Petalidium is a genus of flowering plants in the Acanthaceae family that comprises about 40 species.

Petalidium barlerioides, well known for being the Nail Dye plant, is the only representative of the genus outside Africa. Most species are centred in the arid southern western parts of Africa with a few species extending into South Africa and Botswana. The principal reasons for the wealth of species are the numerous and various "tracks" that have contributed taxa to its range of distribution since early times. Other factors including environmental diversity, different rainfall regimes, and recurrent climate fluctuations in the past. Having a single species on the Indo-Pakistan subcontinent is not unusual for elements of the Kaokoveld Centre of Diversity.

Petalidium species are noticed, not only for the number of taxa, but also the numerous individuals. Some are considered fodder for domestic stock while game and even rhino have been seen to eat them. Some species have been shown to be promising as garden plants because of their colourful flowers and bracts, however viable seed is scarce as seed predation is very high.

In total, almost 100 names have been published or suggested as well as numerous forms. Several additional unnamed species are known and some hybrids recognised. After extensive field work, GIS and pollen studies the growth form diversity is better understood as well as the factors responsible for the variations.

Variations in growth forms, particularly habit and hairiness, is found in a number of species from unrelated taxa with similiar distribution ranges, like Commiphora, Sesamothamnus, Cordia species. These tend to become taller and less hairy further north, while erect species of Petalidium in Namibia, may be prostrate north of the Kunene River.

The phylogenetic study which is essential for placing patterns of growth form in an evolutionary context is ongoing. This should compliment the work on systematic diversity via nucleotides of genes that are not implicated in growth form development and differentiation.

For an inventory and details of the taxa, the search engine on Plants of Namibia must be used. Accepted names and their respective synonyms are databased with type specimens, descriptions and illustrations. The georeferenced collections can be used to map distributions.

This website is a resource for additional information. The tabs above, access links on the history (taxonomic and collecting); references, names, and will include an illustrated key. The tabs on the left provide summaries and elucidation of the data in the report fields in the database.

This site would not exist without Denis Filer, the man behind BRAHMS and Herta Kolberg. Their input and encouragement is greatly appreciated. Thje images in this website are the property of the author and Herta Kolberg. If you wish to use ant photo or information from the website, please contact me for permission petalidium@iway.na