NISAW: Destructive Invasive Plants

Be on the lookout for these two destructive invasive plants.

Biologists from the Maine Department of Agriculture, Conservation and Forestry (DACF) are asking for the public's help finding and reducing the spread of two severely invasive plants: Japanese Stiltgrass (Microstegium vinimeum) and Perennial Pepperweed (Lepidium latifolium). Populations of these species are only known from a handful of sites in Maine. Early Detection/Rapid Response efforts show significant reductions in the size and extent of stiltgrass and pepperweed infestations. The public can help in efforts to eradicate these species from Maine by reporting any suspected locations and preventing them from spreading to new areas.

Stiltgrass is an annual grass that takes over the forest floor. Once established, stiltgrass has a high potential to inhibit seed germination of neighboring plants by releasing allelopathic chemicals into the soil. Stiltgrass thatch builds up, making it even more difficult for native trees, shrubs, and wildflowers to regenerate and grow. Stiltgrass infestations grow rapidly as each plant can produce up to a thousand tiny seeds that can be spread by moving water, deer, contaminated soil, dirty boots, or dirty equipment. Stiltgrass was first detected in York County in 2020 and is currently present in Sagadahoc and York Counties. These populations are being actively managed, but new populations have been found and/or reported every year due to seed dispersal and additional populations going undetected.

Key traits to identify Stiltgrass: Leaves are alternate along the stem, 2-4" long and ½" wide, pointed at both ends Stripe of reflective hairs along the leaf midrib Leaf edges and surfaces feel smooth to the touch Trails along the ground, branching from nodes where it produces "stilts" (roots) Shallow-rooted and easy to pull up Flowers and sets seed late in the season (September-October); one to three seed spikes at the top of the plant that resembles crabgrass Stems often develop a reddish tint late in the season

Stiltgrass invading a forest floor. Photo by DACF

Perennial Pepperweed (aka Perennial Pepperwort) is an herbaceous plant that can take over sensitive habitats in salt marshes and sand beaches. These are rare habitats in Maine and are home to rare wildlife. When Perennial Pepperweed takes over, it can make the habitat unsuitable. Perennial Pepperweed can also grow on roadsides, riverbanks, and freshwater marshes. There are dense infestations of Perennial Pepperweed in Massachusetts and New Hampshire. Still, only a handful of sites are known in Maine. Newly detected Stiltgrass or Perennial Pepperweed should be reported and removed to keep these invasive plants from damaging Maine's treasured forests and wetlands. DACF will offer support and guidance to affected landowners.

Key traits to identify Perennial Pepperweed: Leaves are alternate along the stem, lance-shaped, and get smaller closer to the top of the plant Lower leaves have gentle teeth around the edges; leaves higher on the stem may lack teeth Stems are smooth and lime green, usually 1-4 feet tall, often multiple stems from one base Small white flowers with four petals in dense clusters at the top of the plant (late June to late July); tiny fruits are reddish-brown, flat and disc-like Roots have a strong smell, like horseradish

Perennial pepperweed invading a natural area by the ocean. Photo by Leslie J. Mehrhoff, University of Connecticut, Bugwood.org

If you think you have found Stiltgrass or Perennial Pepperweed on your property or public land, please review these species on the MNAP Web Gallery and GoBotany website. If you believe you have found either of these plants, please send an email with photos and location to [email protected] or map the area and provide a picture using the online mapping tool iMapInvasives. iMapInvasives also allows you to see other mapped locations, explore invasive species near you, and set up email alerts for your area or species of concern. Please visit our website, www.maine.gov/dacf/mnap/, under the Invasive Species tab to sign up for a free iMapInvasives account and to view iMapInvasives help resources.