The genus Gastrolobium consists of over 100 species*, all but two of which are found only in Western Australia. Recent botanical revision has seen all members of the genus Brachysema transferred to Gastrolobium. They are generally small shrubs ranging from prostrate in habit to about 2 metres. Many species of Gastrolobium are known to be poisonous and some have been associated with stock poisoning. Because of this, few gastrolobiums are cultivated, the exceptions being a number of species previously in Brachysema.
Gastrolobium sericeum is reasonably common in cultivation, often under its older name of Brachysema sericea. It is a variable species in habit, but is usually a low, spreading shrub up to half a metre in height and about a metre wide. Leaves vary from elliptical to round in shape and are about 50 mm long.
The typical “pea” flowers consist of 4 petals; the “standard”, the “keel” and two “wings”, as shown in the diagram. The flowers of G.sericeum differ from the typical shape in having a much shorter standard than is typical. The flowers may be cream, red or almost black, about 25 mm long and occur in the leaf axils in winter and spring.
G.sericeum appears to be reasonably adaptable to a wide range of soils, provided they are well drained, in sun or semi shade. It will grow in humid areas of the Australian east coast where many other plants from Western Australian do not thrive, however it is better suited to less humid areas. Its flowers produce nectar attracting honey-eating birds and it tolerates at least moderate frosts.
Propagation is easy from seed following pre-treatment to break the physical dormancy provided by the impervious seed coat. Pre-treatment can be carried out by abrasion or by the use of boiling water (further details can be found in under the “Propagation” tab of the “Pea Family” page). The seed retains viability for many years. Cuttings strike well using firm, current season’s growth.
* Chandler G T, Crisp M D, Cayzer L W and Bayer R J (2002), Monograph of Gastrolobium (Fabaceae: Mirbelieae). Australian Systematic Botany 15 (619-739).
Black-flowered form of Gastrolobium sericeum
Photo: Brian Walters