Yellow iris control in Thorp Brook and Lower Lewis Creek

Yellow iris (Iris pseudacorus), with its striking yellow flowers, provides a colorful addition to the Vermont landscape—and a threat to our priority natural communities. In recent years, it has gained a foothold in many Lake Champlain wetlands andsmall streams that drain directly into the lake. Both are priority natural communities, as documented by the Vermont Fish & Wildlife Natural Heritage Program. The Lewis Creek Association, in partnership with Lake Champlain Basin Program and Habitat Restoration Solutions, began studying this problem in 2015. Control efforts were initiated in 2016 for lower Lewis Creek and this past summer for Thorp Brook.

This year’s control work builds upon two seasons of studying and mapping iris infestation, threat and spread, and validating control methods in the lower reaches of Thorp Brook. Lower Thorp Brook is a diverse, beaver-influenced corridor and serves as the primary iris seed source for the wetlands at its confluence with Lake Champlain. These wetlands are considered state waters, comprising a 53-acre matrix of wetland natural communities, whose value has been acknowledged by, among others, experts at the Vermont Department of Environmental Conservation and The Nature Conservancy.

The lake-influenced lower reaches of Lewis Creek contain important floodplain forests, buttonbush swamps and a range of state significant emergent communities. Some lands are state-owned (e.g., the Little Otter Wildlife Management Area), others are private. While the extent and nature of the infestation is understood and mapped, a mutually agreeable management approach is needed to begin addressing this growing infestation in an ecologically significant area of high public value.

Iris clump growth rates have been observed to double from season to season (2015-17). Iris will raise elevations of wetlands over time, effectively eliminating emerging plant communities. In addition to disrupting natural riparian communities in Thorp Brook, these iris clumps serve as the primary seed source for Thorp/Kimball wetlands.

This past summer, an invasive-species management plan was developed for Thorp Brook. Iris clumps were identified in early summer by Habitat Restoration Solutions and a team of volunteers. With landowner permission, clumps were treated with foliar herbicide in the fall, yielding 90 percent control. In lower Lewis Creek, property management options were identified by reaching out to landowners and technical experts. This project initiated a broader discussion in Lewis Creek about taking care of this valuable natural asset. Thank you to the landowners along Thorp Brook and Lewis Creek for participating in this important work.

Krista Hoffsis is the program coordinator for the Lewis Creek Association. This project was funded by an agreement awarded by the Great Lakes Fishery Commission to the New England Interstate Water Pollution Control Commission in partnership with the Lake Champlain Basin Program. The viewpoints expressed here do not necessarily represent those of NEIWPCC, the LCBP Steering Committee, or GLFC, nor does mention of trade names, commercial products, or causes constitute endorsement or recommendation for use.