Shrubs, to 0.5(-0.7) m, rhizomatous. Stems prostrate to erect; bark ± smooth to vertically furrowed, shredding; twigs with straw-colored to ferruginous, multicellular, broad-rimmed, peltate scales and unicellular-hairy. Leaves persistent; petiole with broad-rimmed, glandular-peltate scales; blade oblong-elliptic to elliptic, ovate, or obovate, 0.4-2(-2.5) × 0.2-0.7 (-0.9) cm, coriaceous, margins entire, plane to revolute, with broad-rimmed scales, apex obtuse to rounded or mucronate, surfaces with conspicuous, straw-colored or golden to ferruginous, broad-rimmed, glandular-peltate scales and unicellular-hairy abaxially. Floral bud scales ferruginous-lepidote, sometimes also unicellular-hairy abaxially, margins ± ciliate, unicellular and multicellular, elongate eglandular-hairy. Inflorescences fasciculate racemes, 3-6-flowered; bracts similar to bud scales. Pedicels 2.5-14 mm, with ferruginous scales and often unicellular-hairy. Flowers opening before development of leaves, erect to horizontal, fragrant; calyx lobes 0.5-2 mm, with ferruginous scales and unicellular-hairy, margins ciliate, unicellular and multicellular eglandular-hairy; corolla rose to purple, sometimes white, without blotch, broadly funnelform, (6.5-)7.5-14(-15) mm, glabrous on outer surface, petals connate (for 3/4+ their lengths), lobes 4.5-8.5 mm, tube gradually expanding into lobes, 1.5-6.5 mm; stamens 5-10, slightly exserted, ± unequal, 7-13 mm; filaments glabrous or proximally unicellular-hairy. Capsules borne on erect pedicels, 4-7 × 2-3 mm, with ferruginous, peltate scales and unicellular-hairy. Seeds without distinct tails; testa closely appressed. 2n = 26, 52. Flowering spring-summer. Arctic and alpine tundra, rocky barrens and heaths or thickets, raised beach ridges, sandy stream banks; 0-1900 m; Greenland; Alta., B.C., Man., Nfld. and Labr. (Nfld.), N.W.T., N.S., Nunavut, Ont., Que., Yukon; Alaska, Maine, N.H., N.Y., Wis.; Europe (Russia, Scandinavia); Asia (Siberia). Rhododendron lapponicum is known to hybridize with R. tomentosum in Greenland, producing R. ×vanhoeffenii Abromeit.
Freely branched low shrub 1-3 dm, sometimes depressed and forming loose mats; lvs leathery, evergreen, elliptic, 10-15 mm, densely lepidote, especially beneath; fls few in a terminal, umbel-like cluster, bright purple, campanulate, 1.5 cm wide; stamens 5-10; fr 5 mm; 2n=26, 52. Circumboreal, s. to the higher mts. of N. Engl. and N.Y., and rarely to c. Wis. June.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.