Diascia (Link & Otto, 1820), commonly known as twinspur, is a genus of approximately 70 species of herbaceous annual and perennial flowering plants in the family Scrophulariaceae (order Lamiales), native to southern Africa. The genus was described by Johann Heinrich Friedrich Link and Christoph Friedrich Otto in 1820, with Diascia bergiana as the type species, and is placed within the tribe Hemimerideae.
Most species are low-growing, straggling plants that reach no more than 30–45 cm in height, though Diascia rigescens can reach 60 cm and D. personata up to 120 cm. Flowers are carried in loose terminal racemes with a five-lobed corolla, typically pink or rose-coloured in the perennial species most commonly grown in cultivation. Some species spread by stolons; others produce multiple lax stems from a central crown. A distinctive feature of the genus is a pair of downward-pointing spurs at the back of each flower. These spurs contain oil, which is collected by bees of the genus Rediviva — notably R. longimanus — that appear to have coevolved with the plants, possessing unusually long forelegs adapted for reaching into the spurs.
The genus is concentrated in southern Africa: perennial species are found mainly in the summer-rainfall KwaZulu-Natal Drakensberg, while around 50 species — mostly annuals — occur in the winter-rainfall Western Cape and Namaqualand. In cultivation, Diascia hybrids and cultivars have become popular garden bedding plants valued for their prolific, long-lasting flower display.
Etymology
The name Diascia derives from the Greek di (two) and askos (bag, pouch, or sack). Somewhat surprisingly, the name does not refer to the two characteristic spurs, but rather to two translucent pouches or "windows" found in the upper part of the corolla of the original type specimen, Diascia bergiana. These windows are thought to help oil-collecting bees orient themselves correctly within the flower when gathering oil from the spurs.
Distribution
Diascia is native to southern Africa, including South Africa, Lesotho, and neighbouring areas. The distribution divides broadly along rainfall seasonality: perennial species are concentrated in the summer-rainfall KwaZulu-Natal Drakensberg highlands, while approximately 50 species — predominantly annuals — occur in the winter-rainfall Western Cape and Namaqualand regions.
Ecology
Diascia flowers are pollinated by oil-collecting bees of the genus Rediviva. The spurs at the back of the flower contain fatty oils rather than nectar; bees of this genus, particularly R. longimanus, have evolved unusually elongated forelegs that allow them to collect oil from deep within the spurs. This relationship represents a notable example of plant–pollinator coevolution in southern Africa.
Cultivation
Diascia cultivars and hybrids — largely derived from crossing several South African species — have become widely grown as bedding and container plants in temperate gardens. They are valued for their colourful, floriferous habit and relative ease of cultivation. Most cultivated forms are treated as half-hardy annuals or short-lived perennials.
