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Swamp Thistle

Cirsium muticum

Notes:

This wildflower is a biennial that forms a rosette of basal leaves during the first year, followed by a flowering plant about 2-7' tall the following year. The basal leaves are up to 16" long and 6" across, although they are normally closer to about one-half that size. Aside from a tendency to be larger in size, they are similar to the alternate leaves of a flowering plant (as described below). A typical flowering plant is unbranched below and occasionally branched above. The central stem and any lateral stems are light green, terete, and longitudinally furrowed; they are finely hairy above, becoming more glabrous below. Alternate leaves are up to 10" long and 4" across, becoming gradually smaller in size as they ascend the stems. In outline, these leaves are lanceolate-oblong to elliptic-oblong; they are also pinnatifid with several pairs of lanceolate to linear-lanceolate lobes that have spiny tips. The lobes of the alternate leaves usually have smaller secondary lobes or sharp teeth that may also have spiny tips. The spines are golden yellow and up to 1/8" (3 mm.) long. The upper leaf surface is medium to dark green and glabrous to sparsely short-pubescent. The lower leaf surface is usually whitened from a dense coating of short white hairs while young, but it becomes more pale green with age from a thinning of such hairs. The central stem and any upper stems terminate in either individual flowerheads or open corymbs of 2-5 flowerheads. The peduncles of these flowerhead are 1-6" long, light green (less often pale purple), terete, finely grooved, and finely hairy to nearly glabrous. There is usually a single leafy bract (up to 3" long) at the base of each peduncle and, less often, there may be a whorl of a few leafy bracts (up to 1½" long) underneath the flowerhead. Otherwise, the peduncle is naked. The leafy bracts resemble the alternate leaves, except they are smaller in size and their spiny lobes are more narrow. Each peduncle terminates in a flowerhead that is about 1¼-2" across and similarly in length. At the flattened top of the flowerhead, there is a dense cluster of numerous disk florets; there are no ray florets. The disk florets are perfect and fertile. The corollas of the disk florets are narrowly cylindrical below, becoming slightly wider above with 5 narrow lobes. These corollas vary in color from pink to pinkish purple. The sides of the flowerhead consist of overlapping floral bracts (phyllaries) that are appressed together in several series. These floral bracts are lanceolate to ovate in shape; their tips taper into erect minute spines, or such spines may be absent. The sticky floral bracts have narrow mid-ribs that are pale green, otherwise they are blackish green; appressed woolly hair usually occurs between the floral bracts. The blooming period occurs from mid-summer to early fall, lasting about 1½ months. Afterwards, the ray florets are replaced by small achenes with tufts of white hair. Individual achenes are about 5 mm. long, oblongoid, and dark brown to black with bands of yellow at their upper ends. They are distributed by the wind. The root system consists of a stout taproot.

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Aarongunnar
Spotted by
Aarongunnar

Wisconsin, USA

Spotted on Aug 22, 2015
Submitted on Oct 15, 2016

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