Thorncroftia longiflora
N.E.Br.

Family: Lamiaceae
Common names:
None found

Shrub in flower


Thorncroftia longiflora is both rare and remarkable. It's a versatile and attractive semi-succulent shrub which makes an excellent and showy garden subject, well suited for hot, dry and sunny gardens.

Description
Thorncroftia longiflora is technically a succulent due to its semi-succulent stems and leaves, however, it behaves and grows much like a herbaceous shrub forming a well-branched bush up to 80 cm high. It forms numerous fleshy stems which arise from a woody rootstock. These stems eventually become a grey-brown and semi-woody as they mature and are covered in fine hairs.

The opposite green semi-succulent leaves are elliptic to obovate in shape, 1020 mm long, mildly toothed in the upper two-thirds, covered with short hairs and attached to the stem by short stalks.

The pale pink flowers are 3040 mm long and tubular with petals flaring at the mouth with the lowermost bearing dark maroon spots or lines. The flowers are carried in dense terminal spikes forming an impressive display during autumn from March to May.

Close up of flowers

The small round dark brown or black seeds are nested in the base of the calyx, visible once the corolla tube has fallen off. They take a couple of months to ripen. Under warm and sunny conditions this species will grow fairly quickly becoming a well-shaped shrub in one year, and they will last many years in the open ground.

Conservation status
According to the latest assessment by SANBI's Threatened Species Programme, this species is recognised as Rare.

Distribution and habitat
Thorncroftia longiflora has been well known for a long time from its type locality around Barberton in Mpumalanga Province. With subsequent fieldwork, new populations have been discovered in Swaziland and as far south as northern KwaZulu-Natal. It is usually associated with rocky outcrops in grasslands, often growing in humus-filled crevices.

Derivation of name and historical aspects
The genus Thorncroftia is named in honour of George Thorncroft (18571934) who collected extensively botanically in the Barberton area. The specific name longiflora refers to the long corolla tube of the flowers. There are a total of 5 species in the genus Thorncroftia and all are endemic to South Africa. They were originally thought to be members of the closely-related genus Plectranthus. T.succulenta is also cultivated in gardens.

Ecology
Because of its semi-succulent nature, this species is well adapted to growing in shallow soils and rock crevices. With the remarkable length of the floral tube, it is suspected that pollination is carried out by a unique pollinator the long-proboscid fly Prosoeca ganglbaueri with a proboscis that matches the length of the tube. This unique pollinator requirement may account for its rarity in the wild.

Uses and cultural aspects
There are no known medicinal or cultural uses for this plant.

Bush in flower

Growing Thorncroftia longiflora

This is possibly one of the easiest and hardiest of all herbaceous South African garden plants to grow. It will survive under a vast spectrum of soil types and climatic conditions and is even frost-hardy. It prefers sunny positions, particularly afternoon sun and thrives in subtropical temperatures which can be simulated by planting it amongst rocks or boulders which retain heat. It prefers wet summers and dry winters but will tolerate water during winter and will survive summer with minimal water. It does not require rich composted soils but will thrive if compost or true fertilizers are provided.

Due to their fragrant sage leaves, they naturally have very few pests and seem to be resilient to fungal and other pathogens. In their natural habitat is a weevil which attacks all species of Thorncroftia, called Apion rectangulum which causes thickened swellings in the stems. This is seldom seen in cultivation, however.

Propagation is easiest by cuttings which can be soft or hardwood, and rooted without the need of rooting hormone at any time of the year, but spring and summer are best. The rooting medium should preferably be sandy and well-drained but the cuttings should strike in any soil medium. Providing light shade and plastic covering to increase air humidity will speed up the rooting. One could also root them in water.

Seed should be collected during July or August and kept until September/October (spring) for sowing (in the southern hemisphere). They should be mixed with fine dry sand and sprinkled evenly over a suitable germination medium in a tray. Watering with a fungicide is advisable but not essential. Depending on the ambient temperatures they should germinate within 24 weeks. Seedlings grow vigorously and will quickly outgrow cuttings of the same age.

References and further reading

  • Codd, L.E.W. 1985. Lamiaceae. Flora of southern Africa 28, 4:237239.
  • Potgieter, C.J. & Edwards, T.J. The occurrence of long, narrow corolla tubes in Southern African Lamiaceae. Systematics and Geography of Plants 71, 2, Plant Systematics and Phytogeography for the Understanding of African Biodiversity (2001), pp. 493502. National Botanic Garden of Belgium .
  • Leistner, O.A. (ed.). 2000. Seed plants of southern Africa: families and genera. Strelitzia 10. National Botanical Institute, Pretoria.
  • Red list of South African plants. http://redlist.sanbi.org/species.php?species=1663-1

 

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