National Park Service Japanese Knotweed (Polygonum cuspidatum), Giant Knotweed (Polygonum sachalinense), Bohemian Knotweed (Polygonum x bohemicum)Within the Upper Delaware River Valley, there are three knotweed species including Japanese Knotweed and Giant Knotweed. The third species, Bohemian Knotweed, is a hybrid between Japanese Knotweed and Giant Knotweed. Originally, knotweed species were introduced to the United States from Eastern Asia as an ornamental in the late 1800s. Invasive knotweed species quickly spread and became established throughout North America, including in the Upper Delaware River Valley. Knotweeds are upright, shrubby, herbaceous, bamboo-appearing perennials having ovate, or truncated, leaves, and small-greenish white flowers from August to September. Knotweed is abundant in many riparian areas and islands along the Upper Delaware River. Like most invasive plants, knotweed spreads rapidly and forms dense thickets, displacing native vegetation. The bare soil under knotweed thickets is highly suspectable to erosion, threatening the stability of riverbanks. Although it is difficult, knotweed can be controlled and eradicated through mechanical or chemical methods. Small young populations of knotweed can be pulled by hand, while larger populations may require the use of herbicides. Knotweed tends to proliferate in disturbed areas; thus, it is important to ensure disturbed habitat is restored with native vegetation to prevent its establishment. 
National Park Service Common Mugwort (Artemisia vulgaris)Common Mugwort is an invasive perennial that was originally used as a medicinal herb. It was transported from Eurasia to North America in ship ballast from where it quickly spread. Mugwort’s vast rhizome system helps it to spread aggressively and outcompete native species. Rhizomes are the underground horizontal plant stems from which the roots and new shoots of a plant originate. The extensive rhizome system of Common Mugwort also makes it difficult to control and manage. Both pulling and mowing are ineffective. For small infestations of mugwort, chemical control may have some effectiveness. However, a combination of methods may be best for controlling mugwort, and it is important to restore areas with native plants to prevent a new mugwort infestation. Eurasian Watermilfoil (Myriophyllum spicatum)Eurasian Watermilfoil is a rooted, submerged invasive plant species that can be found in the slow-moving back-eddy portions of the Upper Delaware River. Native to Europe and Asia, it’s suspected that Eurasian Watermilfoil was first introduced intentionally for aquarium and aquatic nursery trade. It has white to reddish stems and thin thread-like green leaves whorled around the stem. As Eurasian Watermilfoil grows, it can form dense canopies at the water surface, which reduces light penetration. These dense canopies not only allow Eurasian Watermilfoil to outcompete native species for sunlight and other resources, but can also discourage visitors from wanting to recreate in the river resulting in economic losses. Fragments of Eurasian Watermilfoil can easily be caught on boats or other water related equipment, and then transported to new areas. Thus, it is important to remember to clean all watercraft and gear of aquatic plants to avoid unknowingly spreading invasive species. Once established, Eurasian Watermilfoil can be controlled and managed by either cutting and pulling up the plant with rakes and cutting blades or by using herbicides. Currently, there is some research being done to determine if Eurasian Watermilfoil could be controlled biologically with native insects. Purple Loosestrife (Lythrum salicaria)Purple Loosestrife is an erect, branching perennial with showy purple flowers that can be found along the riverbanks and on islands of the Upper Delaware River. Native to Europe, Asia, and Northern Africa, Purple Loosestrife was introduced to North America in ship ballast and as herbal medicine. Even today, some people continue to plant Purple Loosestrife as an ornamental. Although the bright purple flowers are quite charming, some call Purple Loosestrife the “purple plague” because it can devastate natural ecosystems. Purple Loosestrife forms dense stands that outcompete and replace native plants, leading to lower quality habitat for aquatic wildlife such as amphibians and turtles. Stands of Purple Loosestrife may even deter waterfowl and other birds from nesting. Small infestations of Purple Loosestrife may be pulled by hand or controlled by chemical means. However, these methods may be intensive and take years to remove the infestation completely. There are also some beetle species that can be used to biologically control Purple Loosestrife. The beetles lay eggs on the plant, and their larvae feed on the plants after hatching. This feeding aids in slowing growth and reducing seed production. Japanese Stiltgrass (Microstegium vimineum)Japanese Stiltgrass is an invasive annual grass native to Japan, China, central Asia, and India. It was originally introduced to North America because of its use as packing material for imported china. Japanese Stiltgrass resembles bamboo, except for its thin stems that can be green, purple, or brown. The seeds of Japanese Stiltgrass are compact and very mobile, allowing it to easily disperse to new areas. The seeds also float and are often transported by the movement of water, like along the Delaware River. Japanese Stiltgrass grows into dense stands of grass that prevent other native, more desirable species from growing. To control Japanese Stiltgrass, mechanical or chemical methods may be used. Small infestations can be easily pulled, and string trimming may be used for larger infestations. It is important to remove the plant before it goes to seed if using mechanical control methods. Pre-emergent herbicides (chemicals that prevent seeds from germinating) and post-emergent herbicides (chemicals used to control the plant after germination) have been successfully used to control and help eradicate Japanese Stiltgrass. Restoring treated areas with native plants helps to prevent a re-establishment of Japanese Stiltgrass. 
National Park Service Mile-a-minute (Persicaria perfoliata)Mile-a-minute is an herbaceous, annual, trailing vine. Native to India and Eastern Asia, it was first introduced to North America accidentally in a shipment of holly seeds to a plant nursery in York County, Pennsylvania, in the 1930s. Mile-a-minute can grow up to six inches a day and forms dense mats that grow over other plants and block out sunlight, thereby weakening them. Mile-a-minute is spread by water and animals. The bright blue fruits are buoyant and can travel along streams and rivers. Birds eat the berries but don’t digest the seeds which allows the seeds to be transported long distances. Chipmunks, squirrels, and deer all consume the berries and also contribute to the spread of the seeds.Mile-a-minute can be controlled and managed in several different ways depending on the level of infestation. Vegetation buffers along streams and avoidance of openings in existing vegetation help to prevent the establishment of Mile-a-minute. On young stems and leaves, before fruiting bodies develop, pulling may be an effective control method. Pre and post-emergence herbicides can also be used to control Mile-a-minute. Biological control of Mile-a-minute using the weevil Rhinoncomimus latipes has been shown to be effective at preventing the vine from fruiting but not eradicating infestations. Female weevils lay eggs on Mile-a-minute leaves and stems. After the eggs hatch, larvae bore into and feed on the stems, thereby damaging plants. |
Last updated: January 12, 2021