Note: Numbers given in square brackets in the text refer to the images presented above; image numbers are displayed to the lower left of each image.

General: Twinflower is classified as a dwarf shrub, although some authors describe it as a perennial vine. It often forms extensive mats or colonies [1–3] in the understorey of boreal forests, but may also occur on bog hummocks and in forest clearings. In the literature, there is disagreement as to what subspecies should be recognized in North America. Some sources have subsp. longifolia (Torr.) Piper & Beattie restricted to western North America, with eastern regions represented by subsp. americana (J.Forbes) Hultén. However, the currently accepted nomenclature has subsp. americana reduced to synonymy under subsp. longifolia, which is now considered to be the accepted subspecies that occurs across Canada (Brouillet et al. 2010+). The typical twinflower, subsp. borealis, is largely Eurasian in range, but also occurs in the Yukon and British Columbia.

Key Features: (numbers 1–2 refer to the illustration [4]; for number 3, see image [5])

1. Leaves are opposite, sparsely hairy, ovate, obovate, or orbicular, tapering to rounded at the base, and have 1–4 pairs of crenate teeth near the blunt apex.
2. Erect flowering shoots have terminal peduncles that fork near the tip, each of the two pedicels bears a single nodding pink flower.
3. Flowers are subtended by pairs of bracts that are densely glandular pubescent; peduncles and pedicels are also glandular pubescent [5].

Stems/twigs: Stems trailing, to 1–2 m long, perennial, reddish- brown, finely hairy, slender, and 2–3 mm in diameter; older stems are slightly woody. The prostrate stems bear pairs of leaves, root at the nodes, and have upright flowering shoots to 10 cm tall [6–7]; extensive patches of twinflower often develop over old logs, stumps, or rocks [8–9].

Leaves: Opposite, simple, evergreen, 1–2 cm long, short-petiolate, and sparsely hairy [10–12]. Leaf blades are broadly ovate, obovate, to orbicular; bases are cuneate to rounded, the apex is blunt to rounded, and the margins have 1–4 pairs of crenate teeth toward the apex. Flowering shoots arise from the axil of the leaves on trailing stems.

Flowers: Bisexual; flowering shoots have a slender herbaceous glandular-pubescent peduncle, 5–10 cm long, that divides into 2 divergent pedicels, which are about 1/4 to 1/3 the length of the peduncle [13–15]. Peduncle bears a pair of nodding fragrant pink flowers, 8–15 mm long. Each flower has a green calyx with 5 linear-lanceolate acuminate lobes that are glandular pubescent; the pink corolla is funnelform, with a tubular base that gradually flares into 5 rounded [16–17]; overlapping portions of the corolla lobes in bud result in white patches on the expanded corolla lobes. The inside of the corolla is pubescent with long white hairs [18]. The 4 stamens, 2 of which are shorter, are adnate to the inside of the corolla, near the base; the single pistil has an inferior ovary, a slender style about as long as the corolla, and a capitate stigma. The calyx and ovary bear straight white hairs as well as shorter glandular hairs. Subtending each flower is a pair of bracts that are ovate to rounded and densely covered with secretory glandular hairs [19–20]; this pair of bracts often persists in fruit, protecting the developing capsule. Additional pairs of small bracts occur at the division of the peduncle and where the pedicels begin to nod. Flowers bloom in mid-summer. Pollination is primarily by native bees and syrphid wasps (Howard 1993).

Fruit: A small single-seeded capsule, covered with glandular-pubescent hairs; the calyx lobes persist for a short time at the apex of the young fruit [21–24], but are eventually deciduous, leaving the glandular obovoid capsule nodding at the tip of each pedicel [25–26]. The small glandular-covered capsules are well adapted to dispersal by animals (epizoochory), although sexual reproduction is uncommon (Howard 1993).

Ecology and Habitat: Twinflower is a prostrate creeping dwarf shrub that is found throughout Newfoundland and Labrador. Although most commonly found in forest and woodlands [27], it occasionally occurs in barrens [28–29] and peatlands. Because of its small size, twinflower rarely covers more than 10–15 % of the forest floor. It occurs in a wide variety of forest types, with highest frequency and abundance in the moist to somewhat wet balsam fire sites, with a more sporadic distribution in dry black spruce forests or wet alder swamps.

Edaphic Grid: See image [30]: the Edaphic Grid for Linnaea borealis.

Forest Types: Twinflower is most commonly encountered in the following forest types:

• Abietum dryopteretosum (Dryopteris-Balsam Fir Forest Subassociation)
• Abietum gaultherietosum (Gaultheria-Balsam-Fir Forest Subassociation)
• Abietum hylocomietosum (Hylocomium-Balsam Fir Forest Subassociation)
• Abietum rubetosum (Rubus-Balsam Fir Forest Subassociation)
• Abietum taxetosum (Taxus-Balsam Fir Forest Subassociation)

Twinflower can also be found sporadically in a wide range of forest, barren, and peatland habitats and therefore is rarely used as an indicator species.

Succession: Twinflower can occur in a large range of light regimes, from shaded forest understories to open sunlight on cutovers and burns. Its occurrence on burns appears to be primarily from seed, since the shallow root system cannot withstand fire. This may explain its low abundance or absence from most of the forest associations of fire origin. It spreads primarily through vegetative reproduction by stolons. The seed does not persist in soil seed banks. Although rarely abundant, it appears to be able to persist on a wide range of soil conditions, light regimes, and stand ages.

Distribution: The species, Linnaea borealis, has a circumboreal distribution, occurring in subarctic, boreal, and montane forests. In North America, this species is represented by subsp. longifolia, which has a range that includes all provinces in Canada, as well as most of western, north-central, and northeastern United States. With the taxon L. borealis subsp. americana now included within subsp. longifolia, the latter's range has been expanded to include the transcontinental ranges of the combined subspecies.

Similar Species: The twin nodding pink flowers of Linnaea borealis are distinct enough not to be confused with other species. There are a few trailing herbaceous plants with opposite leaves that may initially be mistaken for twinflower, but a closer look at the flowers will quickly differentiate these species. The trailing plant most similar to twinflower is the two-eyed berry, Mitchella repens L., which is commonly called partridge berry in mainland Canada. This plant has opposite, ovate, cordate, or orbicular dark green leaves with entire margins and pale markings along the midrib and sometimes lateral veins; the fragrant flowers are terminal on prostrate stems and have 2 white, 4-lobed corollas, very hairy inside, attached to a single ovary (formed by 2 connate ovaries). The bright red berry-like drupes are small and bear 2 sets of persistent calyx lobes, hence the common name "two-eyed berry." Mitchella repens is rare in Newfoundland, absent from Labrador, and most frequently found in boggy margins in Newfoundland.