Plumbago Tourn. ex L.

Colourful and long-flowering, woody plumbagos are important garden ornamentals on a world scale. However, this is primarily a tropical and subtropical genus. Only one woody species, P. auriculata, is sufficiently hardy to be grown unprotected in our area, and even that is restricted to milder areas.

The family Plumbaginaceae straddles the divide between herbaceous and woody plants. Plumbago itself includes both herbs and shrubs, as do other genera such as Limonium and Ceratostigma. Plumbago belongs (with Ceratostigma and some small genera of little horticultural significance) to Subfamily Plumbaginoideae, an uncontroversial grouping well supported by morphological, biochemical and molecular evidence (Hernández‐Ledesma et al. 2015; Koutroumpa et al. 2018). It is distinguished from Ceratostigma by the conspicuous stalked glands on the calyx. What molecular evidence there is suggests that Plumbago is not monophyletic, the small genera Plumbagella and Dyerophytum nesting within it (Koutroumpa et al. 2018). The taxonomic consequences of this have not yet been worked through, and more thorough sampling of Plumbago species would be required before this could happen.

The name Plumbago dates from classical times, the herbaceous P. europaea L. being native to south eastern Europe. Pliny the Elder used the name (Pliny 1855), from the Latin plumbum (lead, the metal) and agere (to drive, direct or simply do), which might relate to the idea that it could be used to treat lead poisoning and perhaps also (by the doctrine of signatures) to the lead-blue of its flowers. It was in use as a generic name in the era of Latin polynomials, certainly by Tournefort (1700), and adopted by Linnaeus (1753).

We describe the blue-flowered P. auriculata, native to a swathe of southern Africa from Mozambique to the southern Cape. Two other woody evergreen species requiring warm summers and frost-free winters are widely grown to the south of our area, for example in Texas and on the French Riviera. P. indica L. (syn. P. rosea L.), from subtropical Asia, has flowers of a vivid pinkish red, and is usually well under 1 m in height. The pantropical P. zeylanica L. (including P. scandens L.) is rather taller, with straggling stems and white flowers. Far less well known, P. wissii Friedrich, a low shrub spreading by underground stolons, with glaucescent, linear leaves and small violet-pink flowers in significantly sized inflorescences, is endemic to the Brandberg in northern Namibia but takes well to dry Mediterranean-climate gardens in the Cape (Jaarsveld 2006); if it could be obtained, it would be worth trying in dry, sunny situations on the mildest fringes of our area.

The flat-faced flowers, producing nectar at the bottom of a long, narrow corolla tube, suggest pollination by long-tongued insects. Butterflies and long-tongued flies were the major pollinators of a wild P. auriculata population in KwaZulu-Natal, South Africa (Ferrero et al. 2009). Heterostyly is very common in the Plumbaginaceae, but only well studied in Subfamily Limonioideae (Limonium, Armeria etc. – Baker 1966; Costa et al. 2019). This is the occurrence of two or more floral morphs with different style lengths, and usually with anthers positioned to match the style length of another morph, an arrangement encouraging outcrossing, familiar to gardeners in the ‘pin’ and ‘thrum’ morphs of most Primula and Pulmonaria. Morphs are genetically determined, each individual producing flowers of only one type. At least P. europaea (Dulberger 1975) and P. auriculata (Ferrero et al. 2009) are heterostylous, with reciprocal anther and style positions, coupled with strong self-incompatibility in at least the second of these.

Leadworts are poisonous plants. Plumbagin(e) is a toxic, pharmacologically active quinoid usually extracted from Plumbago indica (Sigma-Aldrich 2021). but found widely in Plumbago and its relatives. There is ongoing research on its possible use as a drug against a remarkably wide range of conditions, and many Plumbago species have been used as traditional medicines in bewilderingly diverse ways (Singh, Naidoo & Baijnath 2018). It is not distributed uniformly in the plant: highest concentrations were found in the roots of P. indica, leaves of P. auriculata, and stems of P. zeylanica (Mallavadhani, Sahu & Muralidhar 2002).

Hybrids between species seem not to have been recorded.