Prostanthera junonis

Somersby Mint Bush

Family: Lamiaceae

A low spreading or straggling shrub to 1 metre high and up to 1 metre in diameter.

It grows naturally in sclerophyll forest and woodland in sandy loamy soils on sandstone. It is restricted to the Somersby Plateau in the Gosford area of NSW and has a north-south range of approximately 19 km on the Plateau in the Gosford and Wyong local government areas. It grows at places such as the Australian Reptile Park at Somersby. Also reported from Mangrove Mountain.

It is a listed threatened species.

Its habit can be variable; in open sites, branches appear wiry and are often prostrate. However, where vegetation is denser, the plants have long spindly branches which weave through other vegetation and can grow up to 1 m long. Branches moderately covered with long hairs. Plants are generally non-aromatic.

Leaves are in opposite pairs (rarely in 3-leaf whorls), oval in shape, dull green above and paler below, to 14 mm long and to 5 mm wide.

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 flowers are to 12 mm long by 10 mm wide, pale-mauve to almost white with brown spots in the throat. The petals are fused into a tube for about 3 mm. Flowers are produced singularly, at the base of paired leaves and are produced from October to February.

Fruits – 4 tiny nutlets (mericarps) produced at the base of the calyx.

In the garden

Not readily available for cultivation due to its threatened status. It may become available in the future. It likely needs a sandy soil to thrive with good drainage. This plant grows more readily when not competing with the cover of other plants.

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

Propagation

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.

Other information

Synonym: Prostanthera sp. ‘Somersby’ (before it was formally published).

Very difficult to identify when not flowering.

There are approximately 100 species of Prostanthera, endemic to Australia. They occur in all states.

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 hybridising 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.

Positioning of prostantheras as border plants or near pathways is recommended as the mint odour is released when brushed against. Oil from the leaves of some species is distilled for use in cosmetics and as soap additives.

This plant is listed as threatened under both State and Commonwealth legislation with the category of endangered. Conservation status in NSW: Endangered Commonwealth status: Endangered

Most Prostanthera species will regenerate from seed after fire with some species exhibiting an ability to reshoot from basal areas and stem buds. Mature plants appear to be incapable of resprouting after fire. This editor has heard observational evidence that plants regenerate from seed in the 12 to 24-month period after fire (similarly to Prostanthera densa).

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.

junonis – honours Mrs June Gay (d. 1997), a volunteer at the National Herbarium of NSW, who assisted botanist Barry Conn with research and curatorial help on Prostanthera.

https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=gn&name=Prostanthera
http://anpsa.org.au/APOL2007/apr07-1.html

https://www.environment.nsw.gov.au/threatenedspeciesapp/profile.aspx?id=10678

https://en.wikipedia.org/wiki/Prostanthera_junonis

By Jeff Howes, edited by Dan Clarke