Japanese Knotweed

Invasive SpeciesFound in 17 lakes

Japanese Knotweed appears on state infested-waters lists for 17 surface waters. Boaters and anglers moving gear in or out of those lakes carry a clean-drain-dry obligation under state law. 17 lakes — narrow enough that aggressive containment is still on the table. Most upper-Midwest invasives that established here did so quietly over a decade before anyone noticed, so a low number today is not a guarantee for tomorrow.

The presence of Japanese Knotweed does not directly change a lake's letter grade — the grade measures the water itself, not the species living in it. Invasive presence is still worth reading alongside the grade.

Total Lakes

Wisconsin

Lakes with Japanese Knotweed

Lake	County	State	Grade	Depth
Balsam Lake	Polk	WI	B	37 ft
Long Lake	Bayfield	WI	B	28 ft
Deer Lake	Polk	WI	B	46 ft
Tabor Lake	Burnett	WI	B	28 ft
Lake Nebagamon	Douglas	WI	B	56 ft
Big Roche A Cri Lake	Adams	WI	B	20 ft
Springville Pond	Portage	WI	B	10 ft
Alpine Lake	Waushara	WI	B	18 ft
Mcdill Pond	Portage	WI	C	19 ft
Yellow Lake	Burnett	WI	C	31 ft
Wapogasset Lake	Polk	WI	C	32 ft
Amherst Millpond	Portage	WI	C	7 ft
Cedar Lake	Polk	WI	D	32 ft
Tainter Lake	Dunn	WI	D	37 ft
Mason Lake	Adams	WI	D	9 ft
Balsam Lake Little Balsam Site	Polk	WI	D	37 ft
Swan Lake	Columbia	WI	NR	82 ft

Frequently Asked Questions

How many lakes have japanese knotweed?

Japanese Knotweed has been documented in 17 graded lakes: 17 in Wisconsin.

Does japanese knotweed affect water quality?

Invasive species can significantly affect lake ecosystems. They may alter nutrient cycling, displace native species, and change water clarity. Lakes with japanese knotweed should be monitored for changes in water quality over time.

Invasives Found With Japanese Knotweed

Curly-Leaf Pondweed

Co-occurs in 15 lakes · 420 total

Chinese Mystery Snail

Co-occurs in 11 lakes · 494 total

Rusty Crayfish

Co-occurs in 9 lakes · 247 total

Purple Loosestrife

Co-occurs in 8 lakes · 275 total

Zebra Mussel

Co-occurs in 6 lakes · 667 total

Banded Mystery Snail

Co-occurs in 5 lakes · 362 total

More on invasive species and lake health

Zebra Mussel Spread: How Many Lakes Are Affected?

Zebra mussel counts from the lakes LakeQuality grades, the counties that show up most often, and the Clean, Drain, Dry rules that still matter.

Lake Water Quality Grades Explained: What A–F Means

How lakes are graded A–F on nutrients and algae using EPA ecoregion condition classes, what each letter means in the water, and why an A is not a clearance.

What to Check Before Buying Lakefront Property

Before you buy lakefront, check the water: clarity trend, nutrient impairment, algae advisories, invasive species, and depth — a data-backed checklist.

Source: EPA National Aquatic Resource Surveys, 2026.