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.

Flowering Jun-Sep. Sandy to rocky slopes and flats, mixed grassland and sagebrush communities, juniper and montane conifer woodlands; 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.

FNA 2005, Heil et al. 2013, Allred and Ivey 2012, Carter 2012, Martin and Hutchins 1981

Common Name: Spreading Wild Buckwheat Duration: Perennial Nativity: Native Lifeform: Shrub General: Spreading, diffusely branched shrubs or subshrubs, 15-70 cm tall and 50-150 cm wide, from a compact to spreading caudex; stems numerous, grayish, and covered with woolly tangled hairs (floccose) or hairless (glabrous). Leaves: Alternate along the stems, on petioles 2-7 mm long; blades linear to oblong, 1-3 cm long and 3-7 mm wide, densely covered with whitish to grayish woolly hairs, with the upper surface often less hairy and greener than the lower surface. Flowers: White to pink or yellow, arranged in clusters of a few flowers held together by a cup-shaped structure called an involucre; involucres arranged in a diffusely branched and often congested flat-topped inflorescence, 10-40 cm long and 10-40 cm wide; each individual flower is 2-4 mm high, with 6 petal-like tepals, these white, yellow, or pinkish. Fruits: Achenes trigonous, 2-3 mm long, brown, glabrous. Ecology: Found on dry plains, slopes, and mesas, from 4,500-6,500 ft (1372-1981 m); flowers June-September. Distribution: MT, WY, SD, NE, CO, and NM Notes: Various sources do not agree as to the flower color of this species. FNA lists it as having yellow flowers and almost all other sources say the flowers are white to pinkish. It is possible that the flower color is variable, or that there is some confusion based in mis-identification of herbarium specimens. FNA notes that the species is similar to E. microthecum and E. leptocladon var. ramosissimum, which are generally described as having white to pink flowers (though Martin and Hutchins list E. microthecum's flower color as yellow.) Distinguish E. effusum from those species based on its leaves, which are flat around the edges; and its densely branched, cymose (flat-topped) inflorescence (E. microthecum has revolute, or rolled-under leaf edges and a sparsely branched inflorescence; E. leptocladon has inflorescences composed of racemes or spikes, i.e. with flowers all along the branches.) Ethnobotany: Unknown, but other species in the genus have uses. Etymology: Eriogonum comes from the Greek erion, wool, and gony, knee, alluding to the hairy nodes of the first species described, Eriogonum tomentosum; effusum means spread out or unrestrained, alluding to the many spreading branches. Synonyms: Eriogonum microthecum var. effusum Editor: AHazelton 2017