Prostanthera granitica is a small, spreading shrub that reaches a height of about 1.5 metres, spreading to 2 metres wide, creating large clumps.
It grows over a large geographic range, almost confined to NSW, growing mainly on the western slopes, from as far south as Albury, then disjunctly north-north-west around Bathurst, Orange and Lithgow; Dunedoo-Coolah and Scone-area; then from Coonabarabran to Narrabri (and east of); then extending just into Queensland around Boggabilla and north thereof.
It is found in heathlands and dry sclerophyll woodlands and shrublands, usually on rocky substrates and ledges/crevices which includes sandstone as well as granite.
Prostanthera have simple, opposite and usually odorous leaves. In this species, aromatic leaves are to 15 mm long by 5 mm wide, ovate to narrow-ovate, moderately crowded; mid green and feel like sandpaper to the touch due to hispid hairs.
Flowers have a shape described as labiate (applied to all Lamiaceae flowers) with petals varying in their size, purple to lilac, produced in leaf axils. One of the identification features for Prostanthera is that the 5 calyx parts (sepals; basal whorl of the flower) are fused into 2 lips. In this species, flowers are 1 cm across by 1 cm long; mid-violet to purple and carried in the upper leaf axils, conspicuous and profuse during the spring flowering period.
Fruits are mericarps (can be called nutlets). Four are produced produced at the base of the calyx.
Prostanthera granitica is an interesting mint bush with unusual foliage and colourful flowers.
This shrub would be an interesting addition to cottage gardens and rockeries.
It is not that widely known in cultivation but is known to be cultivated to an extent.
It grows naturally on rocky substrates. In September 2022, APS members took in some beautiful large clumping plants on the pre-conference tour, at the Sandstone Caves in the Pilliga State Forest. From a distance, plants look a little like Prostanthera densa (see photos from this visit).
Likely needs a well-draining soil to thrive. Can likely tolerate warm gardens with good drainage. Prune after flowering to encourage a nice rounded shrub with more flowers the following season.
A Prostanthera in full bloom is a magnificent sight and there are so many colours to choose from for your garden. These plants are found in all states in varied soil conditions and climate and thus while it may be a challenge to grow some species many are easy in a garden situation.
A few basic growing tips are:
• Good drainage is essential. Raised beds ensure this
• Water new plants until established, weekly or as required.
• Do not over water, as this can induce root rot and fungal infestation.
• They prefer moist root runs.
• Plant drooping is an indicator of dryness
Positioning of prostantheras as border plants or near pathways is recommended as the mint odour is released when brushed against.
Propagate from cuttings.
Plants may be grown from fresh seed. However, cuttings are frequently and reliably used, usually semi-hard wood or soft tip material, which strike well in spring or autumn.
Prostanthera is a diverse group of about 100 species, endemic to Australia, occurring in all states. There are still many unresolved taxa and species complexes, with new forms regularly being found. Natural hybrids occur between several species and most species appear to be capable of hybridizing when in cultivation. NSW currently has about 52 species, some of which are species-complex and others which are threatened with extinction.
Some 80% of mints contain aromatic oils within their leaves with oil of cineole being a major component. Prostanthera sieberi, P. incisa and P. staurophylla are quite pleasantly overpowering in their exudates when crushed. Oil from the leaves of some species is distilled for use in cosmetics and as soap additives.
Likely regenerates from seed after fire, possibly after 12 months.
Prostanthera – from the Greek prosthike (προσθήκη) which translates to “addendum” (root of prosthetic), and anthir (ανθήρ) meaning anther – referring to the anthers which have an appendage of tissue.
granitica – Latin meaning “granite-associated” or “found on granite”, referring to the rocky habitat of this species. It is, however, also found on sandstone.
This species is not considered to be at risk of extinction in the wild.
NSW Flora Online (PlantNET) – Prostanthera granitica profile page https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Prostanthera~granitica
Walcott Garden – Prostanthera page https://www.walcottgarden.com/blog/prostantheras
Wikipedia – Prostanthera granitica profile page https://en.wikipedia.org/wiki/Prostanthera_granitica