What to look for when idenitfying Eurasian watermilfoil.

Chip Welling; Coordinator, Eurasian Watermilfoil Program; Minnesota Department of Natural Resources; 500 Lafayette Road; Saint Paul, MN 55155-4025; 651-297-8021Many Minnesotans are familiar with Eurasian watermilfoil, or simply, milfoil. Milfoil is an invasive and non-native, submersed aquatic plant that causes problems when it produces mats of vegetation on the water’s surface. These mats, which are more extensive than those produced by native plants, can interfere with recreation and access to open water.

Controlling milfoil in lakes where it has been introduced can be a big challenge. Our experience over the past ten years with this plant is that permanent eradication or elimination of the non-native plant from a lake is not a realistic goal. A realistic goal is to manage problems caused by milfoil.

Physical methods such as cutting and harvesting can be effective in controlling milfoil. The use of herbicides is believed by many people to be the easiest, least expensive, and most effective strategy. One product that has generated much interest among Minnesotans interested in control of milfoil is fluridone, which is the active ingredient in Sonar™ or Avast!™ herbicides.

My purpose in writing this article is to describe the background within which the Department of Natural Resources (DNR) is evaluating the potential use of fluridone in Minnesota.

There are two categories of herbicides allowed for control of submersed aquatic plants: contact herbicides or systemic herbicides. Generally, contact herbicides act more quickly than do systemic herbicides.

Contact herbicides only affect the plant parts contacted, usually just leaves and stems. As a result, control produced by contact herbicides often is of shorter duration than control produced by systemic herbicides. The latter can be absorbed by the plant and moved within the plant. This means that systemic herbicides can control roots and other underground plant parts, as well as stems and leaves. Generally, control of submersed aquatic plants in Minnesota is repeated annually.

Drawing of watermilfoil.

It is important to remember that any use of herbicides in Minnesota lakes requires a permit from the DNR. In Minnesota, herbicides are usually applied as ‘spot-treatments’ to control submersed plants in limited areas adjacent to privately-owned shoreline. These areas may extend along 50 to 100 feet of each property’s shoreline and 100 to 150 feet lake-ward. On a whole-lake basis, the cumulative total of spottreatments is not allowed to exceed 15 percent of the littoral zone, the area that is 15 feet deep or less. This limit is necessary because submersed plants, even milfoil, can provide habitat for fish and wildlife, protect water quality, and limit erosion of shorelines. This limit allows sufficient control for access and recreation on most lakes and a variance can be issued to allow larger treatments, if necessary.

With this background in mind, let’s consider fluridone, a systemic herbicide. The principal difference between fluridone and other herbicides is that fluridone cannot be used effectively for spot-treatments, but must be applied to whole bays or lakes. The principal reason for considering fluridone is that milfoil is highly susceptible to this herbicide, which can provide lake-wide and multi-year control of this non-native plant.

At present, the DNR does not allow operational whole-lake treatment with fluridone to control milfoil due to the lack of conclusive information on the selectivity of fluridone. If this herbicide could remove only milfoil and allow native plants to survive or, better yet, increase, then the DNR would have fewer reservations about use of this product. The purpose of the DNR’s continuing evaluation of fluridone is to increase our understanding of the effects of this product on native plants, as well as possible indirect effects on water quality and perhaps other aspects of lake ecosystems. The overall challenge for users of Minnesota’s lakes and the DNR is to determine whether the benefit of controlling milfoil by whole-lake treatment with fluridone is worth the risk of possible harm to lakes. In a following article, I will provide an update on the DNR’s continuing evaluation of this herbicide.

Curly-leaf pondweed has been identified in over 500 water bodies in Minnesota. This non-native aquatic plant is often characterized as an invasive nuisance species and during the past few years many shoreland property owners have experienced increasing problems caused by its growth and spread. Requests for information and assistance with managing curly-leaf pondweed infestations have increased over the past two years. In response, the University of Minnesota Extension Service Shoreland Education Program recently offered three workshops to help property owners, local units of government, and lake association leaders better understand how to manage this nasty invader.

Workshops in Big Lake, Nisswa, and Richfield attracted nearly 150 participants who learned about the value of preserving native aquatic plants, the life cycle and characteristics of curly-leaf pondweed, various management methods (cutting, harvesting, chemical and physical options), recent research, and the permitting process for curly-leaf pondweed control. Speakers included representatives from the University of Minnesota Extension Service, Minnesota Sea Grant, Minnesota DNR, local governments, Minnesota Lakes Association, and private consultants and lake management professionals. Lake association leaders also shared their experiences in managing curly-leaf pondweed. The take-home messages for attendees: Preventing the introduction of curly-leaf pondweed into a water body is the only real “control.” There is, to date, no “silver bullet” method of eradicating curly-leaf pondweed once it has invaded a water body. Several factors need to be considered (with the help of natural resource professionals) before selecting an appropriate management method for a lake. Management of curly-leaf pondweed is costly, ongoing, requires a DNR permit, and may alter the ecology of the lake.

These workshops were co-sponsored by the Water Resources Center, Sea Grant Program, Minnesota Extension Service, Minnesota Lakes Association, and the Initiative Foundation.

Included in this issue of From Shore to Shore is a curly-leaf pondweed identification handout for you to use, make copies, and share.