Prostanthera densa is a medium shrub, potentially having branches to 4 metres tall or long, spreading up to 1 metre wide.
Its natural distribution is in six known areas in NSW, all very close to the coast: Nelson Bay, South Cronulla, Royal National Park (Marley and Nioka Ridge), Helensburgh and Beecroft Peninsula, Currarong (northern peninsula of Jervis Bay). The population at Nioka Ridge was re-discovered in 2020 (it had not been monitored since 1972).
It is found growing on a range of soil types – volcanic clay (Nelson Bay), sandstone outcrop (South Cronulla and Royal National Park), shale/sandstone transition soils (Helensburgh) and coastal sand plains (Beecroft Peninsula).
It is listed as threatened with extinction in the wild.
Prostanthera have simple, opposite and usually odorous leaves (a typical Lamiaceae feature). In this species, leaves are shortly ovate, about 1 cm long and wide (at base), narrowing at the tip, with a dense covering of hairs giving a raspy or velvet-like texture (depending on where the plant is observed or the origin of propagation material).
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 light mauve to purple, produced solitarily in leaf axils, typically larger than the leaves and very attractive.
Fruits are mericarps (can be called nutlets). Four are produced produced at the base of the calyx.
A great plant as a gap filler or supplementary plant to create foliage contrast or texture variety, Lends itself to be useful in topiary gardens as a short-term plant. Nice soft foliage and purple flowers. Useful for attracting insects.
Plants are known to live at least 5 years in the wild.
Prune after flowering to encourage a dense round shape.
Will tolerate a sunny position – probably best if not too hot. Give adequate drainage.
Allow good air flow. Likely to be frost intolerant. Will likely need a well-draining sandy soil to do well.
Possibly affected by insects (galls) that invade the stem, based on observations in the wild.
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
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. Some plants have been observed to reshoot from snapped basal stems.
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.
densa – Latin. meaning “dense” – referring to the dense foliage.
This species is listed as threatened with extinction, at the State and Commonwealth level, with the category of vulnerable.
NSW Flora Online – PlantNET – Prostanthera densa profile page http://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Prostanthera~densa
Surveys conducted by Australian Plants Society (coordinated by Dan Clarke) for the NSW Saving our Species Program: https://austplants.com.au/Sutherland-Conservation
NSW Office of Environment and Heritage – Threatened Species Profiles – Prostanthera densa profile page https://www.environment.nsw.gov.au/threatenedspeciesapp/profile.aspx?id=10676
Australian Native Plants Society Australia – Prostanthera information page