Aquatic Plants

Aquatic Plants in Minnesota Lakes

Ideally, Cross Lake and the Snake River would have a variety of aquatic plants growing along the margins. Native plants stabilize the shoreline and anchor fine sediments on the bottom, help clarify the water and provide food, shelter, and spawning habitat for fish. Submerged plants provide food for waterfowl and provide habitat for insect larvae, snails, and other invertebrates, which are part of the aquatic food chain. Oxygen produced by aquatic plants provides enormous benefits to all the life forms in the lake and river system. Just like familiar land plants, such as grass and trees, aquatic plants must absorb nutrients including phosphorous and nitrogen. Aquatic plants absorb these and other nutrients from the bottom or from the water. Much of the available nutrients are the result of runoff from land.

Distribution of Aquatic Vegetation

 Free-floating plants such as algae and duckweed can be found anywhere on the lake or river. Larger aquatics, classified as submerged, emergent, and floating leaf, are found in the littoral zone, which in Minnesota lakes extends to a depth of about 10 feet. Distribution if closely tied to light penetration, which is controlled by water clarity. A Secchi disk reading in Cross Lake may be only 3 to 3 ½ feet. As a rule, the maximum depth to which plants grow is twice the Secchi disk reading for midsummer, which means a depth of 6 to 7 ½ feet in Cross Lake. The width of the littoral zone in Cross Lake varies depending on the depth of the water. A shallow bay may have a littoral zone, which extends 100s of feet from the shore; while along other shorelines with a steeply sloping bottom, the littoral zone may only extend 20 feet off shore.

The type of bottom sediment influences the distribution of rooted aquatic plants. In general, plants are more likely to be found in muddy sediment with organic matter in it than in a sandy bottom without organic matter. Development of your property, especially along the shoreline, has already caused fine sediment to be added to the lake bottom. Most of the littoral zone in Cross Lake can easily support abundant aquatic vegetation.

Managing Aquatic Vegetation

Cross Lake has too much of the wrong kind of vegetation. That is a rather sweeping generalization that I will try to clarify so that you can make decision that affect you and the shoreline property you are trying to manage.

Last year the Cross Lake Association focused on the specific problem of too much curly-leaf pondweed, an exotic plant that has dominated the littoral zone for the last few years. We helped you identify it, educated you about it unusual life cycle, which includes being able to grow under the ice, its tendency to crowd out favorable native plants in the spring, and its early decay leading to a release of phosphorous and a worsening of the algae problem in the summer.

We bought 10 manual weedcutters with a cutting width of up to 60 inches. These were loaned to members-only through nine monitors, volunteers living around the lake and river. The cutters worked very well.

This year our goal is to have each cutter used at least 10 times. Contact me for the name of the volunteer nearest you. He or she will provide a time slot for you. Cutting time is approximately from mid-May to mid-June (about a month to six weeks). Plan ahead. This year we are trying another tactic to encourage more people to remove curly-leaf pondweed from the amount of area permitted by the DNR. As you should know from the membership letter, the Association is selling a good-quality weed cutter at a discount by dealing directly with the manufacturer. You and a neighbor might share the cost and have the convenience of having a cutter anytime you need one.

In addition to the problems previously mentioned, the rather dense surface mats of curly-leaf pondweed interfere with swimming and boating and tend to hold surface algae until the mats begin to decay in July.

The goal of the Association is to reduce the amount of curly-leaf pondweed and at the same time encourage its replacement with diversified native aquatic plants that will improve the oxygen content of the lake, promote fishing, reduce the phosphorous content of the water during the summer, and, thereby, help reduce algae growth.

Permits for Removing Submerged Vegetation

You do not need a permit if you remove submerged vegetation from an area less than 2500 square feet, no more than half the shoreline length of your property and no more than 50 feet, whichever is less. You may remove the vegetation by hand, with hand tools (rake, manual weed cutter) or with equipment that does not significantly alter the course, flow or current of the water. You do not need a permit to maintain a 15-foot wide boat channel to open water through floating leaf vegetation. The Association insists that you make every effort to collect and remove the cut or pulled plants from the water. Do not let your mess become some else’s problem. Composting the vegetation can be beneficial to your garden or flowerbeds next year.

A permit is needed if you intend to:

Use any type of herbicide or algaecide. Some chemicals may be purchased and applied only by licensed applicators. Treatment during cold water temperature is preferred for control of curly-leaf pondweed.
Control any type of emergent vegetation, e.g., cat tails or bulrushes. Remove vegetation from an area larger than 2500 square feet, or from an area along more than 50 feet of shoreline or more than half the property width. Control floating-leaf vegetation in an area larger than a 15-foot wide boat channel to open water.
Use an automated plant control device, e.g., a Crary weed roller.
You are permitted as a shore land property owner to keep your 2500 square feet (or less) of waterfront free of all aquatic plants. Some of you who use the lake often for swimming may choose to do this. Should everyone clear out all aquatic plants in their zone, all summer? For ecological reasons, it is a better idea to tolerate as much native plant growth as you can.

For additional information, click: www.extension.umn.edu/topics.html