Starry stonewort confirmed in Medicine Lake in Hennepin County

The Minnesota Department of Natural Resources has confirmed the invasive algae starry stonewort in Medicine Lake, located within the city of Plymouth in Hennepin County. This is the first new confirmation of starry stonewort in Minnesota in 2018. There are now 12 lakes in Minnesota where starry stonewort has been confirmed.

A DNR-trained Three Rivers Park District watercraft inspector recognized starry stonewort on a boat propeller and notified the DNR. DNR invasive species specialists confirmed a widespread growth of starry stonewort around the public access administered by Three Rivers Park District at the north end of the lake.

Three Rivers Park District and DNR staff surveyed the lake to determine the extent of the infestation. They found starry stonewort in about 14 acres of the 924-acre lake. An initial treatment at the access is planned for early next week, and the DNR and Three Rivers Park District plan to treat the access through the open water season.

Inspection efforts have increased and a decontamination unit is available at the access. The DNR is working with Three Rivers Park District to develop and partner on long-term viable management options. Starry stonewort has never been eradicated from any U.S. lake, but treatment can help reduce the risk of spread and provide nuisance relief for water-related recreational activities.

“Starry stonewort can be difficult to identify without the presence of the tiny star-shaped bulbils,” DNR invasive species specialist Keegan Lund said. “We ultimately used a microscope to examine the algae and confirm that bulbils were present.”

Since starry stonewort was first confirmed in Minnesota in 2015, most new populations have been reported in the month of August, when the telltale star-shaped bulbils are most abundant and visible. Now is the best time of year to look for it. Information on how to identify starry stonewort can be found on the DNR’s website. If people think they’ve found starry stonewort, they should report it to the DNR.

Starry stonewort is an alga that looks similar to other native plants and can form dense mats, which can interfere with use of a lake and compete with native plants. It is most likely spread when fragments have not been properly cleaned from trailered boats, personal watercraft, docks, boat lifts, anchors or other water-related equipment.

The DNR reminds boaters and anglers to follow Minnesota laws to prevent the spread of aquatic invasive species:

Clean aquatic plants and animals from watercraft.
Drain all water by removing drain plugs and keep drain plugs out while transporting watercraft.

Dispose of unwanted bait in the trash.

Some invasive species are small and difficult to see at the access. To remove or kill them, take one or more of the following precautions before moving to another waterbody, especially after leaving infested waters:

Spray with high-pressure water.
Rinse with very hot water (120 degrees for at least two minutes or 140 degrees for at least 10 seconds).
Dry for at least 5 days.

Details about starry stonewort and other aquatic invasive species are available at