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COSEWIC assessment and status report on the Bog Bird’s-foot Trefoil in Canada

COSEWIC Status Report
on the
Bog Bird’s-foot Trefoil
Lotus pinnatus
in Canada

Species Information

Name and Classification

Scientific name:
Lotus pinnatus Hooker
(from Hitchcock et al. 1961; Scoggan, 1978)
Hosackia bicolor Douglas ex Benth.
Hosackia pinnata (Hook.) Abrams
Lotus bicolor(Douglas ex Benth.) Frye & Rigg
Common names:
bog bird’s-foot trefoil, bog deer-vetch, meadow deer-vetch,meadow bird's-foot trefoil
Fabaceae; Legume family
Major plant group: 

During classical times, the Greek word ‘lotos’ was applied to four different plants, including Nymphaea lotus L., the sacred water lily used by the Egyptians as a food source (Kirkbride 1999). In Homer’s Odyssey, the hero Ulysses almost lost his crew when they became intoxicated by the honey-sweet fruit of Zizyphus lotus L. in the land of the Lotus-Eaters. Camarario (1588) was the first to apply the Latin name ‘lotus’ to a trefoil in his “Hortus Medicus”.

Considerable disagreement has existed among systematists about whether North American species of Lotus should be included in the genus Lotus or kept distinct in a separate genus Hosackia. As a result, specimens of North American species have been classified under both genera and may be found under either of these names in many herbaria (Zandstra and Grant 1968).

Lotus pinnatus was first described by Hooker in 1829. Later that same year, with a wealth of new material collected by David Douglas from western North America, Bentham re-described the species as Hosackia bicolor Dougl. In Bentham’s view, L. pinnatus had the habit, inflorescence and fruit of Lotus, but the position of the wings, capitate stigma, foliaceous stipules and pinnate, not ternate, leaves were sufficiently distinct to justify the placement of several North American species, formerly referred to Lotus, in Hosackia. Bentham established a new section, Microlotus Benth - for the New World species of Lotus with single-flowered inflorescences and 3- to 5-foliate leaves without stipules (Bentham 1929).

Subsequently, Torrey and Gray (1838), Gray (1863) and Watson (1876) placed all North American species in Hosackia, though each treatment varied as to the number of sections and the taxonomic position of certain species. Greene (1890) supported the broad and inclusive Linnaean concept of Lotus and transferred all the New World species back to Lotus.  Taubert (1894) and Brand (1898) emphasized different vegetative and floral characteristics in distinguishing Lotus from Hosackia, but both placed all North American species in Hosackia. With the exception of Abrams (1944) who maintained the Pacific northwest species in Hosackia, all subsequent floristic treatments (Pojar, 1999; Polhill, 1981, 1994; Isely, 1981; Hitchock et al. 1961, Callen 1959; Ottley 1923, 1944, 1951) have maintained the North American species in Lotus, sensu lato of Linnaeus, as there were no definitive characters to distinguish them generically from the European species of Lotus.


Bog bird’s-foot trefoil is a low-growing, perennial herb from a thick taproot and short rhizome with many erect to spreading stems, from 15-60 cm long (Figure 1, Pojar 1999). Ottley (1923) and Hitchcock et al. (1961) also provide a detailed description of L. pinnatus with illustrations. The lower stems are often spongy-thickened and whitish. The leaves are 4-8 cm long, alternate, stalked and pinnately compound with 5-9 elliptic, oblong or narrowly egg-shaped leaflets, each 1-2.5 cm long. The stipules are membranous, obovate and 3-10 mm long (Pojar 1999).

Figure 1: Illustration of Lotus pinnatus

Figure 1: Illustration of Lotus pinnatus.

Line drawing in Hitchcock et al. 1961 and Douglas et al. 1999.1

The inflorescence is a compact, stalked, axillary umbel of 3 to 12 pea-like flowers. The umbel stalks are bractless or with a single membranous bract. The corollas are from 10-15 mm long with a yellow banner and keel and creamy-white wings. The keel petals are fused along one edge that is elongated into a well-defined beak towards the outside of the inflorescence. The ten stamens are diadelphous with 9 stamens fused into a tube enclosing the ovary and a single free stamen opposite the banner petal. Five stamens, including the free one opposite the banner petal, have slender filaments and their anther is dorsofixed, that is, attached to the filament on the back near the base. They are shorter than, and alternate with, the other five stamens belonging to the fused group. The filament of the longer stamens is dilated below the anther, which is basifixed (attached to the filament at the base). The anther and dilated region of the five longer filaments are enclosed within the beak of the keel petals. Prior to anthesis, pollen is released into the beak of the keel petals (Kirkbride 1999).

The calyx is tubular and 4-8 mm long with triangular-lanceolate teeth that are all much shorter than the tube. The upper two calyx lobes are joined most of their length and are merely notched between them. The lower calyx lobes are awl-shaped. The linear seed pods range from 3-6 cm in length and 1.5-2 mm wide and contain 5-20 glabrous seeds (Pojar 1999). The seeds are cylindrical, glossy, dark brown to black, mottled with olive and about 1.5 mm long. For a technical description of the seed, see Arambarri (1999).

In the field, there are several species that superficially resemble L. pinnatusVicia species are often present in similar habitats, but the leaves are smaller in size than those of L. pinnatus and the terminal leaflet in Vicia spp. (vetches) is represented by a tendril. Lotus formosissimus (seaside bird’s-foot trefoil) looks very similar to L. pinnatus except the wing petals of L. formissimus are pinkish-purple, while those of L. pinnatus are cream-colored. Though a trifoliate bract usually subtends the flowers of L. formosissimus, bracts were sometimes absent in field specimens or only a unifoliate bract was present (Ryan and Douglas 1994). Although L. pinnatus and L. formosissimus could be confused if the plants are immature or not in flower, the two species do not overlap in their distribution in British Columbia. Listed as Endangered by COSEWIC, Lotus formosissimus is known only from the Victoria area and nearby islands where it occurs in various xeric habitats, ranging from open exposed grass-dominated meadows to exposed steep rocky sites with Quercus garryana (Garry oak) (Ryan and Douglas, 1994). Lotus pinnatus is found only in the Nanaimo area on Vancouver Island, where it occurs in moist soil on exposed, coastal lowland areas. Lotus corniculatus (bird's-foot trefoil) occasionally grows in wet places, is an introduced species and is usually found in drier, disturbed sites. Also, L. corniculatus has completely yellow flowers and its leaflets are smaller and more blunt than the other trefoils (Guard 1995).


1 This illustration has been reproduced with permission from University of Washington Press.