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Hooker's Thistle, White Thistle - Cirsium hookerianum


Family: Asteraceae - Aster family Genus Common Name: Thistle Native Status: NativeDicot Perennial Herb
Cirsium hookerianum - Hooker's Thistle, White Thistle. This plant is thought to be monocarpic - it flowers and forms seeds only once, and then dies (although there is some question about that.) However, it is also considered perennial, because it may live for several years before flowering.

Similar species Cirsium longistylum is found only in Montana, and has long, fringed involucre bracts.

All Cirsium species are listed as noxious weeds in Arkansas and Iowa, although this particular species is not known outside of a few states in the U.S. northwest, and in Alberta and British Columbia in Canada.

Found in:
ID, MT, WA, WY

NOTE: A comment from Bruce Bennett put my 5-year-old identification of this plant as Cirsium hookerianum in question. While I haven't yet had time to research my reason for discounting other white-flowered thistles known in Idaho, some quick research on Bruce Bennett's credentials implies that he can speak with some authority pertaining to plants in the area, making his identification of this plant as Cirsium undulatum (Wavy-leaf Thistle) almost certainly correct.

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Cirsium hookerianum

Distribution of Cirsium hookerianum in the United States and Canada:
USDA Plants Distribution Map temporarily unavailable.
Blue=Native; Grey=Introduced

Map from USDA Plants Database:
USDA, NRCS. 2017. The PLANTS Database (http://plants.usda.gov, 27 Jul 2017). National Plant Data Team, Greensboro, NC 27401-4901 USA.

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Site: Boise Foothills, Ada County, ID Date: 2011-June-21Photographer: Gerald C. Williamson
Nikon D7000
Tamron SP 90MM f/2.8 AF Macro
The blossom of Hooker’s Thistle is white, or occasionally pink-hued. The phyllaries may have glutinous (sticky) ridges - the white lines on the phyllaries in this photo. The phyllaries are spiny-tipped.
Cirsium hookerianum

Site: Boise Foothills, Ada County, ID Date: 2011-June-12Photographer: Gerald C Williamson
Nikon D7000
Hooker’s Thistle is not usually branched except within the inflorescence. The plant may have a single blossom, or it may have many blossoms.
Click on the photo for a larger image
Cirsium hookerianum

Site: Boise Foothills, Ada County, ID Date: 2011-June-12Photographer: Gerald C Williamson
Nikon D7000
The leaves are usually variably lobed, although occasionally partly entire or toothed. They have spines at the tips of the lobes. The surfaces are usually (but not always) hairy. The lower leaves normally have winged petioles; the upper leaves may be sessile.
Click on the photo for a larger image
Cirsium hookerianum

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All content except USDA Plants Database map Copyright Gerald C. Williamson 2017
Photographs Copyright owned by the named photographer