Learn to Spot Invasive Plants Growing in Allegheny County
Reduce the spread of harmful plants
Japanese Knotweed Fallopia japonicaJapanese knotweed was brought to the US from East Asia in the late 1800s to stabilize stream banks in riparian zones. It is often found along streams and rivers, as well as roadsides.
This herbaceous perennial is a highly successful invader, growing up to 11 feet tall. Knotweed also spreads horizontally through an extensive network of underground root systems and sprouting offshoots.
The stems of the knotweed are similar to that of bamboo’s jointed, hollow stem structure. Covered in red or purple nodes, the stems are otherwise smooth and bright green. Some portions of stems feature leaves in a zigzag pattern from each node, creating the plant’s signature density.
photo credit: U. S. Fish and Wildlife Service - Northeast Region CC PDM 1.0
The leaves of the knotweed are characteristically heart-shaped and up to 6 inches long. It should be noted that giant or hybrid knotweed leaves will grow much larger, up to 1 foot long, and feature a rounded leaf base.
During the late summer months, knotweed will produce white or pale green flower clusters from the nodes. The firework clusters are about 3 to 4 inches long and consist of aromatic flowers.
Japanese knotweed’s key to its invasive success is its ability to reproduce through its underground horizontal root system. They are prone to splitting when disturbed through flooding events along waterways. Trimming back the plant still allows it to regrow new plants if it remains in contact with moist soil.
Tree of Heaven Ailanthus altissimaTree of Heaven was introduced to PA in the 1700s for its unique, fast-growing nature. By the 1900s, the tree’s popularity declined due to its root sprouts and unpleasant odor. Today, the tree has begun popping up along highways, on the edge of fields and in forests.
The tree produces more than 300,000 seeds per year. The seeds are easily dispersed through the wind, allowing them to travel long distances. The root system produces a chemical that prevents other plants from growing near the tree.
The tree tolerates poor soil and air quality, making it a prime candidate for survival in Allegheny County. It has become invasive in urban, agricultural and forested areas. It can grow up to 80 feet tall and 6 feet in diameter.
photo credit: "Tree-of-heaven" by NatureServe is licensed under CC BY 2.0
The leaves of the Tree of Heaven have a central stem with leaflets attached on each side. One leaf can be 1 to 4 feet with 10 to 40 leaflets in total. The leaflets are lance-shaped with smooth margins. When all plant parts are crushed, they emit a strong, offensive odor.
The bark of the tree is smooth and brownish-green during its youth, eventually turning light brown to grey. The bark forms a distinct pattern similar to that of a cantaloupe.
The Tree of Heaven is the host tree of the invasive Spotted Lanternfly. Reducing the tree population can slow the spread of the insect and prevent it from further damaging PA agriculture.
photo credit: "Spotted Lanternfly" by wendylefkowich is licensed under CC BY-NC-ND 2.0
Poison Hemlock Conium maculatumPoison hemlock is a highly toxic weed commonly found along roadsides, creeks, drainage ditches, and low-lying areas of fields and pastures. Properly identifying the weed is the first step to manage it. The biennial plant takes two years to complete its life cycle.
During the first year, it only grows to 18 inches in height with its leaves arranged in clusters at the base of the plant. This phase is most similar to a dandelion’s growth cycle. During the second year, from March to April, the plant will still appear to be in its first stage. However, as spring progresses the plant will shoot up and begin to bloom. During this phase, the hemlock can grow up to 6 feet tall.
The flowers are umbrella shaped and bright white. Hemlock is commonly confused with wild carrot or Queen Anne’s Lace during its second phase. Wild carrot is usually much shorter than hemlock, however. The key difference is hemlock has splotchy purple coloration along the stem and complete absence of hairs. The stems of hemlock are similar to that of bamboo stalks. They are hollow and sometimes full of water. When crushed, the stems emit a distinct odor.
Remember to wear gloves when handling poison hemlock, especially if you are prone to allergens and have sensitive skin!
ACCD staff clears invasive poison hemlock at a riparian buffer site in Valley Park in Monroeville.