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July 1999

Curbing knapweed: Sweat, toil, and GPS

Rich Johnson, Davis Soule, and John Pereira discuss knapweed on the mesa. (Photos by Carlye Calvin.)

An outlaw arrived on the NCAR mesa about five years ago. The criminal may have flown in or may have accompanied some unknowing hikers. Since then, the original offender has been joined by offspring and relatives. The outlaw's name: diffuse knapweed.

"There are certain weeds that are legislated by the state of Colorado that if they're on your property you have to do something about them," says John Pereira, head of Physical Plant Services (PPS). Diffuse knapweed is on this public enemy list. It is a nonnative biennial in the sunflower family (Asteraceae) that grows one-and-a-half to two feet tall, with finely divided leaves and small white or lavender flowers in the summer. Untreated, it spreads rapidly, crowding out native plants until it dominates an area. It's unappetizing to cattle and other animals.

John first spotted knapweed on the mesa some five years ago at a newly constructed (and now defunct) horseshoe pit just east of the parking lot. Now it's found along most of the trails. "Knapweed spreads easily when the ground is disturbed," he explains. "Our biggest problem comes from people walking off the trail. Typically we find it on community trails and at the edges of established trails."

Diffuse knapweed growing next to the Mesa Lab.

Knapweed is a problem throughout the High Plains. In western Montana, it's ruined thousands of acres of grazing land. It's been a cause of contention in Boulder County for the last couple of years as citizens and open-space managers debate the safest way to get rid of it. "In the [Boulder Valley], the problem has come from development," John explains. Not only do new construction sites offer disturbed soil, but they include topsoil hauled from elsewhere that may already be infested. Another factor is our explosive population growth; many more people are using open space, and some bring weed seeds with them.

The noxious weed isn't found only on NCAR's mesa. Plants can also be found at the Foothills Lab--even in the volleyball court--and the Marshall Field Site has a significant amount. Compared with those in other county open spaces, however, our patches are small. "The weeds are at a manageable stage, and we intend to keep it that way," John says.

Until this year, PPS has controlled knapweed and the mesa's other main weed invader, Canada thistle, mainly by spraying with herbicide. "It's been a hit-or-miss operation because the [herbicide contractors] are backed up," John says. "If we have an exceedingly wet spring, by the time we've been able to get out there, it's too late." That happened two years ago, and John responded by hiring temporary workers to cut all the flowers from the Canada thistles--a gruelling job. "Nobody stayed for the whole time, and some people only lasted a matter of hours," he says.

This winter, John, David Soule, and Rich Johnson (both of PPS) began working with Maureen O'Shea-Stone of Plantae Consulting Services to create a weed management plan. "The program that they've asked me to set up is at the leading edge of how people are doing weed management," says Maureen. This spring, she is mapping all known weed areas precisely, using a GPS system recommended by Ed Manzanares of UNAVCO. "Based on the map and what we find, we'll address each species and prioritize how the plan should work," Maureen explains. "Each site is a little different. Knapweed in the parking lot is different from knapweed on the mesa."

Methods of weed control

According to Maureen, weed-control techniques can be divided into four types:

For her management plan, she'll probably integrate several of these methods. Biological control looks least promising right now for knapweed because it tends to be more successful on larger patches than appear on the NCAR mesa. However, it may be useful for another weed pest, St. John's wort.

Chemical control is currently under way using an herbicide called Transline. It's specific to only three plant families: sunflower, pea, and buckwheat. Unlike some other herbicides, it can be used under trees and shrubs, as long as they're not members of those three families. Transline isn't toxic to mammals when applied correctly. It doesn't kill the weed outright, Maureen explains. Instead, it causes the plant cells to grow so fast that the plant eventually falls apart. Matt Custer of High Plains Weed Control applies the herbicide in spot treatments.

The weed management plan will look into fall applications of herbicides as well as the current spring treatments. "You've weakened the plant by doing other things [in the summer], then in the fall you give it the coup de grace," Maureen says.

Mechanical control--in our location, this means good old-fashioned weeding--is likely to continue. "We spend a lot of time pulling weeds ourselves," John says. Maureen adds, "It makes you respect knapweed more if you spend a couple of hours pulling it." The taproot must be dug up to keep the plant from coming back; pulling the above-ground growth alone actually stimulates growth. Also, weeders must wear gloves because the prickly stems can irritate the skin. John will make use of the NCAR Rangers, a small group of college students who spend their summers working on the mesa. "We'll have a big sweep, then visit every area of the mesa on rotation," he says. The pulled weeds are bagged and sent to the landfill, where they can safely be burned at a contained site.

When it comes to cultural control, John says, "the problem is controlling the people. One of our goals is to try to get people to stay on the established trails." PPS is adding a trail around the south side of the lab to replace a number of unofficial trails there that were creating a knapweed hazard. "People think, well, I'm just one person, but when you go off the trail you are causing a problem." PPS reseeds around any disturbed area with NCAR's own blend of native grass seed, which comes from Arkansas Valley Seed Co. in Longmont. They spread about 350 pounds of the mix this spring alone.

Maureen notes that even after her plan has taken effect, NCAR properties will need to be monitored. "Weed management has abandoned the idea that any of these weeds will be eradicated. We hope eventually to contain them and make them part of the landscape. In ten years, Rich [Johnson] will still be patrolling the mesa."

Our stewardship of the NCAR site brings a final compliment from Maureen: "If land owners and other land managers throughout the West had been as proactive as soon as NCAR has, we wouldn't have as great a problem in Boulder County as we do right now." •Carol Rasmussen

On the Web:
The U.S. Geological Survey's Southwest Exotic Plant Mapping Program site offers very complete information about knapweed and other nonnative pests, including links to other sites and a bibliography. For more information about Transline, see the National Pesticide Telecommunication Network. For lists of plants that must be controlled by state law and those designated undesirable by Boulder County, see Undesirable Plant Species.


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