Pennsylvania has its share of invasive shrubs--plants that are non-native to the state and grow so densely that they can crowd out native plants. Many of these invasive shrubs escaped from cultivation. This means that at one time they were solely ornamental species, but at some point their seeds germinated in the wild. Birds and small animals, eating the fruit of these shrubs, unwittingly helped to disperse their seeds and allow them to flourish.
Common privet (Ligustrum vulgare) is the perfect candidate to be an invasive shrub. The species became invasive after its introduction from both Europe and China as an ornamental plant in the middle of the 1800s. Privet can multiply through a process known as suckering, growing new offspring plants from its roots until entire thickets composed of privet develop. The shrub, which can grow to 30 feet tall but is usually smaller, produces flowers from April through June. The blooms turn into the fruit, which contains the seeds that birds and animals will eat and carry away, further enlarging the range of the plant. Privet often takes hold along fencerows and invades fields and forest. Privet grows in the shade as well as the sun.
Wineberry (Rubus phoenicolasius) is a thorny shrub that bears berries. The plant is native to parts of Asia and came to America in the 1890s to provide breeding stock for other berry types. However, the wineberry shrub was too aggressive a species for people to contain easily, and the plant made its way into wild areas. Once it gains a foothold, wineberry spreads rapidly and takes over an area. Native species have little chance when competing against this shrub, which features spiny stems known as canes. The leaves are compound, made up of three leaflets shaped like hearts attached to one stem. The canes can grow to be 9 feet long and will arch over. Red hairs cover the stems and give wineberry a reddish hue from a distance. Wineberry will form impenetrable thickets in fields, near streams and along wetland borders in Pennsylvania and in other states where it exists.
According to the Pennsylvania Department of Conservation and Natural Resources website, Japanese barberry (Berberis thunbergii) is invasive in Pennsylvania and 19 other states in the East. Japanese barberry is another escaped ornamental that easily conquers the countryside by producing copious amounts of seeds. The seeds have a germination rate as high as 90 percent, and the shrub can grow new plants where its branches encounter the ground. Japanese barberry is a shrub that deer refuse to eat, a fact that gives it an advantage it really does not need over native species. The shrub grows to 8 feet tall and has brown branches covered with sharp spines, making it painful to traverse a woodland or field where thickets of barberry exist. In addition to crowding out native plants, Japanese barberry also affects the pH level of the soil in which it grows. Japanese barberry flourishes in full sun or full shade with equal abandon.