Proc. Acad. Nat. Sci. Philadelphia 4: 15. 1848.
Shrubs, spreading, not scapose, (1.5–)2–5(–7) × 5–15 dm, grayish- to reddish-brown-tomentose to floccose and gray or, rarely, thinly floccose and greenish. Stems spreading to erect, typically without persistent leaf bases, up to 1/2 height of plant; caudex stems absent; aerial flowering stems erect to spreading, slender, solid, not fistulose, 0.3–0.8 dm, floccose or glabrous. Leaves cauline, 1 per node; petiole 0.2–0.7 cm, tomentose to floccose; blade oblanceolate to oblong or obovate, (1–)1.5–3 × (0.2–)0.3–0.7 cm, densely white-tomentose abaxially, white-floccose to glabrate or green and glabrous adaxially, margins plane. Inflorescences cymose, 10–30(–40) × 10–40 cm; branches dichotomous, white-floccose to glabrate or subglabrous; bracts 3, scalelike, triangular, 0.5–2(–5) mm. Peduncles absent or mostly erect, slender, 0.3–2.5 cm, floccose. Involucres 1 per node, turbinate, 1.5–2.5(–3) × 1–2 mm, tomentose to floccose; teeth 5, erect, 0.3–0.6 mm. Flowers 2–4 mm; perianth yellow, glabrous; tepals connate proximal 1/4, essentially monomorphic, elliptic to obovate; stamens mostly exserted, 2–4.5 mm; filaments sparsely pilose proximally. Achenes brown, 2–2.5 mm, glabrous.
Phenology: Flowering Jun–Sep.
Habitat: Sandy to rocky slopes and flats, mixed grassland and sagebrush communities, juniper and montane conifer woodlands
Elevation: 1200-2500 m
Colo., Mont., Nebr., N.Mex., S.Dak., Wyo.
Eriogonum effusum is rather common on the northern Great Plains and along the eastern slope of the Rocky Mountains in central and eastern Colorado, southeastern Montana, western Nebraska, northern New Mexico, southwestern South Dakota, and southeastern Wyoming. Some specimens from Chaffee County, Colorado, are thinly floccose and greenish (Atwood & Welsh 29689, BRY) and thus similar to E. leptocladon. A collection from Pyramid Lake, Washoe County, Nevada (Frandsen & Brown 182, NESH) is clearly mislabeled. A roadside collection of E. effusum gathered near Little America, Sweetwater County, Wyoming, in 1961 (G. Mason 4025, ASU) was an introduction that has not persisted.
The spreading wild buckwheat is occasionally merged with Eriogonum microthecum even though the two are morphologically distinct and their ranges do not overlap. Plants in New Mexico are sometimes difficult to distinguish from the related E. leptocladon var. ramosissimum. The species is the food plant for the Rita dotted-blue butterfly (Euphilotes rita). A hybrid between Eriogonum effusum and E. pauciflorum has been named E. ×nebraskense Rydberg [E. multiceps Nees subsp. nebraskense (Rydberg) S. Stokes; E. pauciflorum Pursh var. nebraskense (Rydberg) Reveal]. The hybrid is known from Weld County, Colorado; Cheyenne and Kimball counties, Nebraska; and Converse and Platte counties, Wyoming.