half a chance, the broadleaf cottonwoods can hold their own against tamarisk.

Vail, Colorado


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