Garlic mustard is an invasive non-native biennial herb that spreads by seed. It can cross-pollinate or self-pollinate, it has a high seed production rate, it out competes native vegetation and it can establish in a relatively stable forest understory. It can grow in dense shade or sunny sites. Plant stands can produce more than 62,000 seeds per square meter to quickly out compete local flora, changing the structure of plant communities on the forest floor. Garlic mustard is considered allelopathic, producing chemicals that inhibit the growth of other plants and mychorrizal fungi.

Japanese barberry has escaped from cultivation and is progressively invading natural areas. It is a particular threat to open and second-growth forests. An established colony can eventually grow thick enough to crowd out native understory plants. Traversing through dense patches of barberry can be difficult and even painful. Birds eat the red berries, thereby spreading the shrub into new areas.

Asiatic bittersweet poses a serious threat to other species and to whole habitats due to its aggressive habit of twining around and growing over other vegetation. The vines can strangle tree and shrub stems. All types of plants, even entire plant communities, can be over-topped and shaded out by the vine’s rapid growth. Nearly pure stands of this vine are sometimes found in affected areas.

Euonymus is slow growing. The bark is gray-brown and the stems have prominent, corky wings running along both sides. The leaf-buds are brownish-green, and strongly divergent. In autumn the dark green leaves turn a brilliant purplish red to scarlet color before dropping to the ground. In Massachusetts, the flowers bloom in late April to late June. The flowers are small, yellowish green in color and inconspicuous. The smooth, purplish fruit are 1.3 cm long and are present in September through October. Each fruit contains approximately four red to orange seeds.

This good-looking plant has been transplanted into well-watered gardens all over the world and has widely escaped. This tall marginal plant is very common beside rivers and lakes. It blooms from June to August. The stems are often branched, and plants form dense colonies, outcompteing other plants.

Purple loosestrife is an herbaceous perennial characterized by long showy spikes of magenta flowers. Usually under 4 feet in height, the plant may reach up to 10 feet tall in nutrient-rich habitats. Purple loosestrife has flowers with 5 to 7 petals which occur in dense clusters on terminal spikes and which bloom from June to September. According to most reports, purple loosestrife crowds out native wetland vegetation, such as cattails, grasses, sedges, and rushes. The plant thrives in disturbed wetlands but also invades natural wetland communities.

Phragmytes reeds have rhizomes which are invasive and may grow four metres in one year.

Japanese knotweed can tolerate a variety of adverse conditions, including deep shade, high temperatures, high salinity and drought. It is commonly found near water sources, such as along streams and rivers, in low-lying areas, waste places and utility rights-of-way and around old home sites. It spreads quickly to form dense thickets that exclude native vegetation and greatly alter natural ecosystems. Japanese knotweed is an extremely difficult plant to control due to its ability to re-grow from vegetative pieces and from seeds.

Black Locust

Black locust is a fast growing tree that can reach 40 to 100 feet in height at maturity. While the bark of young saplings is smooth and green, mature trees can be distinguished by bark that is dark brown and deeply furrowed, with flat-topped ridges. Seedlings and sprouts grow rapidly and are easily identified by long paired thorns. Leaves of black locust alternate along stems and are composed of seven to twenty one smaller leaf segments called leaflets. Leaflets are oval to rounded in outline, dark green above and pale beneath. Fragrant white flowers appear in drooping clusters in May and June and have a yellow blotch on the uppermost petal. Fruit pods are smooth, 2 to 4 inches long, and contain 4 to 8 seeds.

Multiflora rose grows aggressively and produces large numbers of fruits (hips) that are eaten and dispersed by a variety of birds. Dense thickets of multiflora rose exclude most native shrubs and herbs from establishing and may be detrimental to nesting of native birds.

Common buckthorn is a dioecious species with male and female flowers on separate plants. Flowers are 4-petaled and yellowgreen in color; fruits are black. Flowering takes place from May through June and fruits ripen from August to September. Twigs of common buckthorn are often tipped with short spines. Buckthorn leafs out early and retain leaves late into the fall creating dense shade that helps it to out-compete many native plants.