Rising to Meet the Challenge of Invasive Species

Common buckthorn (Rhamnus cathartica) is a tall understory shrub or small tree found in oak forests, woods, savannas, prairies and similar habitats. Native to Europe, common buckthorn came to North America in the 1800s for use as an ornamental shrub. 

Because it's among the first plants to leaf out in the spring and it keeps its leaves late into the fall, it can create dense shade that eliminates other tree seedlings and understory species. Its long growing season also gives it an edge over native species, outcompeting them for resources and sunlight. 

Common buckthorn also produces chemicals that inhibit the growth of vegetation, which further disrupts the environment and decreases critical food sources for all kinds of insects and other wildlife. 

The plant has simple, dark-green oval shaped leaves. In May through June, small green-yellow flowers bloom along the stem. Beginning in August, the flowers form berries that ripen through September and remain on the plant through the winter. Birds and other wildlife that eat the berries then spread the seeds. 

A relative of garlic mustard, dame's rocket (Hesperis matronalis) is a biennial plant, establishing itself during one year and blooming the next. 

Introduced to North America from Eurasia in the 1600s, dame's rocket is now naturalized across both moist woodlands and open areas.

In its first year, it grows as a low rosette of leaves close to the ground. In its second year, it bolts a stem that's two to four feet tall. Around May to June, the stem will bloom in clusters of white to purple flowers, with each having four petals. 

These flowers form abundant seeds, which allows dame's rocket to quickly spread and dominate habitats. It outcompetes native wildflowers and other plants, creating a monoculture where it grows and reducing critical food sources for native birds, butterflies, bees and other creatures. 

Dame's rocket is often sold in wildflower seed packets, so check seed mixes thoroughly before planting. 

Native to Europe, garlic mustard (Alliaria petiolata) first came over to the United States in the 1800s, likely brought over for food or medicinal uses. 

Garlic mustard is a biennial plant (meaning it has a two-year life cycle) that has triangular, heart-shaped leaves with toothed edges and white four-petal flowers. 

During its first year, garlic mustard leaves are rounder and take on a rosette formation at ground level. In their second year, the leaves grow up a flowering stem, with small white four-petaled flowers emerging from May through June. 

In the late summer and fall, the flowers of garlic mustard produce slender seed capsules that contain a row of small black oblong seeds. These seeds are easily spread by wind, allowing garlic mustard to rapidly colonize habitats from a distance. 

Garlic mustard gains a foothold in fields and forests by emerging earlier in spring than many native plants. By the time native species are ready to grow, garlic mustard has blocked their sunlight and outcompeted them for moisture and vital nutrients.

Japanese knotweed (Fallopia japonica) is a perennial shrub that's typically found along woodland edges, stream or river banks, wetlands and roadsides. It grows three to 10 feet tall and will form large colonies of erect, arching stems that resemble bamboo. 

Introduced from Asia in the late 1800s as an ornamental plant, it escaped cultivation and quickly spread to surrounding areas. Once established in a habitat, Japanese knotweed releases chemicals that are toxic to other plants, disrupting their growth cycle and allowing the knotweed to take over. 

When Japanese knotweed grows along streams or river banks, it can prevent native trees from regenerating, increasing soil erosion around waterways. It also disrupts nutrient cycles along these habitats, making it harder for native plants to thrive. 

Japanese knotweed produces large, somewhat heart-shaped leaves that are about three to six inches long. Beginning in August through September, the shrub will begin to bloom with long stems lined with several small greenish-white flowers. 

After blooming, the flowers produce small, winged fruits transported by wind, water, or wildlife. However, Japanese knotweed will also spread through stem fragments, roots and rhizomes. 

Multiflora rose (Rosa multiflora) is a thorny shrub that forms dense thickets, crowding out native understory plants in open woodlands, forest edges, prairies, savannas and similar areas. It tolerates a wide variety of soil and environmental conditions, giving it an advantage over native plants. 

Native to Japan, multiflora rose was introduced in the late 1700s to the United States. However, because the plant can produce up to 500,000 seeds per year—and the seeds stay viable and dormant for up to 20 years—it quickly escaped cultivation and began to take over wild areas. 

Once mature, multiflora rose can reach up to 15 feet tall, with long flexible green to reddish stems. Each stem is covered with stiff, backward-curved thorns along its length. 

In late May through June, multiflora rose shrubs will bloom in clusters of white to white-pink flowers. By mid-September, the flowers form red berries that persist into the winter months. Birds and other berry-eating animals will disperse the seeds throughout an environment. However, many of the seeds will fall and grow close to the parent plant, which is how this shrub forms impenetrable thickets so quickly. 

Purple loosestrife (Lythrum salicaria) is found in wetlands, where it crowds out and overgrows native species. It will grow both along dry shores and in up to two feet of water, giving it even more of an advantage over native plants that are more particular about where they grow. 

Native to Europe and Asia, purple loosestrife grows up to two to six feet tall, with flower stalks that are six to 12 inches long. Purple loosestrife blooms in early July through September, with several bright purple-pink flowers blooming along each stalk. 

In the fall, purple loosestrife readily forms seeds—more than 2 million per plant. The seeds are tiny, about the size of a grain of sand, and spread easily via water, wind, wildlife and people. Purple loosestrife seeds are also incredibly hardy; while they can germinate in the next season, they can also lay dormant for several years before blooming. 

Reed canary grass (Phalaris arundinacea) is a perennial grass that grows best in wetlands, moist prairies, fields, and along roads and ditches. Native to Europe and parts of Asia,  reed canary grass rapidly grows and spreads, outcompeting native plants. As a result, wetlands can become overrun with this plant as it dominates the areas. 

A cool-season grass, reed canary grass is often the first plant to sprout in the spring. It grows two to six feet tall, with flat, rough grass blades that taper from the base to the tip. Its stems are hairless and stand erect. 

Around mid-May through June, reed canary grass will bloom with dense florets in clusters. These blooms produce seeds in late June, which are easily spread via waterways, animals and people. 

Wild parsnip (Pastinaca sativa) grows in prairies, oak savannas, fields, pastures and other open or disturbed areas. It's an adaptable and fairly hardy plant, growing well in both dry and wet soils. It can be found throughout most of the United States due to its rapid growth and spread. 

As a biennial, in its first year, wild parsnip forms low-to-the-ground rosettes of leaves. The leaves grow across from each other on the stem, with one diamond-shaped leaf on the end. The leaves have jagged (toothed) edges. 

In its second year, wild parsnip will send up a shoot that is about 4-5 feet tall. Then, in June through July, this shoot will develop and bloom in flat disks of tiny yellow flowers. In the fall, the flowers turn to seed, which are easily spread on the wind, water and other means, allowing wild parsnip to spread and colonize new areas easily. 

Wild parsnip is in the carrot family and is closely related to another invasive plant, Queen Anne's Lace, which has white flowers instead of yellow like wild parsnip.

The stem, leaves and flowers of wild parsnip contain chemicals that can increase skin's sensitivity to sunlight and cause rashes or blistering.