Prostanthera rhombea

Sparkling mint-bush

Family: Lamiaceae

An openly-branched shrub to 2 metres tall by about 1 metre wide.

It is found in rainforest and moist sclerophyll forest, often in gullies or near streams, in sandy to loam soils over sandstone.

It grows mainly from Port Macquarie to the Nepean River in New South Wales, with most of its population in the Sydney area. However, it has several disjunct populations further north and into Qld.

In Victoria, it grows in low heath and woodland in rocky places in three isolated areas near Licola, Mount Timbertop near Mansfield and Mount Buffalo.

Branches are strongly aromatic and are covered with glandular hairs, and square.

Leaves in opposite pairs or (rarely in 3-leaf whorls), more or less round to broadly heart-shaped / ovate to diamond-shaped (rhombic) with the edges rolled downwards, to 6 mm long and to 6 mm wide, on a petiole to 1 mm 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 flowers are arranged singularly, in leaf axils, near the ends of branches. The petals are to 8 mm long, fused into a tube for some of their length, with flowers about 8 mm across, and mauve or bluish, often with white tips.

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

In the garden

This species has a history of being cultivated and it is grown by some APS members. There is a pink form available.

It needs moist, well-drained, acidic soil and shelter from hot sun. Sandy soils with regular moisture work best.
Can be pruned regularly to maintain size and shape. A light tip prune after flowering is the best method.

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.

rhombea – from Latin meaning leaves shaped like a “rhomboid” or “rhombic” – diamond shaped with four equal sides, for instance of a leaf, that is roughly diamond-shaped with length equal to width.

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_rhombea
https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Prostanthera~rhombea

By Jeff Howes