Silene acaulis is a nearly ubiquitous and distinctive alpine plant that grows in compact cushions dotted with bright pink flowers. Moss campion cushions can be round or flat, arising from a stout woody taproot. The leaves are densely crowded, linear, a characteristic bright grass green color, the margins ciliated with stiff white hairs. Numerous pink to lavender 5-petaled flowers arise just above the cushion. The sepals are fused into a tubular calyx while the petals are free and notched. Flowers are either bisexual or unisexual but produced on the same plant. The three-styled pistils in female and perfect flowers develop into cylindric capsules which open by six teeth at the top. Called 'moss campion' after its moss-like growth habit, this plant can be distinguished from other cushion-forming, pink flowered alpine plants by the linear, ciliate leaves.
Silene acaulis is a semi-evergreen perennial that flowers mid to late summer.
Silene acaulis is typically monoecious with bisexual flowers, but unisexual, pistillate plants (female) are also seen, and some plants can produce both bisexual and pistillate flowers. It is insect pollinated, probably primarily by bees and wasps. Bisexual plants are both out-crossed and self-pollinated, but selfed plants had lower seedset (Shykoff 1988). In studies in Colorado and Alaska, the female (pistillate) plants produce far more seeds than bisexual plants (Morris and Doak 1998; Shykoff 1988). Seedlings from female plants also had higher survival rates (Shykoff 1988). The fruit is dehiscent and seeds are gravity or wind disseminated. The Alaskan study proposed found plants they estimated to be over 300 years old (Morris and Doak 1998).
The Database of Insects and their Food Plants documents seven insects that feed on Silene acaulis in Europe: 2 flies and 5 macro-moths (Biological Records Centre 2008). Marr (1997) documented a pollinator transmitted fungal disease (anther smut) that affected reproduction of Silene acaulis in Colorado. It is unknown whether this disease is present in Alaska.
Canadian Inuktitut have been documented to eat raw root skins of S. acaulis (Moerman 1998). Wordsworth alluded to moss campion when he wrote: 'Upon it's native bed.../There, cleaving to the ground, it lies/With multitude of purple eyes,/Spangling a cushion green like moss.'
Silene acaulis is a circumpolar species with an arctic-alpine distribution, ranging from North America to Greenland, Iceland, throughout Europe and across Eurasia. In North America, this species ranges from Alaska, east across the Canadian Arctic archipelago and south throughout the Pacific Northwest, and Rocky Mountain states as far south as Arizona and New Mexico. Silene acaulis also ranges south into Maine and New Hampshire in the east. In Alaska, S. acaulis occurs in mountainous regions statewide in suitable habitats. In Denali, moss campion is common and widespread and occurs in alpine areas on both sides of the Alaska Range, including quite commonly along the Park Road.
Silene acaulis is an alpine species that is most commonly found in plots above 1100 m in elevation, although it is also found as low as 591 m, and as high as 1787 m in suitable habitats. This species can occur on all aspects, but slightly prefers south facing slopes. It strongly prefers steep slopes (>20 degrees).