Prostanthera sejuncta, Spiny Mintbush, is a scrambling, more or less prostrate shrub that may reach a height of 50 centimetres.
It grows only in one general location, north of Grafton in NSW, around Banyabba Nature Reserve and Fortis Creek National Park.
It is found in moist to swampy sclerophyll woodlands and forests, usually on skeletal sandy soils and rocky areas.
It is a listed threatened species in the wild.
Prostanthera have simple, opposite and usually odorous leaves. In this species, leaves are small, to 6 mm long and 4 mm wide, ovate and aromatic; pale to deep green above and paler beneath. Unusually for this genus, slender spines are produced in the leaf axils, to around 15 millimetres long.
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, solitary and axillary flowers are about 1.5 centimetres long, pale mauve to pale lilac, appearing from July to December and may be scattered or profuse. Sporadic flowering occurs at other times.
Fruits – 4 tiny nutlets (mericarps) produced at the base of the calyx.
Prostanthera sejuncta would make an interesting ground cover in native garden beds. Currently, not too much is known regarding its cultivation, likely due to it being a threatened species. However, comments have been found online that it can be grown and is cultivated. Check with local native nurseries for availability.
Pruning will keep foliage thick and increase flower production. Likely best grown on a well-draining sandy soil with adequate water.
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.
As with most mintbushes this species propagates readily 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.
The Spiny Mintbush used to be part of the P. spinosa complex. In 1995 P. spinosa was split into three species.
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.
This genus is currently under revision, and several species complexes are unresolved. Natural hybrids occur between several species and most species appear to be capable of hybridizing when in cultivation.
Some 80% of mints contain aromatic oils within their leaves with oil of cineol being a major component. Prostanthera sieberi, P. incisa and P. staurophylla are quite pleasantly overpowering due to 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”, and anthir (ανθήρ) meaning anther – referring to the anthers which have an appendage of tissue.
sejuncta – Latin – modified from sejungo which means ‘separate’ – referring to this species growing well away from other similar species.
This species is listed as threatened in NSW with the category of vulnerable
NSW Flora Online (PlantNET) – Prostanthera sejuncta profile page https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Prostanthera~sejuncta
Australian Native Plants Society Australia – Prostanthera – The Mint Bushes Information Page
NSW Office of Environment and Heritage – Threatened Species Profiles – Prostanthera sejuncta profile page https://www.environment.nsw.gov.au/threatenedspeciesapp/profile.aspx?id=20032