Invasive Plants & Animals

What are Invasive Species?
Invasive species are non-indigenous species whose introduction cause, or is likely to cause, economic or environmental harm or harm to human health.  When invasive species arrive in Polk County they have a competitive advantage over native species because they lack natural predators, parasites, pathogens, diseases, and competitors to keep their populations in check.  As a result, populations of invasive species can explode and outcompete native species by using available resources.  Additionally, many invasive species have life strategies which give them a competitive advantage over native species.  Strategies include high reproductive rates, early seasonal growth and development, and tolerance for a wide range of environmental conditions. 

Invasive species can come from other parts of the United States or from other countries and can be released either intentionally or unintentionally.  Modes and reasons for introduction can vary widely and include ballast water from shipping, food sources, bait sources, and the garden/aquarium plant trade.  Although some species may have been introduced through natural migration, humans are the primary way invasive species are spread.  Invasive species can displace native species; reduce wildlife habitat; and negatively impact property values, recreational activities, tourism, and industries. 

Common aquatic invasive species in Polk County include Chinese/banded mystery snails, curly leaf pondweed, purple loosestrife, and yellow iris.  Smaller populations of zebra mussels, Eurasian water milfoil, phragmites, and rusty crayfish are present in the County.  Additionally, both bighead and silver carp have been caught in the St. Croix River, but never upstream of the City of Stillwater and never in large numbers.  Common terrestrial invasive species in Polk County include Japanese/giant knotweed, wild chervil, and wild parsnip.

For assistance with invasive species identification, bring a sample to LWRD office in plastic bag.

Volunteer Opportunities to Get Involved

The Polk County Land and Water Resources Department offers a variety of trainings for statewide programs where volunteers can learn to identify and monitor for invasive species including:
Staff also provide training for the Clean Boats, Clean Water Program and assistance with raising beetles for purple loosestrife biocontrol.  Polk County is a member of the St. Croix-Red Cedar Cooperative Weed Management Area (CWMA) which is dedicated to fostering multi-generational awareness of invasive species and using partnerships to prevent and limit the intrusive impacts of those species.  The CWMA operates a tool trailer that can be checked out for invasive species management projects.  The trailer is stored in Polk County and is intended for invasive species community workdays, training events and demonstrations, and other similar activities as approved by the CWMA Steering Committee.

Presentations and Recordings


Online Mapping Resources
Many mapping tools exist for aquatic and terrestrial invasive species in Wisconsin.

For more information, contact:

Katelin Anderson
Polk County LWRD
100 Polk County Plaza, Suite 120
Balsam Lake, WI 54810
(715) 485-8637

Visit these websites for more information: