Widely Known Established Invasive Plants


The above ten "weeds to watch" have become well established. However, each is better known in certain regions. For example, the Knapweeds are well established in the West and are not considered threatening in the East. Yet there the plant is, along a roadside in Connecticut.
Kudzu is likely the mother of all weeds, a mega flora, that catches attention. In the seventies, when I learned about this invasive plant, I scoffed at the thought that it would ever establish in the snowbelt. Yet it has! It is now in northern Illinois and Wisconsin is guarding its borders. ALL WEEDS CONTINUE TO ADAPT AND MOVE.
Black locust is native to the southern Appalachians and the only native plant in the group. It was moved to the central plains for dust bowl plantings. Once established, and free of its natural competition, it formed monocultures wherever it went. Now oak forests in Wisconsin are displaced by this apparently alleopathic tree.
Purple loosestrife, Lythrum spp. was determined to be a wetland pasture pest in Canada in the 1940's. It now grows in all 50 States. Never underestimate a wetland weed that can be dispersed by flooding, muskrat fur, and ducks' feet. And so it moved from wetland to wetland and through migratory routes. It is recommended that you carefully remove this plant when it is first sighted as an infestation of one plant per acre.
Starthistle already covers millions of acres in the Northwest into California. Grazing becomes impossible on infected lands. Starthistle is moving eastward and is halfway across the country. This European annual tolerates a range of disturbed soils. Its basal leaves look much like those of a dandelion. Its ? inch spines are unforgettable.
Tamarix or salt cedar quietly moves through riverine areas and sucks up more than its share of dwindling water supplies in the West. It also changes the structure of lowland vegetation and habitat. Consider how well it might do in parts of the United States with higher precipitation and learn how to identify it!
Ailanthus or tree-of-heaven, Ailanthus altissima. This fast-growing deciduous tree comes from China. Sometimes planted in urban areas, Ailanthus is now found coast to coast. It prefers disturbed soils, roadsides, woodland edges, and poor soils. Imagine my surprise when I found one trying to camouflage itself in my Sumac border. It is also mistaken for Black walnut because of its pinnately compound leaves. Unlike sumac, the leaves turn yellow in the fall. I have seen it from Maryland to Oregon to Minnesota.
Leafy spurge, Euphorbia esula is already a noxious weed in nineteen States! This chartreuse colored herb grows from 6-36" depending on the environment. Economic impacts of more than $100 million annually in just four of the infested Great Plains States. This deep-rooted Eurasian perennial impacts grazing lands and natural areas. Watch for its earl spring color and act quickly. It is unlikely to be contained in the West.
Russian Olive, Elaeagnus angustifolia. This small Eurasian tree was favored as an ornamental for its silver foliage, contrasting dark bark and tolerance of salt. Known for wilt afflictions which damage its appearance, but not its will to live, the Russian Olive has been spotted in grasslands, roadsides, and stream banks from Minnesota to New Mexico. Because it is easy to pick out in a crowd of plants, control should be easy.
Knapweeds, Centaurea spp. includes: Diffuse knapweed, Centaurea diffusa; Spotted knapweed, C. maculosa; Meadow knapweed, C. pratensis; Russian knapweed, C. repens; Squarrose knapweed, C. virgata; and possible subspecies, possibly a hybrid reported in Minnesota. Yellow starthistle, C. solstitialis and Bachelor's button, C. cyanus are closely related. Knapweeds are either annuals or short-lived, perennial herbs, native to Eurasia. Knapweeds were likely introduced accidentally as contaminants in forage seed. Spotted knapweed is on 15 State noxious weed lists and spreading. The white, pink-lavender flowers can be striking; but their fine gray-green foliage can be identified easily. They reproduce primarily by seed and their many seeds germinate throughout the growing season. Knapweed seeds are viable up to nine years. These taprooted plants replace plants with fibrous roots, and so compromises soil stability. Knapweeds have devalued rangelands and pastures throughout the West, and are moving eastward.
The Thistles, Nodding, Plumeless, Musk, Canadian, Bull, or Scotch (Carduus, Cirsium, Centaurea and Onoporuma spp.) group are Eurasian imports, likely seed contaminants more than a hundred years ago. They degrade crops and pastures and spread easily into disturbed soils. Remember there are native thistles worth identifying and protecting.

Bibliography:

Much of this information is from the experience of the author, Bonnie L. Harper-Lore, with these references by my side: Czarapata, Elizabeth J. 2005. Invasive Plants of the Upper Midwest. University of Wisconsin Press, Madison. Whitson, Tom D., et. al. 1992. Weeds of the West. University of Wyoming, Jackson.