“Sulfur Buckwheat (Eriogonum umbellatum) - Also known as Sulfurflower, this plant is especially showy in the rocky, sagebrush flats. The sulphur-yellow blossoms are in an umbrella-like cluster at the top of a 10 to 12 inch flowering stalk, while the numerous leaves grow very near the ground and accumulate bits of organic matter which eventually become part of the soil. A closely related alpine species (E. ovalifolium) has pale yellow flowers which become tinged with pink in drying. The flowering perion extends from mid-June to early August.” {From: "Plants of Yellowstone and Grand Teton National Parks" by Richard J. Shaw pg 96.}

Yellowstone Wildflowers link (this is an Adobe pdf file).