Senna artemisioides

Senna artemisioides, commonly known as silver cassia, is a species of flowering plant in the family Fabaceae and is endemic to Australia, where it is found in all mainland states and territories. It is a shrub with pinnate leaves, the leaflets variable, yellow flowers mostly occurring in winter, and linear pods. Six subspecies, four hybrid subspecies and two subspecies named but not yet described are accepted by the Australian Plant Census.

Description

Senna artemisioides is a shrub that typically grows to a height of Template:Cvt high and has pinnate leaves, the leaflets in pairs of one to eight, with a sessile gland between the lowest pair of leaflets. The leaves have tiny, triangular stipules, but fall off as the leaves mature. The flowers are yellow and borne in clusters of two to fifteen in leaf axils on a peduncle up to about Template:Cvt long, the petals Template:Cvt long, each flower on a pedicel up to about Template:Cvt long. The sepals are Template:Cvt long and greenish, the petals mostly Template:Cvt long. There are ten fertile stamens, the anthers Template:Cvt long. The fruit is a glabrous, linear pod Template:Cvt long, Template:Cvt wide.

Taxonomy and naming

This species was first described in 1825 by Augustin Pyramus de Candolle, who gave it the name Cassia artemisioides in his Prodromus Systematis Naturalis Regni Vegetabilis, from an unpublished description by Charles Gaudichaud-Beaupré of specimens collected by Charles Fraser. In 1989 Barbara Rae Randell raised the genus Senna in the Journal of the Adelaide Botanic Gardens, and transferred C. artemisioides to the new genus as S. artemisioides. The Latin specific epithet (artemisioides) means "resembling Artemisia", a different group of plants often known as wormwood.

Senna artemisioides is known as wormwood senna in the U.K., and as silver senna in the U.S., where it is an introduced species.

As of May 2023, the names of six subspecies of S. artemisioides are accepted by the Australian Plant Census:

• Senna artemisioides subsp. alicia Randell (N.T., S.A., Qld., N.S.W.) has cylindrical petioles, the leaves with up to three pairs of flat, silky-hairy, egg-shaped leaflets at least Template:Cvt wide and held vertically, exposing the lower surface. It has clusters of six to fifteen flowers with petals Template:Cvt long, with ten fertile stamens and pods about Template:Cvt long and Template:Cvt wide.
• Senna artemisioides subsp. filifolia, Randell commonly known as desert cassia, broom bush or punty bush, (W.A., N.T., S.A., Qld., N.S.W., Vic.) has cylindrical petioles that are more than Template:Cvt long, the leaves with up to four pairs of cylindrical leaflets Template:Cvt long. It has clusters of three to ten flowers with petals Template:Cvt long, with ten fertile stamens and pods Template:Cvt long and Template:Cvt wide. Flowering mostly occurs in winter.
• Senna artemisioides subsp. helmsii (Symon) Randell (previously known as Cassia helmsii Symon) – commonly known as blunt-leaved cassia or crinkled cassia (W.A., N.T., S.A., Qld., N.S.W.) has cylindrical petioles, the leaves with flat, woolly-hairy, egg-shaped leaflets at least Template:Cvt wide. It has clusters of four to ten flowers with petals Template:Cvt long, with ten fertile stamens and pods Template:Cvt long and Template:Cvt wide. Flowering occurs in winter.
• Senna artemisioides subsp. oligophylla (F.Muell.) Randell – blunt-leaved cassia (W.A., N.T., S.A., Qld., N.S.W.) has cylindrical petioles Template:Cvt long, the leaflets egg-shaped, Template:Cvt long and Template:Cvt wide. It has clusters of four to twelve flowers with petals Template:Cvt long, with ten fertile stamens and pods about Template:Cvt long and Template:Cvt wide. Flowering occurs in winter.
• Senna artemisioides subsp. quadrifolia Randell (N.T., S.A., Qld.) has cylindrical petioles Template:Cvt long, the leaflets narrowly elliptic, Template:Cvt long and Template:Cvt wide. It has clusters of four to eight flowers with petals Template:Cvt long, with ten fertile stamens and pods about Template:Cvt long and Template:Cvt wide. Flowering occurs in winter.
• Senna artemisioides subsp. zygophylla (Benth.) Randell (S.A., Qld., N.S.W., Vic.) has cylindrical petioles, the leaves with flat, glabrous, linear to elliptic leaflets more than five times as long as broad. It has clusters of three to five flowers with petals Template:Cvt long, with ten fertile stamens and pods Template:Cvt long and Template:Cvt wide. Flowering occurs in winter.

Four hybrid subspecies are also recognised by the Australian Plant Census:

• Senna artemisioides (Gaudich. ex DC.) Randell subsp. × artemisioides (previously known as Cassia circinnata Benth.) (W.A., N.T., S.A., Qld., N.S.W., Vic.) has cylindrical petioles that are less than Template:Cvt long, the leaves with three to eight pairs of cylindrical, hairy leaflets Template:Cvt long, the edges rolled upwards so that the upper surface is not visible. It has clusters of four to twelve flowers with petals Template:Cvt long, with ten fertile stamens and pods Template:Cvt long and Template:Cvt wide. Flowering mostly occurs in winter.
• Senna artemisioides subsp. × coriacea (Benth.) Randell (previously known as Cassia sturtii var. coriacea Benth.) (W.A., N.T., S.A., Qld., N.S.W., Vic.) has cylindrical petioles, the leaves flat with up to six pairs of linear to elliptic leaflets that are sparsely hairy and glaucous, the leaflet held horizontally. It has clusters of six to ten flowers with petals Template:Cvt long, with ten fertile stamens and pods Template:Cvt long and Template:Cvt wide. Flowering occurs in winter.
• Senna artemisioides subsp. × petiolaris Randell – woody cassia (W.A., N.T., S.A., Qld., N.S.W., Vic.) has petioles that are laterally compressed. It has clusters of two to ten flowers with petals Template:Cvt long, with ten fertile stamens and pods Template:Cvt long and Template:Cvt wide. Flowering usually occurs in winter.
• Senna artemisioides subsp. × sturtii Randell (previously known as Cassia sturtii R.Br..) (W.A., N.T., S.A., Qld., N.S.W.) has cylindrical petioles, the leaves flat with linear to elliptic leaflets that are densely silky- or woolly-hairy, but never glaucous. It has clusters of four to eight flowers with petals Template:Cvt long, with ten fertile stamens and pods Template:Cvt long and Template:Cvt wide. Flowering occurs in winter.

A further two subspecies are named, but as of May 2023, not formally described:

• Senna artemisioides subsp. James Range (P.L.Latz 18528) NT Herbarium (N.T.)
• Senna artemisioides subsp. Kuyunba (B.Pitts 113) NT Herbarium (N.T.)

Distribution

Silver cassia is endemic to Australia, occurring in all mainland states and territories, and has been introduced to California, the Canary Islands, Cyprus, India, Iraq, Spain and Zimbabwe.

Ecology

S. artemisioides is a recognized larval food plant for several species of butterfly and moth, including the small grass yellow, icilius blue, twig looper, blotched satin moth and bag-shelter moth. Seed dispersal is aided by ants which eat the arils.

Use in horticulture

Senna artemisioides adapts to a wide range of climatic conditions, but is susceptible to frost, especially when young. It prefers dry, well-drained sites with full sun. As an ornamental plant, it is propagated readily from seed, which should first be briefly immersed in boiling water.

This species has gained the Royal Horticultural Society's Award of Garden Merit.