Prostanthera rugosa

Mint bush

Family: Lamiaceae

An openly-branched aromatic shrub to 1.5 m tall, to less than 1 metre wide.

It grows in wet and dry sclerophyll forests, typically on sandy substrates or sandstone outcrop, and is confined to the southern highlands of NSW, growing around Moss Vale, Avoca, Bowral, Bargo and west towards the Wombeyan Caves.

Branches are densely hairy, at least when young and have more or less sessile glands.

Leaves are always in opposite pairs (rarely in 3-leaf whorls), somewhat crowded along stems, egg-shaped to narrow egg-shaped, hairy on the upper surface, have two or three lobes on each side, to 9 mm long and to 5 mm wide, on a petiole 0.5 to 2 mm long. The leaves have a crinkled-contorted appearance with undulating margins and are strongly glandular.

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 arranged mostly singularly in the leaf axils, near the ends of branches with bracteoles less than 0.5 mm long at the base. The petals are mauve with a white tinge, to 13 mm long and are fused for part of their length, with overall flowers about 10 mm wide. Flowering occurs from September to October

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

In the garden

This species can be cultivated and there are records of it online being grown at the National Botanic Gardens in Canberra. An interesting plant due to its clustered and contorted leaves. Attractive in flower and frost hardy.

Plants may be difficult to source. However, it grows best in dry or moist very well-drained soils in filtered shade, making a good container or rockery plant.

Most prostantheras can take some soil enrichment through organic matter addition.

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

There is a tip from the publication: “Letters to Garden Lovers”, Australian Home Beautiful, April 1938, to regularly and lightly prune branches all through the year rather than give plants a heavy prune once a year.
Be careful not to eliminate flower buds, however.

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

There are approximately 100 species, 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 hybridizing when in cultivation.

Some 80% of mints contain aromatic oils within their leaves with oil of cineol being a major component. Prostanthera sieberiP. 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.

Most Prostanthera species will regenerate from seed after fire with some species exhibiting an ability to reshoot from basal areas and stem buds. The exact response for this species is unknown.

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.

rugosa – Latin meaning “wrinkled”, referring to the wrinkled / contorted appearance of the leaves.

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://en.wikipedia.org/wiki/Prostanthera_rugosa
https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Prostanthera~rugosa

By Jeff Howes