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Improving Riparian Vegetation at St. Vrain State Park

by Alan T. Carpenter

St. Vrain State Park Manager, Bob Finch, with a juvenile tamarisk removed from the edge of a pond.

One of the objectives of the Park has been to increase the ecological condition of the plant communities in the Park by controlling invasive plants and increasing native plant diversity by seeding and transplanting. Thus, a major focus of Park management over the past three years has been controlling the numerous species of noxious weeds that had colonized the Park. The worst weeds included Canada thistle, hoary cress, perennial pepperweed, Russian knapweed, field bindweed, diffuse knapweed, common teasel, Russian olive, and tamarisk. All of these except common teasel and diffuse knapweed are long-lived perennials that reproduce prolifically from rhizomes or roots. They are persistent and difficult to control.

At this point, the effects of three years of intensive weed management are obvious to those who were familiar with the Park in the past when budgets for weed management were too low to make much headway. One example of recent weed management at the Park is the work of the Weld County Youth Corps in 2006. In cooperation with Park staff, the young men and women searched for and destroyed tamarisk and Russian olive plants in the riparian areas of the Park, mainly along St. Vrain Creek. As with nearly any weed management activity, follow-up work is essential to achieve long-term success. Follow-up work during 2007 will include a work day with Wildland Restoration Volunteers. The objective of the work day is to eliminate, in selected areas of the Park, all invasive woody plants (namely, tamarisk, Russian olive, Siberian elm, and honey locust) that are capable of regeneration. Thus, on Wednesday, June 20, about 30 volunteers are slated to search for and destroy the remaining tamarisk and Russian olive plants that are capable of reproduction. In addition, the volunteers will kill Siberian elm trees that have colonized the floodplain. Siberian elm is a widely planted alien species that invades floodplains on the eastern plains of Colorado.

Preliminary survey results of the Park from this spring reveal that established invasive woody plants (excluding seedlings) are relatively sparse within the park's boundaries due to the control activities implemented during the past several years. We do not expect to kill all individuals, because it is probably impossible to locate all of the seedlings of invasive woody plants due to their small size. Nevertheless, we believe it is realistic to locate all individuals that are capable of regeneration due to their larger size. We know that some of the work areas harbor large Siberian elm trees that produced millions of seeds this spring. Killing these trees is important for the project, but the trees are so large that volunteers cannot cut them. Therefore, we will use a sawyer crew from the Park to fell the larger Siberian elm trees and buck them into lengths that can be hauled away.

We will employ the tried-and-true cut-stump method of killing invasive woody plants. It involves cutting the stems with a saw or a pair of loppers as close to the ground surface as possible, then within 30 seconds applying a small amount of Pathfinder herbicide to the cut stump. This method has been shown to be highly effective on these plants. Relatively little herbicide is required and the application method results in essentially no transport of herbicide from the cut stump.

Hopefully, 2007 will be an important turning point for St. Vrain State Park when invasive woody species will no longer be ecologically significant.