Plectranthus ernstii is a slow growing, semi-succulent perennial with swollen stems and a compact habit that resembles a bonsai, ideal for pot plants and rockeries.
Plectranthus ernstii is an erect to decumbent, pleasantly aromatic, semi-succulent herbaceous perennial up to 250 mm tall, branching from the base. The stems are greyish and articulated, i.e. they appear to be jointed, with segments and distinct nodes or joints, and are swollen, particularly at the base where they can reach up to 50 mm in diameter, and become brown and potato-like with age. These swollen, jointed stems make this Plectranthus distinct from all others.
The leaves are aromatic and have 6-13 mm long petioles. The leaf blade is semi-succulent, ovate to broadly ovate to somewhat triangular, 12-30 x 10-25 mm, with 3-5 pairs of pronounced rounded marginal teeth. The upper-surface is sparsely covered in short fine soft hairs (sparingly pubescent), and the under-surface is covered in minute hairs and pale to reddish brown gland-dots. The tip of the leaf ends in a blunt, rounded point (obtuse) and the base ends abruptly, as if cut (truncate).
The inflorescence is simple, 30-120 mm long, made up of small, two-lipped, pale bluish mauve to whitish flowers with darker markings, arranged in whorls of 2-6 flowers spaced 10-20 mm apart up the flowering stem, with the flowers at the tip opening first. The flowers don't have stalks (sessile). The calyx is 3 mm while in flower, enlarging to 6 mm as the fruits develop. The corolla tubes are 4-8 mm long, expanding to form a pouch-like base 4-5 mm deep, which narrows gradually to 2mm wide at the throat. The upper lip is 4-5 mm long and the lower lip 3-4 mm long and concave. The stamens are free and of two lengths, the upper pair is 1.5 mm long and the lower pair 3 mm long. Flowering time is late spring to autumn (October to May) with a peak in autumn (April), and flowers can appear sporadically throughout the year. The leaves turn-rose pink during autumn. The fruit is a 2 mm long, light brown nutlet, up to four per flower, attached inside the calyx.
Known from fewer than 10 locations that are potentially threatened by invading alien plants, Plectranthus ernstii is Near Threatened.
Distribution and habitat
Plectranthus ernstii occurs in Scarp Forest in humus-rich pockets of soil in rock crevices on exposed to semi-exposed, sheer quartzitic sandstone rock faces in river gorges from the Msikaba River in the northern part of the Eastern Cape to Oribi Gorge in southern KwaZulu-Natal. The soil where P. ernstii grows is mineral poor, and well drained. In these areas rain falls in summer and averages between 800-1500 mm per annum. Associated succulents include Crassula perforata (sosatie crassula or concertina plant), Gasteria croucheri (forest gasteria) and Aeollanthus parvifolius (rock sage).
Plectranthus ernstii growing in habitat
Plectranthus ernstii growing in habitat
Derivation of name and historical aspects
The name Plectranthus means spurflower, from the Greek plectron meaning spur and anthos meaning flower, and refers to the characteristics spur at the base of the corolla tube of Plectranthus fruticosus, the first Plectranthus in the genus described in 1788 by French botanist Charles Louis L'Héritier de Brutelle (1746 – 1800).
The specific epithet ernstii is named after the horticulturist Ernst van Jaarsveld (1953- ), collector and authority on this genus, who has made substantial contributions to our knowledge of the genus, currently working at Kirstenbosch National Botanical Garden.
Plectranthus ernstii was first collected in April 1977 on the edge of Oribi Gorge by Ernst van Jaarsveld and his wife Erma. This new species was named by Dr Leslie Codd (1908 – 1999) who was responsible for a major revision of the South African Plectranthus in Bothalia 11 of 1975 and later this work was updated for the Flora of Southern Africa in 1985. Dr. Codd was also responsible for naming ten new Plectranthus species and a new variety. These include P. hilliardiae, P. mutabilis, P. dolomiticus and P. xerophilus. Dr. Codd was the director of the National Botanical Research Institute which has now amalgamated with South African National Biodiversity Institute; from 1963 to 1973 where his main contributions were in the field of taxonomy. He was later awarded the South African Medal by the South African Association of Advanced Science in 1977 and also awarded the Medal for Botany in 1979.
Five cultivars have been introduced into cultivation:
‘Foster's Foley' (K431/89)
Originally collected by local botanist and farmer Tony Abbott, who collected it along the Mtamvuna River on the farm Foster's Foley.
From the Msikaba River; this form has more succulent leaves and stems with drooping habit.
This unique cultivar, which has a decumbent growth form and the longest stems of all the cultivars, was only recently introduced by Adam Harrower, a horticulturist at Kirstenbosch, and originates from the Mtentu River, south of Port Edward.
This is the original form from which the species was named. It was collected in 1977 in the Oribi Gorge in KwaZulu-Natal by Ernst van Jaarsveld. This is the form with a squat habit and compact swollen stems, and it makes the best bonsai of all the clones.
This cultivar has elongated drooping stems which are not as thickly swollen and articulated as ‘Oribi', with darker markings on the flowers.
Plectranthus ernstii grows on sheer rocky ledges out of reach of most predators. It has articulated, succulent stems, which store water and nutrients, and a very compact growth habit, enabling the plants to cope with a dry cliff-face habitat. The aromatic leaves possibly discourage insect predators.
