exotic plants

A brownish hood with a white tongue extending make up the flowers that form an upright cluster on the stem.
Grecian foxglove can be harmful to humans if handled without gloves.


Exotic Plant Control Summary

March 13, 2007, Robin Maercklein

St. Croix National Scenic Riverway is known to contain a long list of exotic plants. The vast majority of these are not problematic as they generally do not interfere with the survival of native plants or animals. However, a fair number of these are problematic. Exotic plants threaten native plant communities primarily through competition with native plants. It is well known that they will replace many native species through shading, allelopathy (releasing of chemicals into the soil), or competition for resources such as water, minerals or space. Through these mechanisms they have the potential of changing the native plant communities such as the forest canopy by creating unfavorable habitat for seedlings of native species.

There will never be enough resources – personnel or money – to successfully eradicate all the problematic species. With this in mind, exotic species in the Riverway have been informally ranked to determine those that receive control activities. The most important factors determining priority include ease of control, likelihood of successful control, size of infestation, stage of infestation, invasive potential of the species and the degree to which the species is likely to impact the native communities. The remainder of this document lists most of the exotic plants that regularly receive some control activities. These are listed roughly in priority order.

Purple Loosestrife (Lythrum salicaria) is a perennial herbaceous wetland plant that can completely take over wetlands, replacing most of the native species. Few animals use this plant and thus the plant’s impacts can be considerable. In most cases this plant is easily removed by mechanical means. The use of herbicide is also easy and effective. The use of biocontrol (Galerucella beetles) has been limited to source populations outside the Riverway boundaries. Minnesota lists this plant as a Prohibited Noxious Weed. Wisconsin lists this plant as a Nuisance Weed.

The National Park Service has had an active program to control purple loosestrife since 1983 when only two sites were known. Control activities have grown with the population with apparent eradication at 683 of the 863 sites through 2005. An attempt is made annually to remove every plant observed. Digging or pulling with bagging for removal is the most common method of control. Herbicide (glyphosate) is used at the most difficult sites. The number of plants removed or treated has been recorded since 1993. That year there were 621 plants removed with 1,906 plants controlled in 2006. At least 11,660 plants have been removed or treated since 1993. Loosestrife was found at 180 sites in 2006. Herbicide was applied at just three of those sites.

Source populations are known to exist outside NPS jurisdiction. All in Wisconsin, these include Boyle Brook and Bull Creek in Washburn County, the Yellow River in Burnett County and a five acre population in the St. Croix River within St. Croix Islands Wildlife Area, St. Croix County, Wisconsin. Galerucella beetles have been released in all these areas.

A full report on purple loosestrife control activities through 2006 is available.

Garlic Mustard (Alliaria petiolata) is a biennial herbaceous plant that invades forest environments. This plant rapidly forms dense carpets shading out most native wildflowers and grasses. The key to control is quick removal of populations before they grow too large for available staff. Biocontrol is expected to be available in 2008. Minnesota lists this plant as a Prohibited Noxious Weed.

Five of the six known sites were reported to the Riverway by visitors. The largest site covers over 200 acres with approximately 80 acres on National Park Service land formerly owned by Glen Brae. Control here has been limited to keeping it away from the driveway that accesses an NPS staff landing in the hope that this will limit or eliminate its spread to other sites. North of Soderbeck Landing on the Seven Islands chain, full control efforts cover approximately 20 acres and began in 2000. Small infestations also occur: on the portage around Coppermine Dam; on the St. Croix Trail just south of Old Railroad Bridge Landing, and; on the Indianhead Trail between Lions Park and Rays Garage. A scattered one acre site was discovered in 2006 in lowland on the west side of Rice Lake across from Franconia, Minnesota. This discovery was made possible by the previous discovery of a single plant found here during a visit from the Great Lakes Exotic Plant Management Team (EPMT) in 2004.

Grecian Foxglove (Digitalis lanata) is an herbaceous biennial with a limited range in Washington County. This plant contains digitalis and digitoxin, both of which can be absorbed through the skin requiring the use of rubber or latex gloves during removal.

Our attention and control efforts began in 2000 when we were alerted to its presence by Minnesota Department of Transportation and Minnesota Department of Natural Resources. A collaborative effort has ensued in an attempt to eradicate this plant. Riverway populations are few but include a small population on the former Foster property below the Soo Line High Bridge, two acres across the road and west of the Boomsite Landing, a roadside population at the entrance to Fairy Falls off County Road 11, a small population on the steep bluff overlooking and east-northeast of Fairy Falls, and a widely scattered, but small population in the 10 acre field north of Fairy Falls.

