Prostanthera caerulea

Lilac Mint Bush

Family: Lamiaceae

An erect shrub growing to 3 metres high by about 1 metre wide.

It is found growing in heath, as well as wet sclerophyll forest in gullies, on sandy soil in rocky areas, from south-east Queensland to Wollongong in New South Wales. It occurs mainly on the coast but extends just into the tablelands subdivisions.

Branchlets/stems are often four-sided (square), densely covered with glands.

Prostanthera have simple, opposite and usually odorous leaves (a typical Lamiaceae feature). In this species, leaves are in pairs or rarely in 3-leaf whorls, narrow egg-shaped, to 65 mm long and to 25 mm wide, dark green, on petioles to 15 mm long, and with slightly to strongly toothed edges.

Flowers have a shape described as labiate (applied to all Lamiaceae flowers) with petals varying in their size, purple to lilac, produced in leaf axils. One of the identification features for Prostanthera is that the 5 calyx parts (sepals; basal whorl of the flower) are fused into 2 lips.

In this species, flowers are arranged in terminal groups on the ends of branchlets with each flower having attached bracteoles to 3 mm long at the base, but that fall off as the flower develops. The petals form a tube, to 12 mm long, and are white to vivid bluish-mauve Flowering occurs from September to November.

Fruits are mericarps (can be called nutlets). Four are produced produced at the base of the calyx.

In the garden

This species might be difficult to source but may be available at some nurseries and there is evidence online of it being available for sale. The shape and size of the leaves as well as the prolific flowering make it a very attractive plant. It is known to be cultivated.

Likely needs a well-drained soil to thrive. Plant in dappled shade rather than full sun. Prune lightly to shape after flowering. Can be shaped into a rounded shrub which will promote density and flowering.

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


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

Likely regenerates from seed after fire, possibly after 12 months. May reshoot from stems at the base.

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 sieberi, P. incisa and P. staurophylla are quite pleasantly overpowering in 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.

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.

caerulea – Latin meaning “turning blue” referring to the flower colour.

This species is not considered to be at risk of extinction in the wild.

NSW Flora Online (PlantNET) – Prostanthera caerulea profile page https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Prostanthera~caerulea

Wikipedia – Prostanthera caerulea profile page          https://en.wikipedia.org/wiki/Prostanthera_caerulea

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

By Jeff Howes. Editing and additional text by Dan Clarke