Diascia mollis Hilliard & B.L.Burtt
Common names: trailing Transkei twinspur (Eng.); horingkies (Afr.)
Sweet, rewarding, ground covering herbaceous perennial for both light-sunny and semi-shaded areas, adding bright colour to the garden and hanging baskets with pretty, pink, twin-spurred flowers, while being irresistible to bees.
Evergreen, fast-growing, low-spreading plant, 200 × 50 mm, with multiple trailing stems. Lush, heart-shaped leaves with serrated edges are arranged in opposite pairs along the square, fleshy stems. Stem tips present sprays of small, rose-pink flowers, each with a striking yellow splotch inside. Flowering happens from spring, throughout summer to early autumn. Flowers are semi-tubular and 2-lipped with 4 small upper petals and a single large, flat and rounded lower petal displaying speckled nectar guide markings. The 4 stamens, attached inside the corolla mouth, grow in 2 unequal pairs, twisted around each other. The short tube behind the cup-shaped corolla has a pair of small protruding pouches (spurs) that are a distinguishing feature of Diascia. The 5-lobed green calyx may enlarge slightly in the fruiting stage. Seed capsules contain urn-shaped seeds which are slightly ridged.
Least Concern (LC)
Distribution and habitat
Diascia mollis commonly occurs from Libode and Port St. John’s on the Eastern Cape coast to Hogsback, in the mountains almost due north of King William’s Town. It grows on cliffs close to the sea, in grassland, and in forest. It is found in damp and somewhat sheltered places among the coarse grasses and bushes that are found marginal to forest patches. It is sensitive to wind, drought and frost.
Derivation of name and historical aspects
There are 70 species of Diascia endemic to South Africa. 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 occurring in the eastern, summer rainfall areas.
The name Diascia is derived from the Greek words, di, meaning ‘two’, and askion, ‘a wine skin or bladder’, and refers not actually to the two spurs on the back of the flowers but rather to the pair of translucent yellow sacs or windows in the corolla tube. However, the double spur is a noticeable diagnostic characteristic of many Diascia species and is why the plant is commonly referred to as ‘twinspur’. The specific epithet mollis meaning ‘soft and flexible’, refers to this plants lush growth habit and trailing stems.
Diascia mollis is similar to other species within its geographical distribution range, such as D. racemulosa, which is distinguished by its glandular-hairy calyx, invaginated corolla tube, palate without dark sessile glands and filaments (3 mm long) which project straight forward (not erect, the anthers partially hidden in the window). Diascia cordata also has the corolla tube invaginated and the filaments directed forwards, but its perennial habit, glabrous calyx, longer spurs (8–9 mm, not 3–5 mm) and shorter filaments (2.25 mm, not 3 mm) easily distinguish it apart from D. mollis which has a 3 mm long, invaginated corolla tube, with two lateral spurs (3–5 mm long) curved downwards, with a patch of dark sessile glands within near the tips and without sessile glands on the palate. Stamens projecting forward, filaments (3 mm long).
The Scrophulariaceae is the large, snapdragon family which consists of about 5 100 species worldwide. In South Africa there are 79 genera of the Scrophulariaceae.
Diascia species have a fascinating symbiotic relationship with the specialized genus, Rediviva, oil-collecting bees, which pollinate their flowers. Diascia mollis flowers have a translucent, thin layer of cells serving as ‘windows’ in the base of the upper corolla lip. These attract the female bees in particular, who are seeking the fatty oil produced by the glands, called elaiophores, found in the two corolla spurs. The forelegs of these bees have evolved to almost three times the normal length, to reach deeply into the flower spurs to harvest the oil. Their legs are made up of a complex system of modified hairs adapted to absorbing the fatty secretions. While the female bee is collecting oil, the anthers deposit pollen on her body and so pollination is carried out as the bee flies from one flower to another. Interestingly species of Diascia which grow together in the same geographical area have also evolved individually different placement of their stamens so that their pollen is deposited on separate parts of the bee's body, by this means avoiding cross pollination between different species.
