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 small, woody shrub with silver-green leaves and yellow flowers.

Description
Senna artemisioides is a small shrub 1-3 m high with grey-green or silvery mostly pinnate leaves, leaflets in pairs of one to eight, variable, 2-4 cm long, equal or increasing in size from the leaf base and sessile. The flowers are borne in clusters in leaf axils on a peduncle about 15 mm long, 1.5 cm in diameter, petals yellow, 4-10 mm long, pedicel about 10 mm long and the sepals 6-8 mm long and greenish. Flowering occurs from April to November and the fruit is a linear-shaped pod 5-10 cm long, 8-15 mm wide, smooth, flattened and straight.

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de Candolle, who gave it the name Cassia artemisioides in his Prodromus Systematis Naturalis Regni Vegetabilis, from an unpublished description by Charles Gaudichaud-Beaupré of specimens collected by Charles Fraser.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 2 mm wide and held verticlly, exposing the lower surface.
 * Senna artemisioides subsp. filifolia Randell (W.A., N.T., S.A., Qld., N.S.W., Vic.) has cylindrical petioles that are more than 15 mm long, the leaves with up to four pairs of cylindrical leaflets 20–40 mm long.
 * Senna artemisioides subsp. helmsii (Symon) Randell (previously known as Cassia helmsii Symon ) – blunt-leaved 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 2 mm wide.
 * Senna artemisioides subsp. oligophylla (F.Muell.) Randell – blunt-leaved cassia (W.A., N.T., S.A., Qld., N.S.W.) has cylindrical petioles 5–15 mm long, the leaflets egg-shaped, 10–40 mm long and 10–20 mm wide.
 * Senna artemisioides subsp. quadrifolia Randell (N.T., S.A., Qld.) has cylindrical petioles 15–25 mm long, the leaflets narrowly elliptic, 20–50 mm long and 2–10 mm wide.
 * 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.

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 15 mm long, the leaves with three to eight pairs of cylindrical, hairy leaflets 15–25 mm long, the edges rolled upwrds so that the upper surface is not visible.
 * 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.
 * Senna artemisioides subsp. × petiolaris Randell – woody cassia (W.A., N.T., S.A., Qld., N.S.W., Vic.) has petioles that are laterally compressed.
 * 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.

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.