Abstract - Manual Control of Canada Thistle: A Reasonable Alternative for Controlling Small Infestations in Sensitive Areas

Curtin, Marie M. 2004. Manual control of Canada thistle: A reasonable alternative for controlling small infestations in sensitive areas. NPS Natural Resource Year in Review - 2004. p. 40-41.

Park staff at Wind Cave National Park (South Dakota) achieves good results controlling Canada thistle (Cirsium arvense) using a simple, old-fashioned method: weed pulling. Manual control was selected over other management alternatives in order to protect diverse native plant communities and the watershed that houses dozens of the park's cave and karst features, including Wind Cave, an extensive maze of more than 112 miles (180 km) of subsurface passages. Manual control is one component of the park's Integrated Pest Management Program, which seeks to control Canada thistle using methods that do not conflict with management goals for the park's natural resources.

Canada thistle, native to Eurasia, arrived in this country during the 1800s. Most of the diseases and parasitic insects that harm Canada thistle are absent from North America. As a result, the invasive plant completes aggressively with native vegetation and can reduce native plant species extent and diversity, and habitat available for wildlife.

Pulling Canada thistle by hand is hard work, but well worth the effort because it preserves native plants that might be harmed by chemicals or other control methods. Weed pulling also protects sensitive cave resources, another key management priority. Chemicals most effective against Canada thistle are capable of easy movement through soil and root systems and into groundwater and caves. Cave ecosystems are relatively closed systems that do not recover quickly from changes to their environments. Chemicals applied within the watershed have the potential to leach into Wind Cave, which could impact cave flora and fauna and water resources used for human consumption.

Many infestations of Canada thistle consist of only one plant, but is is a plant with an extensive root system that acts as the support structure for many aboveground stems, flowers, and seed heads. The goal of weed pulling is to starve the root system. When the entire plant is pulled, removing as much root as possible, the plant draws from root reserves to create new stems and leaves capable of conducting photosynthesis. Repeat pulling exhausts the root system, basically starving the plant to death.

The most intensive weed pulling efforts are directed against infestations occurring in riparian areas, drainages, and otherwise pristine areas throughout the park. To reduce potential for seed dispersal by humans, sites along roads and trails are also a priority. Remaining infestations are kept in check with biological and mechanical control methods. These sites are eventually designated for manual control, replacing sites that no longer require treatment. Park personnel monitor treated sites annually for plants that regrow and new plants that germinate from seed.

During the 2004 field season, dozens of small infestations in sensitive areas were pulled or repulled by park staff. At some locations this was a continuation of weed-pulling efforts initiated in previous years. Each return visit required less time and energy. The sites experienced dramatic reductions in overall size, stem density, or both. At several locations, Canada thistle count not be located upon return visits.

Manual control has many advantages. Equipment is minimal, consisting essentially of heavy-duty leather gloves. Weather is seldom a problem, although a breeze makes the work more pleasant and rain-moistened soil releases roots better than dry soil. No training or licensing is needed to pull weeds, allowing volunteers and park staff alike to participate. The environmental advantages of manual control are also compelling. Weed pulling introduces no exotic biological control agents (insects or pathogens) into the ecosystem. And, as opposed to many biological and chemical control methods, manual control is specific to the targeted species. It does not affect native plant species, except to free them from competition with exotic weeds, preserving native species diversity.

In the absence of Canada thistle, future visitors to Wind Cave National Park will discover diverse plant communities of native grasses, sedges, rushes, wildflowers, shrubs, and trees.

Last updated: April 10, 2015

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