Asteraceae9

Idaho Mountain Wildflowers

Aster (Sunflower) Family: Asteraceae

More rayless composites: Thistles and thistle-like plants

Elk thistle, Cirsium foliosum (Hook.) DC. (left). The elk thistle blooms in early summer along mountain streams and in wet meadows. The young plants are eaten by elk and bears; the peeled stems are edible for humans. The plants are easily identified by their size (it is our largest native thistle), by its many prickly pinnate leaves (foliosum means “leafy”), and by bracts (specialized leaves that cup the flower parts) that extend well above a pinkish flowerhead that turns brown as the plant matures. Both this plant and Hooker’s thistle were gathered by Thomas Drummond (1780-1835), a Scot who collected plants in western America.
Hooker’s thistle, Cirsium hookerianum Nutt. (right). Hooker’s, or white thistle, was given the name hookerianum by Thomas Nuttall in 1841 to honor William Jackson Hooker (1785-1865), professor of botany at Glasgow and later Director of England’s Royal Botanic Garden. The plant grows at high elevations and its white flowerhead makes identication easy. Cirsium is a Greek word for a knot of veins, a condition that thistles were used to treat in the distant past.

Jackson Hole thistle, Cirsium inamoenum (Greene) D. J. Keil (left). The Jackson Hole thistle (Cirsium subniveum is an earlier name) grows along roads and trails, blooming in mid-summer. The plants have a bush-like appearance, growing year after year in the same location. They have hard, serrated, spiny leaves, and pale pinkish or lavender, thistle-like flowers, borne on branching stems.
Canadian thistle, Cirsium arvense (L.) Scop. (right). The Canadian thistle, like the spotted knapweed, is a noxious, spreading, exotic plant, originally a native of Eurasia, but now found in most states and Canadian provinces. It is a serious weed, not easily missed, for it grows tall, with deep purple flowers. It is a deeply rooted, perennial plant that is difficult to eradicate, commonly found growing on disturbed ground, often along roads and railroads. Recently, however, we have seen it growing high in the montane zone, well away from a populated area.

Spotted knapweed, Centaurea stoebe L. (left). The spotted knapweed (previously Centaurea maculosa and later Centaurea biebersteinii) is an import from southwestern Europe that has spread throughout the United States (including Hawaii), Canada and Mexico. There are other species of knapweeds, but this is the most troublesome. Many places in Idaho, as high as the montane zone, are purple with blooming knapweed by mid-summer. As with many weeds, its flower is attractive with purple, three-parted rays. It can be identified by its straggly, sometimes bushy appearance, its purple flowers and the spotted involucre below the flower parts, as shown in the illustration. It has recently been shown—as long suspected—that the roots of knapweed release a plant poison that aids in its spread. Biologists are studying the possibility of controlling the plant, using insects imported from its Old World environment.
Yellow Star-thistle, Centaurea solstitialis L. (right). The yellow star thistle is another thoroughly noxious Eurasian Centaurea that has spread through most of the United States and Canada. Hardy, deep-rooted, woody. and rapidly spreading, it is found in large numbers on open mountain slopes at lower elevations, especially in western Idaho where intensive efforts, both physical and biological, are being made to control the plant. Its appearance is unique and one may identify it at first glance.

Cornflower, Centaurea cyanus L. (right). The cornflower (also bachelor’s button) is, as its appearance suggests, related to the knapweed shown above, although a far more benign plant. A favorite ornamental, it has escaped and established itself as a foothill plant above Boise, and in other parts of the United States. It is much-loved in Europe, where it is the national flower of Poland. For the present, at least, the cornflower seems to be a non-aggressive weed in North America.

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	*Elk thistle, Cirsium foliosum* (Hook.) DC.** (left). The elk thistle blooms in early summer along mountain streams and in wet meadows. The young plants are eaten by elk and bears; the peeled stems are edible for humans. The plants are easily identified by their size (it is our largest native thistle), by its many prickly pinnate leaves (foliosum means “leafy”), and by bracts (specialized leaves that cup the flower parts) that extend well above a pinkish flowerhead that turns brown as the plant matures. Both this plant and Hooker’s thistle were gathered by Thomas Drummond (1780-1835), a Scot who collected plants in western America. Hooker’s thistle, *Cirsium hookerianum* Nutt. (right). Hooker’s, or white thistle, was given the name hookerianum by Thomas Nuttall in 1841 to honor William Jackson Hooker (1785-1865), professor of botany at Glasgow and later Director of England’s Royal Botanic Garden. The plant grows at high elevations and its white flowerhead makes identication easy. Cirsium is a Greek word for a knot of veins, a condition that thistles were used to treat in the distant past.
	*Jackson Hole thistle, Cirsium inamoenum* (Greene) D. J. Keil** (left). The Jackson Hole thistle (Cirsium subniveum is an earlier name) grows along roads and trails, blooming in mid-summer. The plants have a bush-like appearance, growing year after year in the same location. They have hard, serrated, spiny leaves, and pale pinkish or lavender, thistle-like flowers, borne on branching stems. *Canadian thistle, Cirsium arvense* (L.) Scop.** (right). The Canadian thistle, like the spotted knapweed, is a noxious, spreading, exotic plant, originally a native of Eurasia, but now found in most states and Canadian provinces. It is a serious weed, not easily missed, for it grows tall, with deep purple flowers. It is a deeply rooted, perennial plant that is difficult to eradicate, commonly found growing on disturbed ground, often along roads and railroads. Recently, however, we have seen it growing high in the montane zone, well away from a populated area.
	*Spotted knapweed, Centaurea stoebe* L.** (left). The spotted knapweed (previously Centaurea maculosa and later Centaurea biebersteinii) is an import from southwestern Europe that has spread throughout the United States (including Hawaii), Canada and Mexico. There are other species of knapweeds, but this is the most troublesome. Many places in Idaho, as high as the montane zone, are purple with blooming knapweed by mid-summer. As with many weeds, its flower is attractive with purple, three-parted rays. It can be identified by its straggly, sometimes bushy appearance, its purple flowers and the spotted involucre below the flower parts, as shown in the illustration. It has recently been shown—as long suspected—that the roots of knapweed release a plant poison that aids in its spread. Biologists are studying the possibility of controlling the plant, using insects imported from its Old World environment. *Yellow Star-thistle, Centaurea solstitialis* L.** (right). The yellow star thistle is another thoroughly noxious Eurasian Centaurea that has spread through most of the United States and Canada. Hardy, deep-rooted, woody. and rapidly spreading, it is found in large numbers on open mountain slopes at lower elevations, especially in western Idaho where intensive efforts, both physical and biological, are being made to control the plant. Its appearance is unique and one may identify it at first glance.
	*Cornflower, Centaurea cyanus* L.** (right). The cornflower (also bachelor’s button) is, as its appearance suggests, related to the knapweed shown above, although a far more benign plant. A favorite ornamental, it has escaped and established itself as a foothill plant above Boise, and in other parts of the United States. It is much-loved in Europe, where it is the national flower of Poland. For the present, at least, the cornflower seems to be a non-aggressive weed in North America.