At first glance, Cassiope tetragona can look like a clubmoss, due to its spreading habit and stems obscured by scale-like leaves, but the white urn-shaped flowers reveal it as a member of the heath family (Ericaceae). It is a dominant plant of heath tundra and snowbeds in the mountains of Denali. Cassiope tetragona grows prostrate to ascending, the tips of stems erect up to 5-30 cm. The backs of the leaves are deeply grooved, distinguishing it from the other Cassiope species of the park, C. lycopodioides. Leaves are leathery, evergreen, and ovate to lanceolate; arranged opposite of each other in four rows. The outer surface is short-hairy and the margins are ciliate. The square shape of the stems is the source of the scientific name 'tetragona', meaning four-angled. Two to six flowers grow at the end of ascending stems. The pedicels are upright, reddish, curved at the tip, flowers nodding. The calyx is yellow green or reddish, appressed to the corolla, five-toothed. The corolla (fused petals) is bell-shaped, white, the edge with five flared semi-triangular lobes. The fruits are dry capsules, globose, which split open to release small seeds.
Four-angled mountain heather is a perennial plant, and its leaves are evergreen. Plants flower at the end of May to August, depending on yearly and local environmental variables. In Sweden, plants flower 25-40 days after snowmelt, and flowers last for 7-12 days. Fruits are produced approximately one month later. The species is slow-growing. A study in Eagle Summit found plants produced an average of 14 new leaves a year, leaves on average lasting for three years (Murray and Miller 1982).
Cassiope tetragona is monoecious with bisexual flowers. Four-angled mountain heather is mainly self-fertile, according to studies in the Scandinavian arctic (Hagerup 1954). However, the flowering morphology also attracts pollinators, particularly bumblebees, and some out-crossing occurs. Seeds are minute, spread by wind and gravity. Plants also expand vegetatively, spreading prostrate along the ground to form large mats.
Because it burns well when still green, C. tetragona has been used as a fire starter by Natives, mountain climbers and other travelers in the arctic or alpine.
Cassiope tetragona is a circumpolar species that occurs widely in the arctic and alpine regions of the northern hemisphere. In North America, this species has an arctic-alpine distribution, occurring in the mountains of the Alaska, the Yukon and Alberta southwards to Washington and Montana, and also across Arctic Canada. In Alaska, C. tetragona occurs in alpine areas across the state except for the Alaska Peninsula and Aleutians (it is also rare in southwest Alaska). In Denali, Cassiope tetragona is a common alpine species, occurring on both sides of the Alaska Range, as well as the foothills in the northern corner of the park.
This species is generally uncommon on south-facing slopes and has a high level of cover on northern slopes. Four-angled mountain heather is also most abundant on steep slopes (20-28 degrees), its frequency of occurrence dramatically declining on gentler slopes. It is an alpine specialist, cover and frequency peaking above 1100 m elevation and plants are almost nonexistent below 600 m.