Mat-forming dwarf shrub with extensively branched rhizome and rooting aerial shoots, forming large stands of procumbent stems. Aerial branches narrow, ca. 2 mm broad, densely covered with needle-shaped leaves, moss-like (note both the vernacular and scientific species names).
Leaves alternate, evergreen, retained on the shoots for 4 years or more, minute, 1.5–2 × 0.2–0.4 mm, imbricate, needle-like, boat-shaped with a rounded back and the mid vein impressed in a groove on the upper surface, dark green, minutely pubescent along the margins with patent or forward-pointing, small, stiff hairs.
Flowers singly in shoot apices.
Pedicels ca. 5 mm in flowering stage, straight for most of their length but curved immediately beneath the flower, stretching out to ca. 10 mm and becoming erect beneath the fruit, pubescent with minute, stiff, patent hairs. Flowers nodding, radially symmetric with 5 sepals and petals. Sepals free, ca. 2 × 1 mm, lanceolate to ovate, acute, with more or less distinct mid vein, dark purple with broad, pink hyaline margins. Corolla ca. 4–5 × 5–6 mm, broadly bell-shaped with petals fused for 1/3–1/2 its length with petal lobes not recurved, white but often with pink lobe ends. Stamens 10; filaments short, ca. 0.8 mm, flattened; anthers reniform, ca. 0.3 × 0.4 mm, with two long, slender spurs opposite the pores. Gynoecium of 5 carpels; style short, ca. 1 mm, with a bulbous base.
Fruit a one-roomed capsule with numerous seeds. Capsule erect in fruit stage, nearly globular, 1.5–3 × 1.5–3 mm, opening by 5 valves diverging from a central axis (topped by the style). Seeds minute (ca. 0.5 mm), pale brown.
Sexual reproduction by seeds; restricted vegetative reproduction by rooting shoots at a very local scale (within less than 1 m²) and with no dispersal effects. Flowers are strictly adapted to insect pollination. The spurs on the anthers function as levers; the anthers only empty pollen through the pores when an insect moves the spurs. Seed set is regular in Svalbard, vouching for efficient insect pollination.
Seeds are very small and light and dispersed by wind.
When not flowering, Harrimanella might be mistaken for a moss but no mosses have rooting shoots and firm leaves of several cell layers with an impressed mid vein on the upper surface. This species was previously included in the genus Cassiope. There is some superficial similarity with C. tetragona but that species has much coarser and ascending to erect shoots with much coarser leaves in 4 distinct rows, and the flowers are lateral on the shoots (not apical as in Harrimanella) and with revolute, cream white petal lobes (straight and often pink in Harrimanella).
Growing in dry snowbeds and heaths with not too tall-grown vegetation, on moist but not soaked or indundated ground. Harrimanella is probably fairly indifferent as to soil reaction (pH) but found only in districts with circumneutral to basic substrates (the districts with only acidic substrates may be too cold).
Thermophilous. Harrimanella hypnoides is known from three areas in Svalbard, all in W Spitsbergen: Berzeliusdalen at Bellsund on the north side of Van Mijenfjorden, Grønfjorden on the south side near the entrance to Isfjorden (both in Nordenskiöld Land), and several places along the eastern and northern side of Kongsfjorden (Ossian Sarsfjellet, Feiringfjellet, Blomstrandøya, Kapp Guissez) and by Krossfjorden (Ebeltofthamna), in Haakon VII and Albert I Lands. This is a coastal distribution pattern shared by, e.g., Cerastium alpinum, Luzula arcuata, Ranunculus glacialis, Rhodiola rosea, and Salix herbacea.
The global distribution is northern amphi-Atlantic, from NE Canada across Greenland, Iceland, Svalbard, and northern mainland Europe to NW Siberia.
Harrimanella has for long been suspected to be a genus apart from Cassiope, based on morphological evidence (Stevens 1970). This is now supported by molecular evidence (Kron et al. 1999). The two genera are not very close in a molecular Ericaceae phylogeny.
Kron, K.A., Fuller, R., Crayn, D.M., Gadek, P.A. & Quinn, C.J. 1999. Phylogenetic relationships of epacrids and vaccinioids (Ericaceae s. l.) based on matK sequence data. – Plant Systematics & Evolution 218: 55–65.
Stevens, P.F. 1970. Calluna, Cassiope and Harrimanella: A taxonomic and evolutionary problem. – New Phytologist 69: 1131–1148.