Shrubs, round to erect, 3-20(-35) × (3-)5-20(-35) dm, tomentose to floccose or glabrate, grayish to reddish. Stems spreading to erect, occasionally with persistent leaf bases, up to 4 or more height of plant, often with a distinct main trunk up to 1 dm thick; caudex stems absent; aerial flowering stems erect to spreading, slender to stout, solid, not fistulose, 1-4 dm, tomentose to glabrate. Leaves cauline, 1 per node; petiole 0.5-4 cm, tomentose; blade oblong-ovate to ovate or lanceolate to narrowly oblong, 2.5-7(-10) × 1-5 cm, white-tomentose abaxially, sometimes slightly less so or cinereous to somewhat glabrate and greenish adaxially, margins plane, sometimes crisped.    Inflorescences cymose, open or compact, 5-30 × (2-)5-50(-80) cm; branches dichotomous, tomentose to floccose or glabrate; bracts 3, scalelike, broadly triangular, and 1-2 mm, or leaflike, oblanceolate to elliptic, and 5-30 mm. Peduncles absent or erect, slender, 0.1-0.5 cm, tomentose. Involucres 1 per node, campanulate, 3-5 × 2.5-4 mm, tomentose; teeth 5, erect, 0.3-0.8 mm. Flowers 2-4 mm; perianth white to rose, white-villous abaxially; tepals connate proximal 1/ 4, monomorphic, obovate; stamens exserted, 2-4 mm; filaments pilose proximally. Achenes brown, 2-3.5 mm, glabrous.

The California Department of Transportation is planting members of this insular species, especially var. giganteum, on the mainland, where it readily hybridizes with both E. fasciculatum and E. cinereum. Every effort should be made to remove the introduced E. giganteum from coastal California.

The Avalon scrub-hairstreak butterfly (Strymon avalona), which is endemic to Santa Catalina Island, uses Eriogonum giganteum as a food plant.