Petalidium saxatile: A new species of Petal-bush from Namibia.

Petal-bushes, Petalidium spp., are woody perinal shrubs in the Family Acanthaceae found in arid sandy or stoney areas of Africa, India, and the Mascarene Islands, although most species are found in areas of Southern Africa with summer rainfall and no frosts. There are currently 35 species recognised from Southern Africa, of which 33 are found in Namibia, 13 in Angola, and six in South Africa. Despite the name 'bush', they are diverse in form, ranging from scrambling herbs to large, robust shrubs. Many species of Petalidium are fast-growing and produce attractive  flowers, leading to some species being cultivated by gardeners.

In a paper published in the journal Phytotaxa on 3 September 2025, Wessel Swanepoel of the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, Kyle Dexter of the School of GeoSciences at the University of Edinburgh, the Royal Botanic Garden Edinburgh, and the Department of Life Sciences and Systems Biology at the University of Turin, Martino Adamo, also of the Department of Life Sciences and Systems Biology at the University of Turin, Erin Manzitto-Tripp of the Department of Ecology & Evolutionary Biology, & Museum of Natural History at the University of Colorado, and Abraham Van Wyk, also of the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, and of the South African National Biodiversity Institute, describe a new species of Petalidium from the Kaokoveld Desert of northwestern Namibia.

The new species is described on the basis of four populations of Petalidium in the Kaokoveld discovered by Wessel Swanepoel while carrying out research for a monograph on the genus in Southern Africa. The populations had been observed by him before, and assumed to belong to the species Petalidium canescens, which is also found in the region. However, in May 2018 Swanepoel visited a population near Palmwag while it was in flower, suggesting that it was a different species, something which was later backed up by generic analysis.

The new species is named Petalidium saxatile, where 'saxatile' derives from the Latin 'saxatilis', meaning 'found amongst rocks', from the habitat where it lives. These Plants form a hemispherical woody shrub about 50 cm tall, with bifurcating stems and elongate oval leaves, both covered with white hairs. Flowers are roughly tubular, with four lobes, the lower of which is yellow and the others pinkish brown (unlike the all pink flowers of Petalidium canescens).

Petalidium saxatile, morphology of flowers. (A), (B) Newly opened and faded flowers. Anterior corolla lobe inside (adaxially)  bright yellow and without nectar guides. (C) Flower viewed from above, showing puberulous abaxial surface of posterior corolla lobes. (D) Flowers viewed obliquely from above, all corolla lobes discolorous with the outside (abaxial) surfaces puberulous and notably paler in  colour. The long white trichomes next to the flower on the right do not belong to the plant, but is a wind-blown feathery awn (arrowed) of  a member of the Grass genus Stipagrostis. Wessel Swanepoel in Swanepoel et al. (2025).

Petalidium saxatile is known from only four sites, three in an approximate line from Palmwag southwards to Bergsig, and one on Welbedacht Farm, to the south of Khorixas. It has been found growing among rocks from the Etendeka Group basalt, on arid hillsides and along drainage lines at elevations of 860–1130 m above sealevel, between 70 and 150 km from the sea. Searches of other areas with similar conditions did not reveal any further populations. It is calculated that the area of potential habitation for Petalidium saxatile is 3337 km², but the area of actual habitation is less than 20 km². All of the areas where the species is found are prone to prolonged droughts. As such, Swanepoel et al. recommend that Petalidium saxatile be classified as Endangered under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species. 

Known distribution of Petalidium saxatile (black dots). Swanepoel et al. (2025).

See also...

Petalidium hoarusibense: A new species of Petal-bush from Namibia.

Petal-bushes, Petalidium spp., are woody perinal shrubs in the Family Acanthaceae found in arid sandy or stoney areas of Africa, India, and the Mascarene Islands, although most species are found in areas of Southern Africa with summer rainfall and no frosts. There are currently 32 species recognised from Namibia, 13 from Angola, and six from South Africa. Despite the name 'bush', they are diverse in form, ranging from scrambling herbs to large, robust shrubs. Many species of Petalidium are fast-growing and produce attractive  flowers, leading to some species being cultivated by gardeners.

In a paper published in the journal Phytotaxa on 8 January 2025, Wessel Swanepoel of the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, and Abraham Van Wyk, also of the H.G.W.J. Schweickerdt Herbarium at the University of Pretoria, and of the South African National Biodiversity Institute, describe a new species of Petalidium from the Kaokoveld region of northwestern Namibia.

During a visit to the Okandjombo area of the Kaokoveld in November 2023, Wessel Swanepoel obseeved a number of distinctive Petalidium Plants, with a dwarf-shrubby habit, pealing white bark, and multiple stems arrising from just below the ground level. At this  time the Plant was not in flower, making it difficult  to confirm that it was not a previously described species, however, during a follow-up visit in May 2024, the plants were found to be producing distinctive pink flowers, confirming it as a  new species. This is named Petalidium hoarusibense, in reference to the Hoarusib River, as it was discovered within the catchment of this river.

