An openly-branched shrub to 2 metres tall by about 1 metre wide.
It is found in rainforest, as well as dry 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.
Prostanthera have simple, opposite and usually odorous leaves (a typical Lamiaceae feature). In this species, leaves are 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 are mericarps (can be called nutlets). Four are produced produced at the base of the calyx.
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.
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.
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 sieveri, 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 snapped basal stems.
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.
This species is not considered to be at risk of extinction in the wild.
NSW Flora Online (PlantNET) – Prostanthera rhombea information page https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Prostanthera~rhombea
Wikipedia – Prostanthera rhombea information page
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