Plant Profile: Hedysarum (Sweet-vetch)
- susan5383
- Aug 12
- 5 min read
By Patsy Cotterill
Among the several species that have flowered prolifically this year has been alpine hedysarum (Hedysarum americanum), making itself conspicuous in our gardens and natural areas. I assume that its congeners, its fellow members in the genus Hedysarum, have fared similarly well.
Hedysarum shares a number of characters typical of the pea family, Fabaceae: its flower is the characteristic pea type, consisting of a standard petal, two wing petals and a keel of two partly joined petals that enclose the stamens and pistil; its leaves are compound, pinnate with several pairs of leaflets and an extra leaflet at the end; and in leaf structure it most closely resembles the vetches (Vicia species) and milk-vetches (Astragalus).
Hedysarum species, however, have a couple of features that make them readily distinguishable from the vetches and other family members. The keel petals are large and end in a straight edge which looks as though they have been cut off, resembling an axe head (the technical term is truncate). Even more distinctive, instead of the typical pod that splits along two sides to release the seeds, the fruit consists of a number of bead-like segments, strung together in a chain like a necklace, each segment containing a single seed. They do not dehisce (burst open), but break apart at the constrictions between the “beads” to scatter the seeds. This type of fruit is called a loment. The leaflets are copiously pitted with tiny white or brown dots on the upper surface and are usually hairy beneath. The large taproots of the species are an important food for bears and small rodents, and those of alpine hedysarum have also been eaten by some northern Indigenous peoples (hence an alternative common name for it is licorice-root).
We have three species of Hedysarum in Alberta (but four taxa because northern hedysarum (H. boreale) has two subspecies). They are: alpine hedysarum (H. americanum, formerly H. alpinum subsp. americanum), northern hedysarum (H. boreale), with two subspecies: northern hedysarum (H. boreale subspecies boreale) and Mackenzie’s hedysarum (H. boreale subsp. mackenziei), and yellow hedysarum (H. sulphurescens). All are perennials.
Distribution of the species
Alpine hedysarum is the most widespread species of the four, occurring all across Canada except for P.E.I. It is found in the lowlands, for example in the North Saskatchewan River Valley in the vicinity of Edmonton, and in our native grasslands, although as its name suggests its populations are most concentrated in the mountains and foothills. Mackenzie’s hedysarum is an Arctic-alpine taxon with a wide trans-Canada distribution, except for some Maritime provinces, but northern hedysarum, i.e., the subspecies boreale, is western, confined to B.C., Alberta and Saskatchewan. The third species, yellow hedysarum, is definitely western, being confined to grasslands in the foothills and mountains of B.C. and Alberta.
Alpine hedysarum (H. americanum) is a tall, erect, and leafy plant. Its flowers are borne in long racemes, on flower stalks that arise from the axils of the upper leaves. They are narrow, 12-18 mm long and typically pink, especially so at higher altitudes, although in our gardens they can be pale to almost white. Characteristically, they hang down from the flower stalk on short stalks. The lobes of the cup-like calyx are not all the same shape. The 15-30 leaflets are broadly lanceolate, dark green above by mid-season and tipped by a tiny point; underneath there may be flat hairs, especially on the midrib, and the veins are prominent (a distinguishing feature from northern hedysarum, in which the veins are not obvious). The beads or loment segments number three or four in the chain and are 3.5-5 mm across with narrow, flat edges called wings.
Alpine hedysarum (Hedysarum americanum) growing on the east slope of the Cardinal Divide, Whitehorse Wildland Park, 2011-07-09. Photo: P. Cotterill.
Portion of a leaf of alpine hedysarum, slightly magnified, showing the conspicuous veins on the underside of the leaf and the single terminal "odd" leaflet, Edmonton garden, 2025-07-34. Photo: P. Cotterill.
Loments of alpine hedysarum showing segments with flat edges or wings. Photo: P. Cotterill.
Northern hedysarum (Hedysarum boreale) is shorter than H. alpinum and has several stems arising from a central base. The leaflets are fewer (9-13), and are narrower, linear-oblong to narrowly elliptic. They lack obvious veins beneath, and are more hairy, in contrast to H. alpinum. The flowers are broader, horizontal or erect on the stems, with equal, narrow calyx lobes, and they are usually of a more intense magenta-pink colour. The loments lack wings.
In subspecies boreale the leaves tend to be slightly bluish-green by mid-summer with hairy undersurfaces to the leaflets and no obvious veins. The flowers are borne in racemes up to 15 cm long. The four to six loment segments are covered in small flat hairs and have no wings.
Northern hedysarum (Hedysarum boreale subsp. boreale) in badlands, showing its sprawling, low-shrub habit, Dry Island Buffalo Jump Provincial Park, 2020-07-29. Photo: P. Cotterill.
Northern hedysarum in flower in Terwillegar Park, 2021-06-13. Photo: P. Cotterill.
Northern hedysarum, showing truncate keel petals and even, narrow-pointed calyx lobes, Terwillegar Park, 2021-06-12. Photo: P. Cotterill.
Mackenzie’s hedysarum (subspecies mackenziei) is more spreading in habit and can become prostrate. Its flowers are slightly larger than those of subspecies boreale and form a compact raceme about 2-4 cm long, shorter than that in subspecies boreale.
Yellow hedysarum (H. sulphurescens) is easily recognized when in flower by its yellow flowers, and when in fruit by its wider loments (6-8 mm) which are hairless and noticeably wing-margined. In vegetative condition it may be distinguished from alpine hedysarum by its fewer leaflets (9-17 compared to 15-30 in alpine hedysarum) which are somewhat broader. It differs from northern hedysarum in that the leaf undersides are conspicuously veined. Its distribution is confined to the mountain and foothills zones in the south. A good place to see it in flower is in Kananaskis Country in June and July.
I have encountered northern hedysarum (subsp. boreale) occasionally and always in the vicinity of rivers where it favours gravelly or clayey river terraces; for example, the North Saskatchewan River in Edmonton, the Red Deer River at Dry Island Buffalo Jump, and the Clearwater River at Cheddarville. There is a population of at least 20 plants in Terwillegar Park, not far from the river. I’d be interested to hear of further records of it.
I am most familiar with Mackenzie’s hedysarum in the alpine tundra at Cardinal Divide (elevation 1980 to > 2000 metres), where it can be so abundant as to turn the tundra a brilliant pink. (In 2017, however, there were several populations of a white-flowered version of it on the tundra. To my knowledge they have not been seen in subsequent years.) Hedysarums can thrive on rocky, infertile alpine soils because they “fix” atmospheric nitrogen in the soil into soluble salts that plant roots can absorb.
Nitrogen-fixing ability is characteristic of all pea family members, and alpine hedysarum is a good plant to have in your native garden. It would be interesting to try growing northern hedysarum from seed from local populations and see if they retain their brilliant colour!
