Helianthus decapetalus, known by the common names thinleaf sunflower<ref>Template:PLANTS</ref> and thin-leaved sunflower,<ref name=BSBI07>Template:Cite web</ref> is a perennial forb in the sunflower family. It is native to the Eastern and Central United States and Canada, from New Brunswick west to Iowa, Wisconsin, and Ontario, south as far as Georgia and Louisiana.<ref>Biota of North America Program 2014 county distribution map</ref><ref name ="USDA">Template:Cite web USDA, NRCS. 2014. The PLANTS Database (http://plants.usda.gov). National Plant Data Team, Greensboro, NC 27401-4901 USA.</ref> It produces yellow composite flowers in late summer or early fall.
The smooth slender stem of H. decapetalus is 60 to 200 cm (2 to 7 ft) tall and branched near the top. The ovate or lanceolate leaves are borne on 2- to 5-cm-long petioles and have serrated edges. They are 7 to 21 cm long and 4 to 10 cm wide. It has three to 10 flowerheads; each flowerhead is composed of 21 to 50 disk florets, and eight to 12 ray florets, which are 2.0 to 2.5 cm long. The bracts are typically 11 to 16 mm long, surpassing the flower disk by at least half their length. The fruit are 3.5- to 5.0-mm-long cypselae with a pappus of two scales.<ref>Britton, Nathaniel Lord & Brown, Addison (1913). An Illustrated Flora of the Northern United States, Canada and the British Possessions: From Newfoundland to the Parallel of the Southern Boundary of Virginia, and from the Atlantic Ocean Westward to the 102d Meridian, Volume 3., p. 484. Charles Scribner's Sons, New York.</ref><ref>Template:Cite web 'eFloras (2008). Published on the Internet (http://www.efloras.org). Missouri Botanical Garden, St. Louis, MO & Harvard University Herbaria, Cambridge, MA.</ref><ref name="NEWFS">Template:Cite web Copyright © 2011-2013 New England Wild Flower Society (http://www.newenglandwild.org)</ref>
Distribution and habitat
In Virginia, it grows in habitats such as floodplain forests and riverbanks.<ref>Template:Cite web Virginia Botanical Associates. (2014). Digital Atlas of the Virginia Flora (http://www.vaplantatlas.org). c/o Virginia Botanical Associates, Blacksburg.</ref> The presence of this species is dependent on appropriate habitat, and it may be eliminated from an area by development, changes in land use, or competition with invasive species.
The flowers attract many kinds of insects, including bees and butterflies, some of which, such as the painted lady and the silvery checkerspot, use the plant as a larval host. The seeds provide a source of food for birds.<ref>Template:Cite web Copyright © 2002-2012 Dr. John Hilty. Illinois Wildflowers (http://www.illinoiswildflowers.info)</ref> Muskrats eat the leaves and stems and use the stems in the construction of their lodges.<ref name="NEWFS" />