Agathosma orbicularis (Thunb.) Bartl. & H.L.Wendl.
Common names: Caledon buchu (Eng.); Caledon-boegoe (Afr.)
A vigorous, low-growing, lilac-flowering buchu, teetering on the brink of extinction.
This is a low-growing, white-stemmed, aromatic, bushy shrublet with small, orbicular leaves. Lilac flowers are presented in lax, terminal clusters from midwinter to spring (July to September). Fruits are 3-chambered.
According to the Red List of South African plants website, the conservation status of this species is Critically Endangered (CR).
Agathosma orbicularis is known, since its discovery, from a small area to the north of the town of Caledon. This species was not seen at the one known location on the Caledon Swartberg since 1932, and was thought extinct because of alien plant invasion, urban expansion and inappropriate fire management. Another population monitored by a local conservation authority in the 1970s, went extinct by 1980. However, a small subpopulation of 150–300 mature individuals was rediscovered in the same area, in August 2008. This subpopulation occurs in a 0.1 ha road reserve and is declining because of habitat degradation, as a result of invasive alien plants and road verge clearing. It is also threatened by potential future widening and road maintenance of the N2 highway, and the possible mining of manganese.
Distribution and habitat
This critically endangered shrublet inhabits lower ferricrete slopes from Caledon to the Langeberg Mountains in the Western Cape, in Greyton Shale fynbos.
Greyton Shale Fynbos can be found south of Riviersonderend and the Caledon Swartberg Mountains, on higher altitude shales between the Theewaterskloof Dam and Stormsrivier, as well as the Bergfontein and Spitskop hills to the north of Caledon. The altitude ranges from 220–550 m. The moderately tall and dense shrubland vegetation consist mainly of proteoid, asteraceaous and patches of graminoid fynbos. Few succulents are present. This fynbos type can be considered vulnerable as only 1% is statutorily conserved in the Riviersonderend Nature Reserve. A similarly named private conservation area provide an extra 6% protection of the area.
In general, shale fynbos comprises about 5% of all fynbos and represents the 4th largest area of the Biome vegetation. This fynbos-type is found in areas with leached soils derived from shale, predominantly on higher altitudes of the southern slopes of some mountains. Shale fynbos also shares several species with granite fynbos. Silicrete and ferricrete cover the shales in several high-rainfall areas.
Derivation of name and historical aspects
The genus name Agathosma is derived from Greek words, agathos and osme respectively, which means ‘good’ and ‘scent’, in reference to the aromatic scents present in the leaf glands. The presence of aromatic oil glands in the leaves is characteristic of all members in the family Rutaceae. The specific ephithet orbiculata is in reference to the round and flat leaves.
The Rutaceae, also known as the buchu or citrus family, is of relative economic importance and well known for its variety of citrus. The family is distributed in all temperate and tropical regions of the world, in particular Australia and South Africa. There are 160 genera, which consist of 1 650 species, found globally.
Southern Africa is home to 21 genera and 301 species. Some well-known South African genera include Vepris, Calodendron, Zanthoxylum, Acmadenia, Adenandra and Agathosma. The highest concentration of species in South Africa is concentrated in the Western Cape. All South African species belong to the subfamily Diosmeae. Species in this particular subfamily are shrubs with simple leaves and unique fruit made up of 1–5 free carpels characterized by horn-like processes at the apex. The carpels undergo a complete separation at maturity, as each carpel splits down an inward-facing line. This ultimately results in the seed being released in an extremely powerful manner because of the hygroscopic properties of the inner layer of the wall.
Very little is known about pollination-biology of this species. Like several other species of Agathosma, it is possible for bees, beetles and butterflies to be involved in its pollination.
No known uses. This buchu shows tremendous potential as a potplant and landscaping plant. Given its severely threatened status, possible re-introduction into the wild to boost its natural wild population could potentially be considered as a priority.
Growing Agathosma orbicularis
Sow the seed in autumn in a light, well-draining medium. Cover the seed with a thin layer of sand or fine compost. At Kirstenbosch National Botanical Garden seed will now be placed under an enclosed cover and be subjected to a smoke treatment. The use of smoke-discs, in which the seeds can be soaked, fulfil a similar function (but this method gets done before seed are sown).
After removal from the smoke-enclosure, the seed trays are then watered and moved to a space that provides good light and ventilation. Seeds must never be allowed to dry out completely and should also not be overwatered. Germination takes between 1–2 months and the onset of the first 4 true leaves, is an indication of the readiness of the seedling to be potted on.