The earliest Diascia cultivars were introduced to ornamental horticulture in the 1980s by Hector Harrison, a keen gardener residing in England. Diascia species and their cultivars are still very popular as colourful and dainty herbaceous perennials in hanging baskets, pots, rockeries and flowerbeds.
Growing Diascia mollis
This twinspur's rambling habit and dense flowering makes it ideal for brightening up those dull sections of your garden with a splash of bright colour and it has a long flowering period, which ranges from spring into winter (October to June).
Diascia mollis thrives in lightly sunny or semi-shaded positions. In the wild, it tends to grow along streams or in the moist shade of rocks or large grass tussocks. Plant it in a well-drained, nutrient-rich, loam soil and keep moist by watering regularly, especially in summer.
Although a very rewarding perennial, it is short-lived and will need to be replaced in the garden approximately every 2 years. The plants are semi-dormant in winter and are often lost if the soil is not sufficiently well drained.
Unless environmentally stressed, Diascia mollis is generally in vigorous growth and relatively pest free. Snails and slugs can damage small plants. Red spider mite, white fly and mealy bug can be troublesome pests which should be treated with a suitable insecticide, either organic or chemical, as soon as noticed.
Diascias should be regularly cut back, especially when they get straggly. Trimming will promote new growth and fresh crops of flowers. Established plants can regularly receive a good slow-release fertilizer to keep them vigorous and healthy.
Diascia mollis can be propagated by seed sown in spring. Lightly broadcast the fine seeds on the surface of the soil and keep protected from wind, in light shade, until the true leaf stage. Pot up and grow on until planting size. Diascia mollis is most easily propagated vegetatively from 50-60 mm softwood stem cuttings done in spring, which root quickly in a sandy or light potting mix without any special treatment such as underheating or rooting hormone. Plant rooted cuttings into small pots in a humus-rich, friable and well-drained potting mix. Sometimes stems will root where they touch the soil and are easily removed and potted up.
- Burtt, B.L. & Hilliard, O.M. 1984. A revision of Diascia section Racemosa. J. S. African Bot. 50: 269–340.
- Foden, W. & Potter, L. 2005. Diascia mollis Hilliard & B. L. Burtt. National Assessment: Red List of South African Plants version 2020.1. Accessed on 2021/05/10.
- Honig, M. 2014. Indigenous plant palettes. Quiver Tree Publications, Cape Town.
- Kearney, A. 2008. Indigenous beginnings. Metz Press, Cape Town.
- McQuillan, M. 2009. Diascia vigilis Hilliard & B.L.Burtt. (Scrophulariaceae). PlantZAfrica. Online. http://pza.sanbi.org/diascia-vigilis.
- Vogel, S. 1984. Diascia flower and its bee—an oil-based symbiosis in southern Africa. Acta Botanica Neerlandica 33: 509–518.
- World Flora Online. Diascia mollis Hilliard & B.L.Burtt. Online. http://www.worldfloraonline.org/taxon/wfo-0000644727. Accessed on 10 May 2021.
- Whitehead, V.B. & Anthony, N.C. 1984. The bee, Rediviva longimanum Michener (Apoidea: Melittidae), collecting pollen and oil from Diascia longicornis (Thunb.) Druce (Scrophulariaceae). South African Journal of Science 80: 286.
Kirstenbosch National Botanical Garden
Acknowledgements: Images by Monique McQuillan and Alice Notten.
Plant Type: Bi/Annual, Ground Cover, Perennial
SA Distribution: Eastern Cape
Soil type: Loam
Flowering season: Spring, Early Summer, Late Summer, Autumn
Flower colour: Pink
Aspect: Shade, Morning Sun (Semi Shade), Afternoon Sun (Semi Shade)
Gardening skill: Easy