Eriophorum viridicarinatum is a species of sedge known by the common names thinleaf cottonsedge, green-keeled cottongrass, and bog cottongrass. It is native to Canada and the United States.
Eriophorum viridicarinatum is a perennial sedge that forms tufts of stiff, erect stems, sometimes just a single stem, and basal leaves up to 30 centimeters long. It grows from a rhizome. The inflorescence is accompanied by two to four leaflike bracts each a few centimeters long. There are up to 30 spikelets, increasing in size as the fruit develops, reaching 3 centimeters in length. Each flower has a tuft of white or brown bristles that are long and cottony, measuring up to 2.5 centimeters long.
Eriophorum viridicarinatum was first described as the variety Eriophorum latifolium var. viridicarinatum by the German-American botanist Georg Engelmann in 1844. Engelmann's description was based on specimens collected in Massachusetts and Ohio. (The name published by Engelmann was hyphenated, as in viridi-carinatum, but the orthographical variant viridicarinatum is now widely used instead.) The American botanist Merritt Lyndon Fernald raised the variety to species rank in 1905., Eriophorum viridicarinatum is a widely accepted name.
Eriophorum viridicarinatum is native to northern North America, where it occurs in Alaska and throughout much of Canada, its range extending into the northern contiguous United States. It is widespread in eastern Canada, with spotty distribution in western Canada and Alaska. In the United States, it is most common in western Montana, the Great Lakes region, and New England.
Eriophorum viridicarinatum is an obligate wetland (OBL) species. Throughout its range, it occurs in marshes, wet meadows, bogs, fens, and wet woodlands, at altitudes up to 6600feet. In the Pacific Northwest, British Columbia, Montana, and Wyoming, it typically occurs in montane and alpine zones. In New England, it prefers fens and high-pH meadows. It is a strict calciphile in Michigan, Pennsylvania, and Vermont, but its habitat broadens further north into Canada.