Wetland Invasive Species

This is a list of the wetland invasive species that are currently in Calumet County.

Japanese KnotweedPhragmites
Purple Loosestrife

Japanese Knotweed

Japanese knotweed is a perennial that quickly grows to heights of 5 to 10 feet in large clones, potentially up to several acres in size. The stems are reddish-brown and are hollow and bamboo-like. When they die back, the stems remain upright through winter. Mature leaves are 3 to 5 inches wide and 4 to 9 inches long and lighter on the lower surface. Young leaves are heart-shaped. Two-inch long clusters of tiny white flowers bloom in late summer. Japanese knotweed can reproduce by seed but spreads primarily by underground rhizomes, which can reach 6 feet deep, 60 feet long, and become strong enough to damage pavement and penetrate building foundations.

Japanese Knotweed showing zig-zag stem growth

Japanese knotweed in bloom.JPG

Japanese knotweed grows in a variety of habitats, in many soil types and a range of moisture conditions. It forms dense thickets that exclude native vegetation. Of particular concern is its ability to invade valuable wetland areas and line the banks of waterways where it forms an impenetrable wall of stems. This crowds out native vegetation and leaves banks vulnerable to erosion when the plants drops its leaves in winter. It is also found along roads, railroads, and utility pathways. Japanese knotweed is often spread through movement of its rhizomes; plant fragments that easily move in the water of streams, by transportation in fill dirt, on contaminated equipment, or through roadside mowing and plowing.

Japanese Knotweed in Flower


Trying to remove Japanese knotweed by pulling or digging is generally ineffective due to its deep underground rhizomes. Pulling and digging may further spread the plant if the pieces are not disposed of properly. Herbicide application is effective when the entire clone is treated repeatedly. Applications of herbicides containing glyphosate are typically used after spring leaf out and on resprouts emerging after cutting. Follow-up for a few years is required. For additional information on Japanese knotweed and its control, visit the Department of Natural Resources DNR website.

View a map of the known Japanese knotweed in Calumet County (PDF).

Japanese Knotweed, massive stand

Japanese knotweed, mound at High Cliff.JPG

Phragmites Australis

Non-native phragmites is a tall perennial wetland grass ranging in height from 3 to 20 feet. Cane-like stems, about one inch in diameter, support narrow leaves which taper to a point. Large, dense, featherlike grayish purple or pale beige plumes, 5-12 inches long, are produced in summer. The plumes turn tan in fall and most of the leaves drop off the stalks. The root system is made up of strong horizontal shoots, or rhizomes, that grow below the ground or along the water surface. These rhizomes can be 6 feet deep and 50-60 feet long.

Phragmites in Winter


Phragmites thrives in sunny wetland habitats. It is particularly common in disturbed or polluted soils with brackish water, but will tolerate a wide range of conditions. In Calumet County it is commonly found along our roadsides, ditches, open wetlands, riverbanks, dredged areas, and in disturbed sites.

Phragmites in Summer


It's an astonishingly destructive invasive plant, quickly displacing virtually all native plants and thereby eliminating the diverse wetland communities that support our native wildlife. It rapidly outcompetes all other plants and creates a monotypic community that is hostile for our native species. This invasive plant occurs throughout Calumet County, though it is extremely problematic in Brown County and the western shore of the Bay of Green Bay.

The nodes of the rhizomes are the primary means of reproduction for phragmites. This makes manual removal of the plants very difficult, because any remaining underground node can sprout a new plant.

Phragmites plume


Do not walk or drive an ATV or other equipment through beds of phragmites. The nodes of the rhizomes can be picked up in the tread of your vehicle, or the soles of your shoes. These small plant fragments can be transferred to new areas on your equipment. If you see phragmites in the field, go around it. If you need to pass close to an infestation, be sure to clean your equipment very carefully to remove any traces of the plant.

Chemical treatments can control phragmites effectively. However, since it is most commonly found in wetland communities, the Department of Natural Resources permits are required for chemical treatments, mechanical treatments, and some manual treatments.

For further information on all aspects of this plant, including chemical controls, visit the DNR website.

View a map of the known phragmites in Calumet County (PDF).

Purple Loosestrife

Purple loosestrife is a perennial plant, three to seven feet tall with a dense bushy growth of 1 to 50 stems. Showy flowers that vary from purple to magenta are positioned along with the numerous long spikes and bloom in mid to late summer. Leaves are opposite, nearly linear, and attached to four-sided stems without stalks. It has a large, woody taproot with rhizomes that form a dense mat.

The plant has a wide tolerance of physical and chemical conditions and is able to reproduce prolifically by both seed and vegetatively. A single flower stalk can produce from 100,000 to 300,000 seeds per year. Seeds can germinate many years after they have entered the soil. The absence of natural predators also contributes to purple loosestrifes proliferation.

Any sunny or partly shaded wetland is susceptible to a purple loosestrife invasion. The plant's preferred habitat includes marshes, stream margins, and wet prairies. It's tolerant of moist soil and shallow water sites such as pastures and meadows, although established plants can flourish in drier conditions.

Purple Loosestrife provided by WI Lakes Partnership


Purple loosestrife displaces native wetland vegetation and degrades wildlife habitat. Eventually, purple loosestrife can overrun and almost entirely eliminate open water habitat. The plant can also be detrimental to recreation by choking waterways.

Purple Loosestrife

Purple Loosestrife.JPG

Preventing the spread is the easiest control method and the best way to stop purple loosestrife. Monitor your wetlands regularly and remove any new young plants. Find pioneering plants or isolated small colonies, especially in areas otherwise free of loosestrife. If you remove plants, do not leave stems or cuttings that can resprout or disperse seed.

Dispose of plants and seeds in a landfill, or dry and burn them. Composting will not kill the seeds. Keep clothing and equipment seed-free to prevent its spread. Rinse all equipment used in infested areas before moving into uninfested areas, including boats, trailers, clothing, and footwear. Remove new plants wherever you see them. Follow-up is essential for all invasive species control methods.

Pulling and digging can be effective, but can also be disruptive by creating disturbing bare spots, which are good sites for invasive plant seeds to germinate. Do not leave root fragments that can grow into new plants. Use these methods primarily with small plants in loose soils, since this does not usually leave behind large gaps or root tips. Large plants with multiple stems and brittle roots often do.

Chemical treatment is a good way to eliminate purple loosestrife quickly, especially with mature plants. Timing is important, so treat in late July or August. Make sure you treat before flowering to prevent seed set. Chemical treatments can control phragmites effectively. However, since it is most commonly found in wetland communities, Department of Natural Resources (DNR) permits are required for chemical treatments, mechanical treatments, and even some manual treatments. For further information on chemical controls, visit the DNR website.

Biocontrol is considered the best option for more control for heavy infestations. Two species of galerucella beetle, which are host specific to purple loosestrife, have been introduced in our area. . The larvae of these beetles chew on the buds, leaves and stems of the plants, preventing seed production. These insects will not eradicate loosestrife, but can significantly reduce the population and prevent further spread.

The good news is that Calumet County Master Gardeners have been engaged in this purple loosestrife biocontrol program for many years. They have been raising beetles in Calumet County and releasing them locally and in surrounding areas wherever purple loosestrife is a threat to native communities. Before using any chemical control methods on your purple loosestrife, contact the Calumet County Master Gardeners to find out if your plants have biocontrol beetles at work. For additional information on biocontrol of purple loosestrife, visit the DNR website.

View a map of the known purple loosestrife in Calumet County (PDF).