People commonly refer to any plants without value in their current locations as weeds, according to the USDA's Animal and Plant Health Inspection Service. The USDA's own definition of weeds is that of plants seriously threatening U.S. agriculture or ecosystems. Within that definition are the classifications of ordinary and noxious, especially threatening, weeds.
Mile-a-minute (Persicaria perfoliata) is vine native to India and Asia. As of 2010 according to the National Park Service Alien Plant Working Group, it grew in 11 Eastern U.S. states and Washington, D.C. Recognizable for its barbed stems and light green, triangular leaves, mile-a-minute thrives in sunny, moist soil. The barbs let it scramble over other vegetation toward sunlight. It eventually kills those plants by cutting off the light they need for photosynthesis.
Common along woodland edges, roadsides and stream banks and in wetlands, mile-a-minute has summer-to-fall white flowers and blue berries. It reproduces by self-sowing and with help from birds, ants, deer and small mammals. Feeding on its berries, they also transport the seeds, which survive for as long as 6 years. Plants growing along waterways drop their berries into the water for long-distance dispersal.
Purple loosestrife (Lythrum salicaria) is an ornamental perennial native to Europe and Asia. Introduced to the United States in the early 19th century, it now grows in every state but Florida. It has taken over marshes, stream and river banks and reservoirs.
Standing up to 10 feet high, purple loosestrife has a strong, woody stem and lance-like green leaves. Popular with gardeners for its brilliant summer display of magenta flower spikes, purple loosestrife can produce between 30 and 50 blooming stems and millions of seeds on a single plant. It also spreads by roots, according to the National Park Service Alien Plant Working Group. Forming vast wetland colonies, it chokes out native grasses and plants that feed and shelter waterfowl and wildlife.
While many people don't associate trees with weeds, 31 U.S. states now classify tree-of-heaven (Ailanthus altissima) as an invasive weed. This Chinese native came to America in the 18th century. Growing up to 80 feet high, it has sumac-like green leaves. Spring clusters of small, yellow flowers give way to flat, winged seedpods, samaras, on pollinated female trees. Tree-of-heaven's distinctive nut-like odor distinguishes it from native sumacs, according to the National Part Service Alien Plant Working Group.
Heavy seed production, more than 300,000 seeds per plant per year, and the release of chemicals that deter other plants from growing near it account for tree-of-heaven’s invasiveness. The tree's spreading roots send up new shoots that can grow through concrete, damaging sidewalks, foundations and sewer lines. It’s as common in urban parking lots as it is along country roads and fences.