An estimated total of 45,335 plants have been removed to date with an estimated 26,500 plants in 2000 and just 327 plants in 2006. Because the plant needs plenty of sunshine, the long term control plan on most of these sites is to allow them to succeed to forest communities and thus shade out these plants.

Japanese Knotweed (Polygonum cuspidatum) is an herbaceous perennial that can grow to nine feet tall each year. It quickly smothers all other plants in its dense shade. Control efforts have been limited to cutting and treatment with herbicide.

Only two sites are known within the Riverway on National Park Service lands. This plant covers a quarter acre at Matrious Landing and used to create a cathedral like tunnel from the landing to the parking area. The second site covered a small area within the campsite at Phipps Landing. Both sites havae been treated since 2004 and are nearing complete eradication.

Two other sites are known on lands adjacent to the Riverway. The first is located south and adjacent to the Excel Energy dam in St. Croix Falls. The second is 100 yards east of MN Highway 95, north and adjacent to the Falcon Ridge development in Chisago County.

Japanese Barberry(Berberis thunbergii) is a small to medium perennial shrub. Its bright red fall color makes it easy to spot on the forest floor in late September. It is easily controlled when still small as it is easily pulled, being careful to avoid the numerous thorns.

The two known locations include the forest northwest of Fairy Falls and an island just south of Cedar Bend and north of the Swing Bridge. Barberry is widely scattered in the 30 acre Fairy Falls site. Removal efforts since 2004 have been quite successful but overlooked plants here continue to be discovered. The island site is more limited and underwent extensive removal efforts in 2006.

Common Buckthorn (Rhamnus cathartica) is one of the most problematic invasive species in the Riverway. It is a perennial shrub/small tree that invades open areas, thrives in shade, and can form dense colonies providing a dense shade that eliminates all other plants underneath. Buckthorn is widespread in the lower half of the Riverway and has been found on an exotic plant survey as far north as Soderbeck Landing. The population density appears to increase southward with the densest populations found between Nevers Dam and Stillwater. Minnesota lists this species as a Restricted Noxious Weed.

Buckthorn control efforts have been limited to a few areas with specific management goals for each in mind. Plants were cut and treated with herbicide in 2005 prior to burning at Sterling Prairie, one mile downstream from Sunrise Landing in Polk County as part of a prairie/savanna restoration. Staff from the EPMT and volunteers from St. Croix Fall School District have assisted in removing buckthorn from the head of the Indianhead Trail since 2004. Buckthorn has been cut and/or treated with herbicide in 2004 and 2006 on the island surrounding Rice Lake and Peaslee Lake in anticipation of prescribed fire in 2007. A local volunteer group helped cut and treat buckthorn in the woods adjacent to Fairy Falls in 2004. Finally, Riverway staff and volunteers from the Osceola High School removed buckthorn along the Ridgeway Trail in Polk County. This area was designated as Osceola Bedrock Glades State Natural Area in 2002 with our occasional restoration efforts beginning in 2003. Buckthorn was removed from the Headquarters landscape in 2006 when the new facility was finished and new landscaping was installed.

Tartarian Honeysuckle (Lonicera tatarica) is a perennial shrub that invades open areas and persists in shaded forests. It is somewhat less aggressive than buckthorn but its multiple stems make it more difficult to control.

Honeysuckle is widespread and found throughout the Riverway. Selected target areas include those sites where common buckthorn was removed. Some eradication efforts took place in a grassland site just southeast of Wild Mountain Recreation Area in anticipation of restoration activities at this site. Additionally we have been working since 2004 to eradicate the most upstream site found on the Namekagon on a granite outcrop.

Spotted Knapweed (Centaurea stoebe) is another problematic invasive species. This short-lived herbaceous perennial is difficult to control. It successfully competes with native plants in sunny areas partly through allelopathy. It is found throughout the Riverway.

Sites targeted for control efforts include campsites, landings and restoration sites. The latter includes the prairie at Trego Visitor Center and Riverside Landing. It is expected that other sites will be added to this list as restoration activities expand.

Musk Thistle (Carduus nutans) is an herbaceous biennial found at limited locations within the Riverway. To date only small populations have been found in two locations with high potential for eradication. These sites include the former Foster property below the High Bridge and in Browns Bluff, the grassland just south of the Lower District Maintenance facility. Efforts to remove the former site began in 2000 and the latter in 2005. This plant is listed as a Prohibited Noxious Weed in Minnesota.

