Prostanthera - The Mint Bushes
Norm McCarthy
It is indeed opportune to remind ourselves of the remarkable qualities of our native mint bushes. Stemming from the family Lamiaceae, prostantheras are purported to reach or even exceed 120 species with some unnamed. The genus is under revision at present. It should be mentioned here that our mint bushes might hybridise if other species are growing in the immediate vicinity. (Several 'Poorinda', man-made hybrids and others are available at nurseries).
Prostantheras can occur as prostrate plants, small, medium or taller shrubs and rarely tree forms as in P.lasianthos. Foliage is frequently crowded and attractive on often squared stems. Leaves are alternate and opposite, mid green, not shiny, while under-leaves present a little paler.
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 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.
The leaves may be terete, lanceolate, ovate or even rotund in shape and relatively small in size e.g. 25mm x 10mm for ovate, with 25mm x 5mm for lanceolate forms. However, while comparatively small individually, leaves are normally multitudinous.
Inflorescence
Flowering occurs from July to January in sun or part shade according to species. Flowers may occur as single axillary flowers or as terminal racemes or panicles. The calyx is two lipped and may be hairy, the corolla is two-lipped, in many shades of the spectrum; upper lip is two lobed while, the lower lips is three lobed, stamens four. Flowers can be somewhat tubular in large numbers. There are two forms of flowers within the genus. The Klanderia section includes P.calycina, P.aspalathoides and P.porcata. These species have long thin tubular flowers and are visited by bugs and insects. Prostantheras such as P.ovalifolia and P.cuneata have more regular tube shaped flowers that are insect attractive. While the latter regular flowered species contain a myriad of colour forms, flowers are blue to purple, pink or white. The Klanderia section is mainly pink, red or orange.
It must be mentioned here with great reverence to the unstinting lifetime dedication with Prostantheras of George W. Althofer and his brother Peter (both sadly deceased). Without their extensive knowledge Australian Plants would be much the poorer. It was my great privilege on several occasions to spend time at Wellington and Burrendong Arboretum (central west of New South Wales), which George and Peter created for the posterity of Australian native plants. The book Cradle of Incense is a fitting tribute to George as a man of vision. The 'Magic Circle' mentioned in this book relates to an area within a radius of 150 km of Wellington which contains some thirty-seven species and forms of mint bushes which are chronicled by this great man as a result of his great love of native plants.
Naturally, I have diverged back to history with good reason. 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 it may be a challenge to grow some species. Certain species are found thriving in seemingly impossible situations but may be short lived.
| Photos: Horst Weber, Brian Walters |
Plants may be grown from fresh seed, however for ease of reproduction, cuttings of half hard material strike well in spring or autumn. For rare species it may be necessary to graft onto suitable rootstock. Prostanthera magnifica and P.megacalyx really need this treatment. Rootstocks recommended are Westringia fruticosa and Westringia glabra. Do not be deterred! Many prostantheras grow readily from cuttings and are long lived. Why not try some! Some plants closely associated with Prostanthera are Hemiandra, Hemigenia, Westringia, and Wrixonia.
Some thoughts on growing prostantheras
- Mints prefer acid soils (Ph 5.5 to 6.5), though some like alkaline situations.
- Do not crowd. Allow for aeration to avoid mould.
- Good drainage is essential. Raised beds ensure this.
- Water new plants until established, weekly or as required.
- Do not over water! It can incur root rot and fungal infestation.
- Mints prefer moist root runs. Plant drooping is an indicator of dryness.
- Mulch to reduce evaporation.
- Pinch prune new plants for steady compact growth.
- Do not prune plants in bud.
- Prune flowers and stems judiciously to encourage new growth after flowering season.
- Fertilise sparingly. Use only native plant Osmocote or similar. Beware of phosphorus-based fertiliser. Seaweed emulsion used monthly is a good growth promoter. Diluted Aquasol may be an option.
From "Native Plants for New South Wales", the newsletter of the Australian Plants Society (NSW), January 2007.
Australian Plants online - 2007
Association of Societies for Growing Australian Plants
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