Non-native Species

Theodore Roosevelt National Park and the State of North Dakota are under attack by numerous plant invaders. These non-native plants are robbing our natural heritage. They claim land at an alarming rate and are the outlaws of the plant world.


Invasive plants, called exotics or noxious weeds, are imported from other parts of the world. Humans move about the earth quickly and easily, making the spread of invasive plants and animals a serious threat to our native ecosystem. Exotics arrive without their natural controls, such as insects, diseases, and competing plants, to keep them in check, and as a result may spread rapidly, crowding or choking out the existing native plants.

Theodore Roosevelt National Park and neighboring states are under attack by numerous plant invaders. Over sixty species of exotic plants have found their way to the park. Several of these plants have become serious invaders and have caused substantial habitat damage and loss. Leafy Spurge and Spotted Knapweed are two aggressive plants that can rapidly invade grasslands. The plants have very few enemies and are not eaten by native species such as bison, elk and deer. They also have the ability to produce a toxin that reduces the growth of neighboring plants.

Intensive management is necessary and is one of Theodore Roosevelt National Park's highest priorities in resource management. Control efforts focus on leafy spurge (Euphorbia esula), spotted knapweed (Centaurea maculosa), Russian knapweed (Centarurea repens), Canada thistle (Cirsium arvense), black henbane (Hyoscyamus niger), absinth wormwood (Artemisia absinthium) and salt cedar or tamarisk (Tamarix ramosissima).

  • Learn to recognize noxious plants in your area. Some city, county and state governments ban the planting of non-native species in their regions.
  • Report noxious plant infestations to the land manager immediately.
  • Be aware that your camping equipment, boots, clothes, pets, and vehicle can all transport non-native plant seeds.
  • When using horses or pack animals, carry only feed that is "certified weed free."
  • Within 96 hours before entering backcountry areas, feed horses and pack animals only food that is "certified weed free."
  • Remove non-native plant seeds from horses and pack animals by brushing them thoroughly and cleaning hooves before transporting to un-infested areas.

Exotic plants cross borders as easily as the wind. Theodore Roosevelt National Park works closely with its neighbors, private landowners and other federal, state, and local agencies to keep these undesirables in check. The park implements an integrated pest management program combining chemical, mechanical and biological control methods to fight these plants. This approach targets a plant, then selects the methods of treatment best suited for the location and habitat type.

In 2002, the park became home to the Northern Great Plains Exotic Plant Management Team (EPMT). The EPMT works with park staff to deal with exotic plant issues. Learn more about the Midwest Region Exotic Plant Management Team.

Biological control can consist of the release of insects that keeps plants in control or stresses them enough to cause their death. After exhaustive testing in quarantine facilities some insect species collected in Europe have been approved for release in the United States. Before their release the insects or pathogens were tested to insure that they would feed solely on the targeted plants.

Control methods are implemented cautiously to prevent damage to native plants, animals, and aquatic resources. Actions are based on research and consultation with field experts.

Leafy Spurge

Leafy spurge

LEAFY SPURGE (Euphorbia esula)

Leafy spurge was introduced to North Dakota in the early 1900s by Eastern European homesteaders who inadvertently carried the plant with them in seed grain and as ship ballast. In North Dakota, leafy spurge infests nearly 1 million acres. It costs North Dakota farmers and ranchers $27 million in direct impacts every year. Total economic impact to the state is estimated at $86 million. In addition to the monetary loss is the loss of valuable and essential grasslands that puts the future of native wildlife in jeopardy.

The leafy spurge infestation is estimated near 4,000 acres (about 10%) in the South Unit of the park. Heaviest concentrations are found along stream beds, drainages and wooded draws. For a first hand look at a leafy spurge infestation, hike along the Little Missouri River or any park trail. In June and July the abundance of yellow-green flowers is a clue that you are standing in the midst of a spurge patch. Leafy spurge is extremely competitive and capable of completely displacing native plants due to its extensive root system that can extend to a depth of 15 feet or more. The root system consists of both coarse and fine roots and contains numerous buds capable of producing new shoots. Older roots are woody and may be as large as one-half inch in diameter. They serve as a large nutrient reserve capable of sustaining the plant for years. Seeds can remain viable in the soil for up to 8 years.

Leafy spurge produces a milky latex that is poisonous to some animals. Thus, most wildlife, such as bison, elk and deer, do not eat it. For people who are sensitive to latex, leafy spurge can cause irritation, blotching, blisters, and swelling if handled.

Theodore Roosevelt National Park uses chemical and biological control methods to fight leafy spurge. Chemical control consists of ground and aerial herbicide applications. Biocontrol by using flea beetles is proving to be very effective and are considered one of the safest ways to control leafy spurge. Flea beetles are released onto plants. Their larvae, which feed on the roots, eventually kill the plants. Through exotic plant management efforts, landscapes are being transformed back into native grassland species.

Spotted knapweed

Spotted knapweed

SPOTTED KNAPWEED (Centaurea maculosa)

Spotted knapweed reduces forage for wildlife and can double the amount of soil erosion when it invades rangeland. It was introduced by settlers from Europe in the early 1900s. Knapweeds are highly competitive and will readily establish on any soil surface. They release a chemical substance which inhibits surrounding vegetation from growing and are not grazed by wildlife because of their bitter taste and high fiber content.

Currently in North Dakota, spotted knapweed is increasing its range at a rate of 175% per year. People are the main reason knapweed is spreading across the country. Seeds are being transported by vehicles, contaminated hay, livestock, and wildlife. The park has spotted knapweed along road corridors and at the Painted Canyon Visitor Center area.

Luckily the plant is easy to control if we can find it early. The most effective treatment for knapweed is to apply herbicide. Biological controls are available but their success has been limited.

Gallfly, used to control canada thistle

Gallfly, an insect used to biologically control the spread of Canada thistle

CANADA THISTLE (Cirsium arvense)

Canada thistle is an aggressive, creeping plant that infests hundreds of acres of park land and over 500,000 acres in the state of North Dakota. Generally, the plant infests disturbed ground, reducing forage for wildlife. One plant can colonize an area of 3 to 6 feet in diameter in one to two years. Established Canada thistle is difficult to control.

Canada thistle is a native of north Africa, Europe, Asia and Scandinavia through Siberia. Canada thistle is not a native of Canada and poses a tremendous threat to the Canadian ecosystem and is considered an unwanted plant there. If your park visit includes a trip to the North Unit in July and August, you will become intimately aware of the presence of Canada thistle. The campground is highly infested with this thorny, prickly plant.

The key to Canada thistle control is to stress the plant. This forces it to use stored root nutrients, which exhausts the supply and eventually causes its death. Chemical, mechanical and biological controls are used to eradicate this exotic plant. The tiny gallfly (Urophora cardui) has been used in the Juniper Campground, laying eggs that produce galls on the stems of Canada thistle plants. The galls harm the plant's growth and development while reducing flowering and root weight. The tiny fly has a conspicuous black and white wing pattern shaped like a V.


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