This attractive, dainty South African perennial will do well in temperate gardens around the world. Easy to grow and long-flowering this is an ideal plant for the front of a mixed border, a rocky bank or a container.

The name Diascia (pronounced Dye-ass-ee-a or Dye-ass-kee-a) is derived from the Greek di (two) and askos (a sac), referring to two pouches/sacs found on the flowers of D. bergiana, the species first named in this genus. There are more than 50 species of Diascia, widespread throughout South Africa and many have horticultural potential. The genus may be divided roughly into annual species which tend to occur in the western, winter rainfall parts of the country and perennial species from the summer rainfall, eastern areas. These plants thrive in Highveld gardens provided they are given enough water. They also do well in the mediterranean climate of the Western Cape, provided they receive adequate water in summer. They are hardy enough to grow out of doors in milder areas of Britain too, where several hybrids have been bred. Diascias are some of South Africa's most attractive soft perennials and annuals. Diascia interregima is another species frequently cultivated in gardens.

The natural distribution of Diascia rigescens extends from KwaZulu-Natal - especially in the Harding and Mount Ayliff districts to the coastal regions of the Eastern Cape - especially in the Keiskammahoek, Stutterheim and Hogsback districts. In these areas it is usually found in damp places on grassy mountain slopes, often among rocks or in full sun at forest margins. It is also found in Lesotho and the Free State. D. rigescens shows some variation in form in its various localities. The most robust plants occur in the Mount Kemp and Stutterheim districts.

D. rigescens is a showy perennial herb that grows up to 600 mm tall. Several stiff stems that are four-angled grow from the crown. The specific name "rigescens" is Latin for 'rather stiff' and refers to the stem. This plant may also sprawl down banks or over rocks. The upright stems are less branched than those that are sprawling. The leaves are simple, sessile, thick-textured, heart-shaped, opposite and have sharp little serrations along the margins. The leaves are mostly crowded at the base of the plant, becoming more sparse towards the top.

Flowers are carried in loose clusters and vary from pink to rose with a deep rose throat. They are especially adapted to attract oil-collecting bees. The main flowering time is summer, however, flowers may appear throughout the year. The flowers last well in a vase, but become pale if they open indoors. The fruit is a dry capsule, which contains many small, brown seeds, which are approximately 1,5 mm long.

Diascias have a fascinating co-evolutionary relationship with oil-collecting bees which pollinate their flowers. The flowers of most of the species in the Racemosae section of Diascia (to which D. rigescens belongs) have translucent "windows" at the base of the upper corolla lip. These attract the bees which are seeking the oil secreted in the two corolla spurs. Modified hairs on the bee absorb the oil while the flower deposits pollen on the feeding bee who then flies away to visit another flower carrying a load of pollen.

Growing Diascia rigescens

The twinspur can be propagated by seed. Although seed may not set in cultivation, if it is available it may be collected when ripe and sown on a fine seedling mix. Cover seeds lightly to ensure that they do not dry out. This plant also grows easily from cuttings. In spring you can root the softwood cuttings; while in summer, semi hardwood cuttings will do better.

The mature plant prefers full sun and will grow well in fertile, moist but well-drained soil. This plant grows out of doors in mild areas in Europe. Established plants can be clipped back if they get straggly during the season. Pruning will promote new growth and a fresh crop of flowers.

The twinspur's sprawling habit, heart shaped leaves and dense flowering makes you sit up and take notice of this beautiful shrubby perennial. It looks great trailing down a rock wall or in the sunny spot of the Highveld garden.

References

• Batten A., 1986, Flowers of Southern Africa, Sandton: Frandsen Publishers.
• Benham S. 1987. Diascia - A survey of the species in cultivation. The Plantsman 9(1)1-17.
• Garbutt S. 1994.The up-and-coming Diascia. The Garden: Journal of the Royal Horticultural Society. 119:18-21.
• Grey-Wilson C. 1986. Diascia rigescens. Kew Magazine 3:99-102.
• Steiner Kim E. 1990. The Diascia (Scrophulariaceae) window: an orientation cue for oil-collecting bees. Botanical Journal of the Linnean Society 102:175-195.
• Van Jaarsveld, E., 2000, Wonderful Waterwise Gardening, Cape Town: Tafelberg Publishers.

Mhlonishwa D. Dlamini & Alice Aubrey
Walter Sisulu National Botanical Gardens
with additions by Yvonne Reynolds
May 2002