A tall, graceful forest shrub (to small tree), highly variable in its habit and appearance. It ranges from a 10-metre high tree growing in sheltered forest to a 2-metre high shrub growing in exposed montane areas. It is the largest member of the genus Prostanthera.
It can be found growing by creeks and in the moist shade of dense, wet sclerophyll forests in Queensland, NSW, Victoria and Tasmania. In NSW, it grows all along the coastal and tablelands divisions and moved into the north-western slopes, from low to high altitude, in dry sclerophyll woodland and forest and well as wet sclerophyll forest to rainforest edged. They can be found as large plants on the headwaters of the Hawkesbury/Nepean River around Kangaloon, NSW, where at first glance, they resemble small willow trees.
Bark is green to dark brown to blackish, shallowly fissured longitudinally. Stems square. In large plants, the stem can be up to 15 cm diameter at the base.
Leaves in opposite pairs (rarely in 3-leaf whorls) on petioles to 1 cm long. They are dark green to green-grey, lanceolate to ovate, to 15 cm long and to 4 cm wide, and tapering to an acute apex. The leaf undersurface is paler. Leaves vary a lot in their shape, size and toothing.
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 white to pale mauve or bluish with purple to mauve spots in the throat, 10–15 mm long and wide, with a bell-shaped tube. Flowers are produced in large open terminal panicles to 12 cm long and 10 cm wide, occurring from November to March.
Fruits – 4 tiny nutlets (mericarps) produced at the base of the calyx.
This has been a popular plant in cultivation. It may depend on which form is sourced or purchased but the larger forms are spectacular in flower and well worth growing.
In a garden situation, with a good mulch and some watering. it is fast growing and will grow in light or stiff soils in sun or shade, but constant wind should be avoided.
It is also tolerant of light frost and snow. It is excellent for hiding a fence or as a tall hedge.
Plants will tolerate some enrichment from organic matter. Can be used to great effect with other shrubs and can likely tolerate heavier pruning than most of its relatives.
Usually, flowers are white but they are produced in large number in panicles, creating an attractive show. Flowers are food for native bees and wasps.
This species will also likely live a fair amount of years if happy.
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, this may give erratic results. Hence cuttings are frequently and reliably used, usually from semi-hard wood or soft tip material, which strike well in spring or autumn. In this species take cuttings of short laterals with heels in February to July; they may be slow to strike. Mist may rot cuttings.
Prostanthera lasianthos var. subcoriacea
Prostanthera lasianthos Labill. variant ‘subcoriacea’
Prostanthera lasianthos var. Typical variant
A number of variants have been identified over the years with uncertain taxonomic status. At the time of writing this profile, a botanical paper had just been published recognising the split of this species into the last four names listed above, as well as the retention of a narrower concept of Prostanthera lasianthos.
Other forms likely still persist – a mauve-pink form, known as ‘Kallista Pink’ occurs in Kallista, not far from Melbourne, Victoria in the Yarra Ranges Shire.
Other cultivars include ‘Mint Ice’, a form with variegated foliage from the Dandenong Ranges. Another tall cultivar, ‘Liffey Falls’, has a lilac flower.
Sapling wood is hard and tough but flexible and has been used for fishing rods.
There are approximately 100 species of Prostanthera, 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 hybridising 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.
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 but it likely has both abilities.
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.
lasianthos – from the Greek lasios (λάσιος) meaning ‘shaggy’ or ‘woolly’, and
–anthos (άνθος) ‘flower’, referring to the woolly flowers.