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An erect shrub to 4 m tall, spreading up to 1 m wide; strongly odourous, pleasing or perhaps unpleasant.
It is found naturally growing in wet sclerophyll forest and on margins of rainforest, along the New South Wales coastline from Mount Warning near the Queensland border, extending all the way along the coast to Victoria. Populations do extend just into the Central and Northern Tablelands regions.
Branches are square with hairs and glands on very short stalks.
Prostanthera have simple, opposite and usually odorous leaves (a typical Lamiaceae feature). In this species, leaves are in opposite pairs or often in whorls of 4, (or rarely in 3-leaf whorls), to 5 cm long and to 1.5 cm wide, green-coloured with the lower surface paler, covered with short, curled hairs at the leaf base. The margins are coarsely toothed with the tips blunt.
Flowers have a shape described as labiate (applies to all Lamiaceae flowers) with 5 petals varying in their size, fused at their base, produced in leaf axils. One of the identification features for Prostanthera is that the 5 calyx parts (basal whorl of the flower) are fused into 2 lips.
In this species, the flowers are pale-mauve to purple, with petals fused into a bell-shaped tube, to 10 mm long by about 10 mm wide. Flowers are produced in leafy to leafless clusters from the upper leaf axils on each branch, occurring in spring.
Fruits are mericarps (can be called nutlets). Four are produced produced at the base of the calyx.
An easy prostanthera to grow and readily available from council bushcare and other nurseries. It is common in cultivation.
Can be shaped into a very nice-rounded shrub to about 1 or 1.5 m tall with tip pruning after flowering. Plant in dappled shade on a well-draining sandy soil for best results. A slight slope works really well. It will appreciate some soil enrichment from organic matter. The entire plant turns from green to purple when in flower. Provide some water when dry; plants can be observed to wilt in dry hot days. Good for bee attracting as well.
One drawback with this plant is it may be short-lived – up to 2 or 3 years. When pruned back to a height of 50 cm it reshoots readily.
There is also a pink flowering form reported.
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.
This species has the following synonymous names
Currently in NSW, P. sieberi has been absorbed (lumped) into P. incisa, pending further study.
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 the basal parts of stems.
Prostanthera – from the Greek prosthike (προσθήκη) which translates to ‘addendum’, and anthir (ανθήρ) meaning anther – referring to the anthers which have an appendage of tissue.
incisa – Latin for ‘incised’ – referring to the leaves having deeply cut and irregular incisions.
This species is not considered to be at risk of extinction in the wild.
NSW Flora Online (PlantNET) – Prostanthera incisa profile page https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=gn&name=Prostanthera
Wrigley, J.W. & Fagg, M.I. (2001). Australian Native Plants – Propagation, cultivation and use in landscaping. 4th edition. New Holland Publishers, Pty. Ltd. Australia.
Australian Native Plants Society Australia – Prostanthera information page http://anpsa.org.au/APOL2007/apr07-1.html
Wikipedia – Prostanthera incisa profile page https://en.wikipedia.org/wiki/Prostanthera_incisa