Chinese Lespedeza (Lespedeza cuneata)
Lespedeza cuneata, unlike many of the plants on this list, is not a threat to forested areas. It prefers open spaces and full sun and thus invades fields, meadows, and endangered prairies. It can form dense stands and smother native plants. It is extremely difficult to control as well, as it establishes a seed bank in the soil that means new plants can sprout years later, even after extreme control measures are used. Chinese lespedeza should never be planted, as the only effective ways to control the plant are intensive mowing or herbicide use. However, neither of these control methods is adequate, and seeds will continue to sprout for years after the initial control measures.
Garlic Mustard (Alliaria petiolata)
Garlic mustard was introduced to the US as a source of food and medicine but has quickly become a severe threat to native ecosystems. It spreads extremely rapidly, producing hundreds or thousands of seeds per plant and forming dense stands across the forest floor. These dense stands choke out native plant species and decrease forest species richness, both through competition for light and nutrients and through the release of growth-inhibiting chemicals that stops plants other than garlic mustard from growing. Garlic mustard infestations are best prevented by limiting forest disturbance, continuous monitoring, and prompt removal of plants that do appear. Existing infestations can be controlled with manual removal or herbicide, provided non-target plants are not present.
Japanese Knotweed (Fallopia japonica)
Japanese knotweed is one of the worst invasive plants in the entire US. The plant quickly spreads by mechanical dispersal of root fragments, often carried great distances by rivers and streams. It is found in 42 states, where it quickly invades disturbed ecosystems and forms dense thickets that prevent native plants from growing. The soil beneath knotweed is often bare due to the chemicals the plant releases in the soil and the dense shade it casts, causing severe soil erosion. The plant is notoriously difficult to control once it establishes, often requiring multiple herbicide applications or repeated mechanical removal to eradicate. The best way to prevent infestations is to ensure disturbances are quickly filled back in with plantings of native plants, which can help prevent Japanese knotweed from establishing itself.