Drawing a line in the mud

Coloradans weed out tamarisk before it takes over

  • , San Juan and San Miguel rivers in Colorado

    Diane Sylvain

NATURITA, Colo. - Mallory Dimmitt reaches down into a boggy area a few yards from the lower San Miguel River. She is surrounded by young willow sprouts, but she comes up with a tiny tamarisk seedling trailing a taproot almost twice as long as the plant is high. This is Dimmitt's nemesis, and she intends to get rid of it.

Like so many desert waterways, the San Miguel is a ribbon of green in a rocky, arid landscape. It runs along the western side of the Uncompahgre Plateau, near the Colorado-Utah border. What's unusual here is what the "green" actually is: Rio Grande cottonwood, coyote willow, skunkbush sumac and other plants endemic to wild, free-flowing rivers.

Relatively little of the San Miguel's riverbank has been taken over by tamarisk, an invasive woody plant which covers 1.5 million acres of Western riverbanks - and is expected to spread to an additional million acres within the next decade. So here on the San Miguel, The Nature Conservancy is drawing a line in the mud. Staffers intend to rid the entire 80-mile length of the watershed of tamarisk and two lesser threats, Russian olive and Chinese elm.

No one has ever tried to clear an entire watershed of tamarisk. If the Conservancy succeeds in the three-year, $250,000 project commencing this summer, it will be a big - and rare - victory against tamarisk, the plant one biologist called "one of the worst ecological disasters ever to befall Western riparian ecosystems."

A virtual desert

Introduced in the 19th century, tamarisk, also called salt cedar, infests riparian areas from Texas to Utah to California. It creates impenetrable thickets along riverbanks. Big tamarisks can transpire 200 gallons of water per day, and the plant concentrates salt in its needles, which fall to the ground and poison the soil for other plants. The result is a near monoculture along the West's riverbanks, a virtual desert compared to the original riparian ecosystems (HCN, 5/25/98: Tackling tamarisk).

In Colorado, riparian areas make up only 1.5 percent of the land area, but researchers have found that wildlife use these native habitats far more than tamarisk-dominated landscapes.

The San Miguel, which drops 7,000 feet from the western San Juan Mountains to its confluence with the Dolores, is one of the few undammed Western rivers. Consequently, much of the riparian habitat is relatively healthy; regular floods wash away tamarisk seedlings and give natives a chance.

The Washington, D.C.-based Nature Conservancy holds title to 10 miles of riverbank along the San Miguel in three preserves, making it the river's largest private landowner. In an analysis last year, Conservancy staff concluded that weeds - especially tamarisk - are the biggest threat to the 11 rare plant communities found along the San Miguel's banks.

This year, Dimmitt, who is the San Miguel Land Steward for the Southwestern Colorado Program of The Nature Conservancy, received a $125,000 grant from the Boulder, Colo.-based Terra Foundation, half the project's budget. Mapping tamarisk sites started this month, but already the project has run into trouble. Nobody, it seems, knows how best to do that. Dimmitt has put out a request for proposals and hopes a private contractor can come up with a good way to map infestations.

"What's essential that we get now is the uppermost extents on the tributaries," she says. Clearing crews will start at the highest-elevation plants and work down; missing just one plant means that seeds can reinfest cleared areas downstream. Because tamarisk doesn't grow much above 8,000 feet, only the lower watershed and tributaries will need control - about 80 miles of stream altogether.

The most effective control method is to cut a plant, then paint the stump with an herbicide containing Garlon 4. The next year, new shoots must be cut and treated again. After that, annual monitoring and light control generally is sufficient. In heavily infested areas, this labor-intensive strategy can cost as much as $3,000 an acre, but the cost should be much less on the relatively healthy San Miguel.

In search of success

So far, local landowners and governments have been cooperative. "We spend two days a year going along the river and cutting down whatever tamarisk we see," says James Thorneycroft, caretaker at the Cascabel Fishing Club, which owns three miles of riverfront on the San Miguel. He thinks salt cedar can be nearly eliminated from the San Miguel. "The spread of tamarisk had only reached here, and there's not a lot out here. It doesn't spread well at these altitudes, so I think we have it under control."

Control doesn't mean extermination. Kacey Conway, a wetlands reclamation specialist from Grand Junction, Colo., says that watershed clearing projects and biocontrols such as tamarisk-eating bugs may beat back the tamarisk invasion. "Maybe we can get it to the point where it is just one of the species in the riparian environment, but it won't be dominant," she says. Conway has helped put together the Tamarisk Coalition, formed in 1999 with a goal of ridding western Colorado of tamarisk. It is, Conway says, a daunting task that will take decades and millions of dollars.

But both she and Dimmitt think many other high-altitude rivers, including Colorado's Gunnison, Yampa and Upper San Juan, could be cleared of tamarisk soon if the right strategy is used.

"I think tamarisk really needs a success story," says Dimmitt. "If we could show success, I think it would inspire a lot of funding for other projects."

Hal Clifford writes from Telluride, Colorado.


  • Mallory Dimmitt, The Nature Conservancy, 970/728-5291;
  • Kacey Conway, the Tamarisk Coalition, 970/256-7853.

Copyright © 2001 HCN and Hal Clifford

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