Petalidium hoarusibense. Habitat and habit. (A) Mature plant about 800 mm high (greyish shrub in foreground), growing among dark grey sedimentary rocks of the Swakop Group along the banks of the Hoarusib River near Okandjombo, Namibia. (B) Base of plant showing several branches arising from the main stem just above ground level, each covered with rough, fissured, greyish white bark. Wessel Swanepoel in Swanepoel & Vak Wyk (2025).

To date, Petalidium hoarusibense, has only been found in m the mountainous area along the Hoarusib River and its tributaries to the east and south of Okandjombo, part of the Great Escarpment of northwestern Namibia, on on arid hillsides and along drainage lines at elevations of between 450 and 700 m above sealevel. This area recieves about 100 mm of rainfall in an average year, and additionally has about five foggy days, which might contribute to the available moisture slightly..

Petalidium hoarusibense. Morphology of leaves and flowers. (A) Shoot with flower and leaves, the latter with dense, greyish white indumentum. (B), (C) Flower in front (B) and side (C) view; note bracteoles (some of spent flowers) with cobwebby indumentum. (D), (E), (F), (G) Flowers in front view, each from a different plant to show variation. Note all corolla lobes of a flower being roughly similarly coloured in various shades of magenta, or lateral and upper lobes slightly darker shaded than the anterior lobe, sometimes, as in (D), (E), & (G) with darker magenta or maroon triangular nectar guides; anterior lobe adaxially with two narrowly triangular yellow nectar guides. Wessel Swanepoel in Swanepoel & Vak Wyk (2025).

All known specimens of Petalidium hoarusibense were found growing within an area of about 20 x 18 km, although within this area there were several areas where it was quite common. There were also a large number of dead Plants on the hillsides, possibly due to a prolonged drought in the area. For this reason, Swanepoel and Van Wyk reccomend that the species be listed as Vulnerable under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species.

See also...

Justicia cubangensis & Justicia eriniae: Two new species of Water Willow from south-central Angola.

Water Willows, Justicia, are perennial herbs and shrubs found in tropical, sub-tropical, and warm temperate regions of the Americas, Africa and India. They typically have heavily veined leaves and white, cream, yellow, orange, or pink tubular flowers, which are attractive to Butterflies, making them widely cultivated as ornamental flowers, though they are not hardy and need to be kept under glass in areas prone to frosts. The genus Justicia is the largest in the family Acanthaceae, and was recently expanded to include the genus Monechma, after that genus was found to contain plants which were morphologically similar to one-another but not genetically monophyletic (i.e. not all sharing a common ancestor which isn't also the ancestor of other plants which are not placed within the genus), and it is likely that other genera will also be subsumed by Justicia in the future.

In a paper published in the journal Blumea on 3 May 2019, Iain Darbyshire of the Royal Botanic Gardens at Kew, and David Goyder of the National Geographic Okavango Wilderness Project at the Wild Bird Trust, describe two new species of Justica from southern Angola.

The first new species is named Justicia cubangensis, meaning 'from Cubanga' in reference to the Cubango River drainage, where it was discovered, part of the upper catchment of the Okavango system in Angola. This plant forms erect or trailing perennial herbs, 15-65 cm high, with annual branching stems arising from a woody base. The stems are quadrangular and hairy, the leaves lance shaped and up to 46 mm in length. Flowers are born on a spike 25-70 mm long and are 12–13 mm long and cream-white in colour, with the palate of the lower lip pink with white veins in raised ‘herring-bone’ pattern. The species was found in Cuando Cubango and Huíla provinces, growing in open grasslands within areas of Miombo (flood-plain) woodland. It was observed at only three locations, and was not abundant at any of these, but may be more widespread as Angola has been under-explored by botanists to date.

Justicia cubangensis. David Goyder in Darbyshire & Goyder (2019).

The second new species described is named Justicia eriniae, in honour of Erin Tripp of the Museum of Natural History at the University of Colorado, who collected the first specimen of this species. This is a brittle-stemmed herb reaching 50-75 cm in height, which has blade-shaped leaves up to 65 mm in length. and produces tubular flowers 18-18.5 mm in length on a flower spike 5.5-8.0 cm high. A single patch of these plants, less than 10 in number, was found growing by a road in Namibe Province, southwest Angola, in an area of lowland thornscrub, though it is unclear if this means the species is rare, since the area is poorly explored.

Justicia eriniae. David Goyder in Darbyshire & Goyder (2019).

See also...

Follow Sciency Thoughts on Facebook.