Invasive Plants

Most non-native plants are not invasive. When a new plant first arrives, it’s impossible to predict with certainty whether it will become invasive. But here are a few common characteristics of invasive plants, along with examples: 

• They have no natural predators in their new habitat. Deer avoid eating Japanese barberry, which gives it an advantage over native plants.
• They produce lots of seeds. One garlic mustard plant can release hundreds or thousands of seeds when it matures.
• They have aggressive root systems. A single black locust tree can create a dense grove of clones, and any damage to the roots causes them to spread vigorously.
• They can thrive on disturbed sites. Phragmites (common reed) is commonly found in disturbed wetlands.
• They release chemicals that are toxic to surrounding plants. Amur cork trees release allelopathic compounds that change the soil microbes and hurt nearby plants.
• They leaf out earlier and drop their leaves later than native plants, which gives them a longer growing season while casting shade on other plants. Buckthorn is often the last green plant in woodlands in late fall (aside from evergreens). 

Invasive plants are not the same thing as weeds. Weeds are plants that people consider undesirable in a certain place or context, regardless of whether they’re native or non-native. For example, white snakeroot is a native plant that benefits woodland ecosystems, but a gardener might consider it a weed if it showed up next to their carrots. 

Although people brought them here with good intentions, we now know better and can stop planting plants that have become invasive. In Wisconsin, the DNR’s invasive species rule (NR-40) makes it illegal to transport, transfer, introduce, and (for some species) possess regulated invasive plants. However, it’s still easy to find invasive plants at nurseries and online stores if you’re not careful. Make sure that any plants you buy for your property are not invasive. 

Today, people commonly spread invasive plants unintentionally. Plant seeds can easily hitch a ride on clothing, boots, vehicles, and equipment. Before you travel from one forest to another, make sure to clean off mud, seeds, and vegetation from everything you bring with you, including your pets.   

Another way that invasive plants spread locally is through wildlife. A bird might eat the fruit of a honeysuckle, fly to a nearby patch of woods, and unintentionally plant the seed with its droppings. 

For example, buckthorn can create too much shade for native wildflowers and tree seedlings to grow, and garlic mustard can hoard water and soil nutrients at the expense of oak seedlings.  

If left alone, invasive plants make forests less diverse and weaken or eliminate the next generation of trees. Some invasive plants also harm mature trees. For example, round leaf bittersweet, a woody climbing vine, can shade out trees’ leaves, weigh down crowns, or even girdle the trunks. All these effects make forests less resilient to other stresses like droughts, extreme storms, pests, or diseases. This is especially concerning in light of the many stresses that our forests face as the climate changes. 

Typically, invasive plants provide worse food for wildlife and pollinators than native plants do. An understory with a diverse range of native plants blooming at different times of year creates a buffet for wildlife and pollinators. In contrast, an understory of just buckthorn and honeysuckle offers a severely limited menu that may not match what animals need. What’s more, when native plants decline, insect populations also tend to suffer, which has ripple effects on the whole ecosystem. For example, bush honeysuckle and Japanese knotweed seem to benefit birds with generalist diets (such as American Goldfinches and American Crows) at the expense of birds with specialist diets (such as many wood warblers).

Invasive plants also get in the way of recreation—literally. By covering trails, they make it hard to hike, bike, or ride on horseback. By creating dense thickets, they make it hard to find a spot to pitch a tent, hunt deer or turkey, or forage for mushrooms.

The negative economic impact of invasive species is substantial, though hard to quantify. In fiscal year 2024, the Wisconsin DNR spent $4.4 million specifically on invasive species management (this figure includes all invasives, not just woodland plants), plus millions more on park maintenance or land conservation projects that necessarily involved managing invasive species. These figures do not include the economic impacts on Wisconsin’s recreation and forest products industries. Across the U.S., invasive species (plants, insects, mussels, and more) had an estimated economic cost of $21 billion per year from 2010 to 2020.

Here are some of the highest priority invasive plants to look for in your woodland. Click a plant to learn more about it. Then learn how to manage invasive plants.

Woody invasive plants

*Round leaf bittersweet was formerly known as Asian or oriental bittersweet.

Herbaceous invasive plants

*Although reed canary grass is not on the DNR’s list of regulated species, this plant is still a problematic invasive species in wetlands. 

**Japanese stiltgrass has been found only five times in Wisconsin so far: first in La Crosse County in 2020, and then in Rock, Walworth, Grant, and Dane Counties in 2025. However, we expect more to arrive throughout the state in the coming years.

If you have questions about invasive plants and your woods or feedback on this webpage, contact:

Page last updated September 2025.

Additional photo credits:

• Japanese stiltgrass: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
• Wild parsnip photodermatitis: Pfc598, CC-BY 3.0
• Japanese stiltgrass: John Zaborsky, UW–Madison Renz Weed Science Lab
• Common buckthorn: anemoneprojectors via Flickr, CC BY-SA 2.0
• Japanese barberry: Richard Gardner, Bugwood.org
• Multiflora rose: Southeastern Wisconsin Invasive Species Consortium
• Round leaf bittersweet: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
• Dame’s rocket: David Cappaert, Bugwood.org
• Japanese knotweed: Jan Samanek, Phytosanitary Administration, Bugwood.org
• Phragmites/common reed: UW–Madison Extension Horticulture Program
• Reed canary grass: University of Main Cooperative Extension
• Japanese stiltgrass: John Zaborsky, UW–Madison Renz Weed Science Lab