The flowers are pollinated by insects. The main flowering of Plectranthus takes place at the end of the growing season, in autumn, when rain is usually abundant and the plants are at their peak, with enough water and energy reserves for flowering and seed development. The bonsai mint has a persistent and relatively short inflorescence. The inflorescence dries during late autumn and winter, when dispersal takes place the nutlets drop close to the parent, so remaining on the cliff to which it is adapted, so the seed is ready to germinate and grow during the spring and summer months.
Uses and cultural aspects
Plectranthus ernstii is a popular pot plant, and highly prized house plant, particularly in Europe and Japan. In South Africa this species is not used in traditional medicine or culture, although several other Plectranthus species are used. e.g. P. esculentus (African potato or wild potato) is used as a substitute for potato or sweet potato and infusions of P. hadiensis var. tomentosus (hairy spurflower) are used as sprinkling charms against evil spirits and as a fish poison.
Growing Plectranthus ernstii
Plectranthus ernstii is easy to grow, although relatively slow growing. It thrives as a pot plant and can be successfully grown in containers or shallow pans on windowsills. Shallow pots enhance its bonsai effect. This species is frost sensitive, it can withstand light frost but not moderate to severe frost. It can be grown outdoors in frost free, subtropical and coastal areas but in frost-prone areas it should be moved indoors during the coldest period of the year. It needs well-drained, humus enriched soil in a position where it will receive full sun, semi-shade or bright shade. Watering should be in summer, and care should be taken not to over water as this will cause the roots to rot. In the winter rainfall Western Cape it needs to be watered in summer and be planted in a very well-drained position, or grown in a container and taken under cover during winter.
Its attractive swollen stems and bonsai-look means that this species is most often grown in containers, but with its ornamental and aromatic foliage and attractive flowers it also makes a handsome addition to a mixed planting, and there is no reason why it can't also be used to good effect in the perennial border and rockeries, in frost-free areas. It should also do well in retaining walls and on steep rocky slopes.
Propagate by seed or by cuttings. Seed is best sown in spring or early summer in a well-drained soil mix containing two parts of sand and compost to one part of loam. Moisten the mixture, sow the seed evenly, and then cover lightly with a thin layer of sand to hold the seeds in place. Place in dappled shade in a warm place and keep damp. Germination occurs within approximately 21 days. Seedlings can be transplanted when they have developed two sets of true leaves.
Cuttings produce roots fairly quickly. Rooting is possible throughout the year, however the best time is in spring and early summer. Take softwood cuttings with 3 - 4 nodes. The rooting medium can consist of sand, perlite, vermiculite, peat or a mixture of any of these. Keep moist and shaded. The cuttings root fairly quickly in approximately 14 days under ideal conditions. Hygiene is of prime importance, when propagating from cuttings. Cuttings are prone to contamination by fungus this can be controlled by fungicide. Rooted cuttings can be transplanted after about a month and grown on in containers.
After planting out in the garden, plants must be well watered, at least twice a week until such a time as the plants are established. Feeding is also a very important aspect as it affects the flower and leaf quality. A balanced fertilizer can be applied every three months in the summer. Annual application of compost should be worked into the soil in late winter or early spring.
Prune P. ernstii frequently to maintain a neat ‘bonsai' shape, although Plectranthus will grow and flower without being pruned, it is an essential practice which improves the quality of plants and flowering. Pruning is a renewal process and occurs in the natural habitat. Dead, diseased and weak growth is removed and the natural form is improved. The optimum pruning time is late winter, once the plants have finished flowering.
Several Plectranthus species are prone to root rot caused by soil-borne eelworms (Nematodes), and if affected, the plants need to be re-established from cuttings and planted in new soil. The green plectranthus caterpillars, slugs and snails can be troublesome at times.
References and further reading
- Codd, L.E. 1982. Plectranthus ernstii. The Flowering Plants of Africa 47: 1855
- Codd, L.E. 1985 Lamiaceae in Flora of South Africa, ed. O.A. Leistner, Vol. 28 Part 4. Botanical Research Institute, Pretoria.
- Glen, H.F. & Germishuizen, G. (compilers) 2010 Botanical Exploration of Southern Africa, edition 2. Strelitzia 26. South African National Biodiversity Institute, Pretoria.
- Hutchings, A. Haxton Scott A., Lewis G., Cunningham Balfour A., 1996. Zulu Medicinal Plants An Inventory. University of Natal Press, Private Bag X01 Scottsville 3209
- Pooley, E. 1998. A Field Guide to Wild Flowers Kwazulu – Natal and the Eastern Regions. Natal Flora Publications Trust, Durban
- Raimondo, D., Von Staden, L., Victor, J.E., Helme, N.A., Turner, R.C., Kamundi, D.A. & Manyama, P.A. (eds) 2009. Red List of South African plants 2009. Strelitzia 25. South African National Biodiversity Institute, Pretoria.
- Van Jaarsveld, E. 1987. The Plectranthus Handbook. National Botanic Gardens, Cape Town.
- Van Jaarsveld, E. 2006. The Southern African Plectranthus and the art of turning shade to glade. Fernwood Press, Simon's Town, South Africa.
- Van Wyk, B. & Gericke, N. 2007. People's Plants a Guide to Useful Plants of Southern Africa. Briza Publications, Pretoria.
Kyle Smith & Alice Notten
Kirstenbosch National Botanical Garden
Walter Sisulu National Botanical Garden
Pretoria National Herbarium