Swainsona formosa

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iSturt's desert pea

Sturt's desert pea, at Melbourne Zoo

Scientific classification

Kingdom:	Plantae
Division:	Magnoliophyta
Class:	Magnoliopsida
Order:	Fabales
Family:	Fabaceae
Subfamily:	Faboideae
Tribe:	Galegeae
Genus:	Swainsona
Species:	S. formosa

Binomial name

Swainsona formosa
(G. Don) Joy Thomps.

Sturt's desert pea is an Australian plant in the genus Swainsona, famous for its distinctive blood-red leaf-like flowers, each with a bulbous black centre, or "boss". It is one of Australia's best known wildflowers. It is native to the arid regions of central and north-western Australia, and its range extends into all mainland Australian states with the exception of Victoria [1].

Specimens of Sturt's Desert Pea were first collected by William Dampier on 1st September 1699. These specimens are today in the Sherardian herbarium at Oxford University [2].

The taxonomy of Sturt's Desert Pea has been changed on a number of occasions. It was initially treated in the 17th Century in the genus Clianthus as Clianthus dampieri [3], and later became more widely known as Clianthus formosus (formosus is Latin for "beautiful"). However it was later reclassified under the genus Swainsona as Swainsona formosa, the name by which it is officially known today.

A further reclassification to Willdampia formosa was proposed in the publication Western Australian Naturalist in 1999; however this proposal was rejected by the scientific community in 2000 [4].

The common name honours Charles Sturt, who recorded seeing large quantities of the flowers while exploring central Australia in 1844; the second version of the scientific name honours the naturalist William Swainson, and the third (rejected) version of the scientific name was intended to honour the explorer William Dampier.

19th century illustration

Sturt's Desert Pea is a member of Fabaceae, subfamily Faboideae, however its flowers are so different from its relations that it is almost unrecognisable as a member of the pea family [5]. The flowers are around 9 centimetres in length and grow in clusters of around half a dozen on thick vertical stalks, which spring up every 10-15 centimetres along the prostrate stems, which may be 1 to 1.5 metres in length. The plant blooms from spring to summer, particularly after rain. There is a natural pure white form, as well as hybridised varieties which can have flowers ranging from red to pink, as well as yellow, with or without the central boss [6]. Approximately 15 silky grey-green leaves rise from each prostrate branch; both branches and leaves are covered with soft hair-like filaments. The fruit is a legume, about 5 centimetres long, and yields several flat, kidney-shaped seeds at maturity.

Most forms of the plant are low-growing or prostrate, however in the Pilbara region of north-western Australia varieties growing as tall as 2 metres have been noted [7].

[edit] Life cycle

A short-lived perennial, it is often treated as an annual when domestically grown. However if the roots are left undisturbed, flowering may resume in the next season.

It is well adapted to life as a desert plant. The small seeds have a long viability, and can germinate after many years. The hard seed coat, which protects the seed from harsh arid environments until the next rainfall, inhibits germination in normal domestic environments. Growers can overcome this by either nicking the seed coat away from the 'eye' of the seed, or by rubbing the seed gently between pieces of sandpaper, or by replicating a desert rainstorm and placing the seed in very warm (but not boiling) water and leaving it to soak overnight. Boiling water should not be used as beneficial bacteria on the seed coat would be destroyed [8].

Once germinated, seedlings quickly establish a deep tap root, vital for desert survival. This means that if domestically grown, they should either be planted in their intended final location, transplanted as soon as possible after germination, or grafted as a seedling on to a different root such as the bladder senna, Colutea Arborescens. They do not tolerate disturbance of their roots but, once established in well-drained soil, require little and infrequent watering, and can withstand extreme heat and sunshine, as well as light frosts.

[edit] Use as emblem

Sturt's Desert Pea (described as Clianthus formosus) was adopted as the floral emblem of the state of South Australia on 23 November 1961. It is not endangered, but it is illegal to collect specimens of the plant from Crown land without a permit. The plants must not be collected from private land without the written consent of the land owner.

Sturt's Desert Pea has appeared in several releases of sets of six Australian postage stamps depicting Australian floral emblems (issued in 1968, 1971 and 1975) [9].