Ixianthes retzioides Benth.
Common names: river bells (Eng.); rivierklokkies (Afr.)
SA Tree No: 670.1
This is a beautiful plant, seldom seen. It is a rare endemic, confined to a few streams of the winter-rainfall Western Cape. It has striking, yellow, bell-shaped flowers. Due to its specialized habitat in the rocky crevices of swift, fast-flowing streambanks, the plant is difficult to grow and seldom cultivated.
The river-bells is an erect, twiggy, sparsely branched shrub up to 2 m tall and about to 1 m in diameter. It has fibrous roots often submerged in water. Its young branches are densely leafy, 4-7 mm in diameter, square, distinctly ribbed when young, green, becoming purplish and round (terete). Its older branches can thicken up to 50 mm in diameter at the base (rarely up to 70 mm), becoming brown and eventually grey and smooth. Its leathery, green, slender and flattened leaves (linear oblanceolate, 65-125 x 5-9 mm) are ascending and carried in whorls (4-verticillate, occasionally 3-verticillate). The petiole is short. Young leaves are soft, shiny and viscous, due to minute glandular hairs soon becoming smooth. Leaf margins are slightly thickened, and bearing 4-8 pairs of small teeth in the upper third of the leaf.
The striking, yellow, tubular flowers are carried in whorls. The broadly tubular flowers are 27-30 x ± 30 mm, the corolla is two-lipped (5 lobed, 2 upper and 3 lower) and spreading, the youngest flowers opening from stem top downwards. The fruit, splits at the sides, dispersing its numerous small seeds. Flowering time: spring to early summer.
Ixianthes retzioides is not threatened.
Distribution and habitat
River bells is confined to the upper reaches of the Olifants River and its tributaries and also above Tulbagh near the waterfall in the Groot Winterhoek Mountains, Western Cape. The climate is hot and dry during summer but moist during the wet winters. The shrubs occur on streambanks in cool, swift and fast-flowing water, often with their roots in the water. The plants grows in association with Metrosideros angustifolia, Brachylaena neriifolia and the palm, Prionium serratum.
Derivation of name and historical aspects
Ixianthes retzioides was discovered in the Groot Winterhoek Mountains above Tulbagh waterfall by Christian Ecklon (1795-1868) and Carl Zeyher (1799-1858) in 1831 while documenting the plants around Tulbagh. Ecklon, a Danish pharmacist and plant collector, settled in Cape Town in 1823 to further his experience. Zeyher, a German botanical collector came to the Cape in 1822. Ixianthus was named by the botanist Bentham in Hookers Botanical Magazine. The genus name is derived from Greek, but the reference is not very clear: ixos is mistletoe or birdlime; the genus Ixia was named for its viscous sap; anthos is flower; retzioides means 'like the genus Retzia'.
Until recently, Ixianthes was included in the Scrophulariaceae, a large family with about 3 000 species with a cosmopolitan distribution, but molecular studies have shown that it belongs in the Stilbaceae, a smaller, mainly African family containing Stilbe and Retzia, and now also Ansastrabe, Bowkeria, Nuxia and Halleria. The best known is the tree fuschia, Halleria lucida, bearing tubular orange flowers and black edible berries. Other tree members include Bowkeria, Freylinia and Anastrabe.
Research work by Dr Kim Steiner (formerly of Kirstenbosch) and Vim Whitehead (South African Museum, Cape Town) during 1996, show that river bells are pollinated by an oil collecting bee, Rediviva gigas. Oil secretion as a pollinator reward is rare among flowering plants and also found in the genus Diascia.
Seed is dispersed in two ways: the small capsules split during summer, releasing large amounts of small seed, which are initially wind dispersed and then fall into the streams and are carried to suitable moist habitats where they germinate.
Growing Ixianthes retzioides
Although it grows readily from cuttings and thrives in containers, also flowering soon, the plants rapidly die when planted out in similar habitats. In spite of several attempts to grow the plants at Kirstenbosch, no success has been achieved even when planted next to strong flowing streams.
Cuttings root rapidly in a sandy mixture under mist propagation during spring and summer. Attempts are now under way to graft stock onto closely related, but easily grown Bowkeria or Anastrabe species from the northern and eastern parts of South Africa and once achieved will be released into gardens (see grafting below).
It is best grown as a container plant. Due to its acidic sandstone habitat the plants are best grown in a fynbos potting mixture (a quartzitic sandstone-based, well-drained mixture). For best results, water frequently during the dry summer months.
Grafting and budding:
Why do we graft or bud a plant? Not all plants are easy to grow and especially those confined to specialized habitats such as a fast flowing streambank. Also some have weak or disease-prone rootstocks. When a useful or attractive (agricultural or ornamental horticultural) plant is difficult to grow on its own rootstock (such as the river bells), grafting is often the answer. By using a closely related plant which is better able to cope with conditions in cultivation as a stock or host plant, the problems associated with growing the plant are sometimes overcome.
Examples include various fruit trees and other ornamentals such as roses and many other tree and shrub species. Success has been achieved with the rare and difficult to grow indigenous marsh rose, Orothamnus flabellifolius, which has successfully been grafted onto Leucadendron conocarpodendron. The impala lily, Adenium multiflorum, from dry, subtropical and tropical regions in Africa has been grafted onto Nerium oleander in Europe. The author has also successfully grafted plants of the large-leafed spekboom, Portulacaria armiana, onto the spekboom, P. afra.
Grafts are usually performed with a close relative or at least a member of the same family. Many difficult members of the stapeliad family have been grafted onto Ceropegia woodii rootstock with great success. There are various methods of grafting or budding. In the latter a single bud is transferred into a slit made into the bark of the host and the bud slipped in, and covered with plastic wrapping. Grafts are made with shoots of the desired plant inserted into slits in the stock plant and held in place until the bark/outer layer of both plants has grown together.
- Angiosperm Phylogeny Group (2009). An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III. Botanical Journal of the Linnean Society. 161 (2)
- Hiern, 1904, Flora Capensis 4:2, 216-217
- Steiner, K. 1996 Plant Systematics and Evolution 201: 131-138
Ernst van Jaarsveld
Kirstenbosch National Botanical Garden
December 2004, updated March 2018
Plant Type: Aquatic, Shrub
SA Distribution: Western Cape
Soil type: Sandy, Loam
Flowering season: Spring, Early Summer
Flower colour: Yellow
Aspect: Full Sun
Gardening skill: Challenging