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.

Leaves are in opposite 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 (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 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 – 4 tiny nutlets (mericarps) 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.

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

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 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.

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.

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_caerulea

By Jeff Howes