Marsh labrador tea is a widespread shrub with leathery leaves and clusters of cream white flowers, common in black spruce muskeg, bogs, heaths, and shrub tundra, even extending into alpine tundra areas in Denali. Plants are prostrate to ascending, up to 50 cm tall. The leaves are alternate, evergreen and linear, with inrolled margins and a strong midvein, 0.3-2 cm long. The top is dark green, contrasting with the hairy rust-brown bottom. The woody stems are densely rusty-haired. Marsh labrador tea flowers early in Denali, and the inflorescence is an umbel-like cluster of 10-35 flowers, growing at the tips of branches. Each perfect flower has five white petals, 2-8 mm long and 10 stamens with white filaments and cream anthers. The lower half of the filaments is hairy. The calyx is short, the margins with long rusty hairs. In fruit the pedicels are sharply bent downwards. The fruits are capsules, which split into five segments from the base, remaining attached at the apex of the fruit, releasing many seeds. The leaves have a very strong, characteristic smell. Rhododendron tomentosum ssp. decumbens may be confused with closely related R. groenlandicum. This species has narrow, linear inrolled leaves and 10 stamens, whereas R. groenlandicum has strap-shaped, wider leaves, and usually 8 stamens.
This species flowers in mid-summer. Leaves are evergreen, in the early spring they are downflexed, with reddish pigments to protect the leaves from excess sunlight when there is little water available for photosynthesis.Flower appear mid-summer.
Marsh labrador tea has bisexual flowers. The pollination biology of marsh labrador tea is not well studied, but a study of closely related R. groenlandicum may provide some insights in to this species' reproduction. Flowers are the color and shape of other insect-attractants. Seeds are wind-dispersed, and plants also spread vegetatively.
R. tomentosum ssp. decumbens is an alternate host for spruce needle rust (Hennon 2001). In 2011, a mass of 'orange goo' covered the coast of Kivalina in arctic Alaska, and was discovered to be spores of this species (Frazer 2012).
The leaves are made into a tea, drank by natives across North America and white settlers. The tea has also been used variously by Native peoples for treating colds/flu, arthritis, constipation, cuts and scrapes, hangovers, indigestion and gas, other stomach troubles and general ill health (Garibaldi 1999). Labrador tea leaves are traditionally burnt as a purifying incense by the Yup'ik (Fienup-Riordan 2000).
Rhododendron tomentosum subsp. decumbens (=Ledum decumbens) is a widespread amphi-Beringian species (with gaps). It occurs across Alaska and Canada to Greenland in North America, and Siberia and the Russian Far East in Eurasia. In Alaska the species occurs widely in the boreal, alpine, arctic, and low tundra regions across the state, through the Alaska Peninsula. It only occurs in the northeastern corner of Southeast Alaska. In Denali, this species is abundant in the northern half of the park, most frequently in permafrost-influenced sites in the lowlands, but also occurs commonly in the subalpine, and also can be found south of the Alaska Range crest in the park.
Marsh Labrador tea occurs on low inclines, its frequency peaks at 4-12 degrees slope, and forming the largest stands on flat areas (<4 degrees). On inclines, it is slightly more abundant on north-facing slopes. Throughout the Park, this species occurs on a wide range of elevations, from 122 to 1517 m. It has two peaks in amount of cover: under 300 meters, and at 500-700 m.