Invasive Thistles
by Sue Bird, Yakima County Noxious Weed Board
Thistle, noun, widely distributed herbaceous plant of the daisy family, typically has prickly stems and leaves, and rounded heads of purple flowers.
In central Washington, it’s time to start thinking about invasive thistle control. These plants are considered noxious weeds because they aggressively spread, displace native vegetation, and can impact agriculture and grazing lands
There are more than 62 varieties of native thistles (Cirsium) in the United States. Native thistles are not considered noxious weeks. They occur naturally in regions where they evolved and are a vital part of that ecology. There are more than 500 species of butterflies and 60 species of bees in the Pacific Northwest alone, as well as other insects, birds, and animals that rely on native pollen producers to survive. With that in mind, it’s critical to protect native species while controlling the invasive plants that threaten our native populations by competing for nutrients, water and space. To do that, we need to recognize a native and protect it from the invasive weeds.
Many of the thistles found in central Washington vary in appearance from the dictionary definition. Flower colors can vary from white to pink to purple. Some thistles have thorns on the stems, while others do not. Most have thorns on their leaves. Yellow Star Thistle is actually a knapweed that has long stiff bracts around the base of the flower, and no thorns on the remainder of the plant. The Wavy Leaf Thistle only has thorns on the leaves, and not the stems.
Eastern Washington’s native thistles include Wavy Leaf Thistle (Cirsium undulatum), Edible Thistle (Cirsium edule), Elk Thistle (Cirsium scariosum), Green Thistle (Cirsium inamoenum), and White/Hooker Thistle (Cirsium hookerianum.
Some are more prolific than others, but all of the above have been found growing in central Washington. A common characteristic of the native thistle is typically (but not always) the presence of finely spun, silk-like or web-like hairs, especially around the bracts of the flower and commonly covering the plants. An exception is the Wavy Leaf Thistle, which is covered in short, soft fuzz. It doesn’t have the finely spun hairs, lacks thorns on the stems, but does have thorns on the leaf margins.
Native Thistles are found in less disturbed areas, higher elevations, and in places that are rarely traveled. I find a lot of Wavy Leaf Thistle in arid rocky areas and dry creek beds in the eastern and southern areas of Yakima County. The Edible Thistles, Elk Thistles, and White Thistles are often found in the foothills and higher elevations of the Cascades like Oak Creek and Rimrock Lake. Green’s Thistle has been found in Kittitas County.
Invasive thistles in central Washington include:
Canada/Creeping Thistle (Cirsium arvense) commonly found in lawns, gardens, and disturbed areas. It has a waxy green leaf and a formidable rhizomatous root system.
Scotch Thistle (Onopordum acanthium has broad leaves grayish in color that are covered in fine fuzz. The stems are thorny, as well as the leaves. It has a tap root that can be cut an inch or two below ground for control.
Bull/Spear Thistle (Cirsium vulgare) leaves are dark green with deeply toothed leaf margins, thorns on leaves and stems, and stiff hairs on the leaves. It also has a tap root that can be cut below ground to control.
Musk/Nodding Thistle (Cardus nutans) has a waxy leaf, thorns on margins of the leaf edges, with a white border and mid-vein on the leaves. When blooming, pineapple-like bracts that fold downward. It also has a taproot that can be cut below ground to control.
Milk Thistle (Silybum marianum) has a very irregular leaf edge/margin with very white veins throughout the leaf. It has a waxy leaf surface and long bracts on large pink to purple flowers.
Yellow Star Thistle (Centaurea solstitalis) is not a thistle but an ornery knapweed. It has spines around the base of the flowers and elongated stiff bracts.
Invasive thistles are most often found where there has been a disturbance and were likely introduced mechanically by hitching a ride on construction equipment, on the undercarriages of vehicles, and as accidental contaminants in seed. Others were transported with animal and human movement.
Musk Thistle is mainly found in the foothills of the Cascades, above Wenas Lake and also near the river in Selah.
Milk Thistle is commonly a nursery contaminant. Canada, Scotch, Bull, and Yellow Star Thistles are common in disturbed areas and agricultural lands throughout the Yakima Valley.
It is very rare to find a native thistle in rural or developed areas of the valley, but it can and does happen. Native plants can remain dormant for several years waiting for the perfect temperature and moisture to stimulate germination. They too will move with animal and equipment movement.
The Yakima County Noxious Weed Board has been helping landowners control problem and noxious weeds since 1975. Contact them at 509-574-2180 or susan.bird@co.yakima.wa.us