Amur Maple (Acer ginnala) is an aggressive perennial shrub/small tree popular for its bright red fall color. Two known populations have been eliminated with one remaining in the grassland just north of Fairy Falls.

Cypress Spurge (Euphorbia cyparissias) is an herbaceous perennial with similar invasive properties as its close relative, leafy spurge. Cypress spurge will tolerate shade and acid soils better than leafy spurge.

This plant is known from two sites in the Riverway. A scattered population is located in 5 acres of native prairie at Tewksbury, just south of Osceola, Wisconsin. The second is a former campsite on the Namekagon River downstream from Hayward. This latter site has had control efforts including pulling (ineffective) and spraying with herbicide in 2005 and 2006.

Siberian Peashrub (Caragana arborescens) is a perennial shrub that prefers open areas. Known from two locations, it has only been cut and treated with herbicide at the campsite at Phipps Landing. The other known site is in the campsite at Webb Creek on the Namekagon River.

A four petaled pink flower
Dames Rocket has spread from flower gardens to the countryside.  While it is an exotic, it is not as threatening as some of the other exotics described.

Robin Maercklein

Dames Rocket(Hesperis matronalis) is an herbaceous perennial and has received annual control efforts since 2002 only at the Indianhead Trail head adjacent to Rays Garage. Another site near government housing at Magney has received occasional control efforts.

Canada Thistle(Cirsium arvense) is an herbaceous perennial listed as a noxious weed in Wisconsin and a prohibited noxious weed in Minnesota and. It is present at selected landings and campsites throughout the Riverway. It has had limited control efforts, mainly aimed at the above sites along the Namekagon River. The spraying with herbicide at Norway Point appears to have been an effective means of control.

Birds-Foot Trefoil(Lotus corniculatus) is an herbaceous perennial popular as a ground cover with highway departments. As such most sites are escapees from adjacent highways. Known also from a site across from the Boomsite Landing, control efforts have been limited to removing it from the restored prairie at the Namekagon Visitor Center.

Ox-eye Daisy(Leucanthemum vulgare) is an herbaceous perennial found in sunny locations. Control efforts have been limited to removing it from the restored prairie at the Namekagon Visitor Center.

Crown Vetch(Coronilla varia) is an herbaceous perennial popular as a ground cover with highway departments. As such most sites are escapees from adjacent highways. Like birds-food trefoil it is found across the road from the Boomsite Landing but control efforts have been limited to removing it from the restored prairie at the Namekagon Visitor Center.

Scots Pine(Pinus sylvestris) is found in the Riverway primarily in plantations. Three known sites include a two acre plantation at the northern edge of the Ridgeview Trail, a three acre plantation near Phipps Bridge, and a spreading population at Sterling Prairie, one mile east of Sunrise Landing. At this latter site, the trees were all cleared and burned as part of prairie restoration. Several plantations are not surveyed and other populations may be present.

Black Locust(Robinia pseudoacacia) is a perennial tree popularly used as erosion control with highway departments. As such most sites are escapees from adjacent highways. Known from the grassland north of Fairy Falls and near McDowell Bridge, control efforts have been limited to removing it from the Fairy Falls site by person or persons unknown.

White Poplar(Populus alba) is a perennial established in a dense, but limited stand at Barker Farm in northern Burnett County along the Namekagon River. No control activities have been initiated.

Leafy Spurge(Euphorbia esula) is an herbaceous perennial much more common adjacent to the Riverway than within. A small population is known to exist near the driveway entrance to Glen Brae at the eastern terminus of the Soo Line High Bridge in St. Croix County, Wisconsin. Minnesota lists this plant as a Prohibited Noxious Weed and Wisconsin lists it as a Noxious Weed. No control efforts have been initiated.

Eurasian Water-milfoil(Myriophyllum spicatum) is an herbaceous perennial submerged aquatic plant found in the St. Croix River. Currently it appears to be limited to the stretch between Marine-on-St. Croix and Stillwater, Minnesota. No control efforts have been initiated.

Many staff members and volunteers have participated in this program and are too numerous to mention. I would like to list the following groups without whom much of this work would not have taken place:

Osceola Schools
St.Croix Falls Schools
MinnesotaConservation Corps
WisconsinConservation Corps
Great LakesExotic Plant Management Team – this entity has been invaluable in jump-starting an almost non-existent exotic plant control program beginning with their creation in 2003.

Last updated: April 30, 2019

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