All potted seedlings now get moved to slightly shady areas to allow for hardening of roughly 3–4 weeks. Thereafter, plants get moved into the full sun. This is also an opportune moment to facilitate more vigorous bushy growth, through the pinching out of the growing tips of the young plants.
Good results are also being yielded through the propagation of cuttings, using tip or semi-hardwood cuttings using fresh material of the current year’s growth. Prepare cuttings of 25–40 mm long, remove about a third of the foliage and cut it below the node. Dip the cuttings in a rooting hormone, firmly position it into ready-made holes with the rooting medium (equal parts bark and perlite) and placed in the cutting tray on a mist-unit with heated benches. Rooting of the cuttings will take 9–11 weeks. Rooted cuttings are subsequently moved to a hardening-off section for 2–3 weeks.
Pot the rooted cuttings in to a well-drained soil mix and place in a shady area for 2–4 weeks to harden off and move into full sun, following this period. After the next 7–8 months plants will be ready for planting out in the garden.
Instead of potting rooted cuttings straight into bags, cuttings can also be potted into Unigro-plugs. Depending on the species, plants can spend between 6–12 months in such plugs. It offers several advantages in that less casualties are being experienced, the young plants grow faster and can be better maintained, with pruning and feeding to improve overall ornamental value. The Unigro-plugs can be reused.
This buchu can be used with several other companion plants, which include: Protea nitida, P. repens and P. lepidocarpodendron, Phylica ericoides, Pelargonium cucullatum and P. betulinum, Mimetes cucullatus, Felicia filifolia, Agathosma serpyllacea, Coleonema album, Leucadendron spissifolium, L. sessile, L. salignum, L. laureolum and L. argenteum, Elegia stipularis, Brunia noduliflora, Watsonia borbonica, Cannomois virgata, Erica plukenetii, E. quadrangularis, E. viscaria subsp longifolia and E. cruenta, Helichrysum petiolare and H. cymosum, Halleria lucida, Kiggelaria africana, Cunonia capensis, Maytenus oleoides and Dodonaea viscosa var. angustifolia.
Plant Agathosma orbiculata in full sun and well-drained soil. The best season for planting is late autumn to early winter, to allow enough time for it to get established in the garden, before the warmer spring and summer months. Planting can be done relatively close together, with spaces of 20–30 cm, enough to encourage growth. Buchus prefer denser plantings, as an aid in moisture retention. Generous mulching of 20–30 mm will further enable moisture retention. Cut plants back very lightly after flowering.
- Gould, M. 1992. The buchus: cultivation and propagation. National Botanical Institute, Cape Town.
- Koekemoer, M., Steyn, H.M. & Bester, S.P. 2015. Guide to Plant Families of southern Africa. Strelitzia 31. 2nd ed., 2nd print. South African National Biodiversity Institute, Pretoria, South Africa.
- Manning, J. & Goldblatt, P. 2012. Plants of the Greater Cape Floristic Region 1: the Core Cape Flora. Strelitzia 29. South African National Biodiversity Institute, Pretoria.
- Mucina, L. & Rutherford, M.C. (eds) 2006. The vegetation of South Africa, Lesotho and Swaziland. Strelitzia 19. South African National Biodiversity Institute, Pretoria.
- Powrie, F. 1998. Grow South African Plants. A gardener's companion to indigenous plants. National Botanical Institute, Cape Town.
- Stearn, W. 2002. Stearn's dictionary of plant names for gardeners. Timber Press, Portland, Oregon.
- Trinder-Smith, T., Helme, N.A., Hanekom, A., Ebrahim, I. & Raimondo, D. 2010. Agathosma orbicularis (Thunb.) Bartl. & H.L.Wendl. National Assessment: Red List of South African plants version 2017.1. Accessed on 2019/07/01.
- Trinder-Smith, T.H. 2003. The Levyns Guide to the plant genera of the south western Cape. Bolus Herbarium, UCT, Red Roof Design CC, Cape Town
Kirstenbosch National Botanical Garden
Plant Type: Shrub
SA Distribution: Western Cape
Soil type: Sandy, Clay, Loam
Flowering season: Spring, Winter
Flower colour: Mauve/Lilac
Aspect: Full Sun
Gardening skill: Average