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Issue 17

The Mint Bush Part 1

Extracts From the 'Australian Plants Online' site. Visit the ASGAP site at: www.farrer.riv.csu.au/ASGAP/sgap.html

The mint bush family, known as the Lamiaceae (formerly the Labiateae) is widespread throughout the world and contains a number of well-known, commercially cultivated plants which are used in cooking and for perfumes.

Worldwide, the family comprises over 200 genera and over 3000 species. There are about 20 Australian genera in the Lamiaceae, the ones of interest here being:

- Mentha; about 6 species native to Australia

- Prostanthera; about 60-70 species, all confined to Australia

One of the most commonly cultivated Australian member of the family is Prostanthera (known as "mint bushes").

Characteristics

Members of the Lamiaceae are generally herbs and small to medium shrubs. Many members of the family have aromatic foliage due to the presence of volatile oils and it is these oils which give exotic members of the family their characteristic taste and aroma which is so valuable in cooking. In Australia, the genus with the most pronounced aromatic foliage is Prostanthera (although not all species are aromatic). The foliage of many prostantheras emits a pleasant aroma when crushed or brushed against. In the Australian bush the aroma of mint bushes can be very pronounced following rain. Despite the relatively high oil content of the foliage of many Australian members of the family, littie use has been made of either the leaves or the oil. Some species are used by bushfood enthusiasts as flavourings in cooking in the same way as their exotic counterparts (eg. Mentha australis, Prostanthera rotundifolia).

Cut leaf mint - Prostanthera incisa - photo Jan Tilden

Most species are found as part of the understorey in open forests and woodlands. A characteristic of the family is a "two-lipped corolla" - in which the five petals are united into two upper lobes and three lower lobes giving the appearance of two lips. Although not a unique characteristic, it is one of the easiest features to help the average person in identification.

Propagation

Members of the mint bush family are usually grown from cuttings as many of the most horticulturally desirable species strike readily.

Seed

Prostanthera seed can be unreliable and fresh seed usually gives the best results in these cases. Pretreatment of seed prior to sowing to improve germination is not normally beneficial with members of this family. However, a method that has been successful for at least some species is the use of smoke or "smoked water" as a pretreatment. This has been reported to be successful in the germination of species of Hemigenia and may be worth considering for other genera.

Cuttings

Most shrubby members of the Lamiaceae strike readily from cuttings using hardened, current season's growth. Cuttings about 75-100 mm in length with the leaves carefully removed from the lower two-thirds seem to be satisfactory. Treating the lower centimetre with a "root promoting" hormone both seems to improve the success rate but is often unnecessary. Members of this family serve as a good introduction to those starting out in propagation from cuttings as success rates are usually high.

Division

Some members of the family (eg. Mentha) produce suckers from their root systems. These can be cut from the parent plant (ideally retaining some of the roots) and potted into individual containers. If placed in a sheltered location and kept moist, these should develop quickly into new plants.

Grafting

Considerable work has been carried out on grafting with Prostanthera to improve the hardiness of the genus in areas where they are difficult to grow. Most species of Prostanthera are compatible with a root stock of Westringia .fruticosa, a very reliable plant suited to many districts. Grafting of Prostanthera species to Wftuticosa is not difficult and, like cuttings, is a good project for those starting out in grafting. A number of other westringias (eg. the cultivar "Wynyabbie Gem") have also been successfully used as rootstocks for Prostanthera species. Little or no work has been done on grafting of other genera of the Lamiaceae. There is plentty of scope for experimentation for the keen amateur.

Cultivation

Of the Australian members of the Lamiaceae, only Prostanthera and Westringia are in widespread cultivation. All species that are in general cultivation perform best in well-drained, moist soils but they rarely succeed in continually wet soils. Generally they are at their best in light shade such as in the dappled light of eucalypts. Prostanthera species should be selected that are native to a climate similar to that where they are to be grown. In humid, summer-rainfall areas, avoid species from drier climates.

Mint bushes are usually quick growing, however, they may be past their best after 6-8 years.

Mint bushes often wilt noticeably when moisture is lacking in a garden. They are excellent "indicator" plants because of this.

They are not demanding as far as fertilizing is concerned but

they do respond to applications of slow release fertilizer applied after flowering.

The Prostanthera and Westringia Study Group

The Prostanthera and Westringia Study Group is one of over 20 such Groups whose aims are to further knowledge about the cultivation, propagation and conservation of specific Australian plants. This Study Group produces informative newsletters describing members' experiences in growing and propagating mint bushes and westringias in various parts of Australia and overseas as well as documenting botanical changes in the genus resulting from research carried out in a number of scientific institutions. Members of the Group are mainly keen amateurs with no formal horticultural or botanical knowledge, although a number of professionals in those fields also participate. As in all study groups, the members' work is mainly carried out in their own homes and gardens and in their own spare time.

Further information Most books dealing with Australian native plants will contain useful information on the botany and horticulture of Prostanthera, Westringia and other Lamiaceae. Some of the most detailed references are listed below. Althofer, G (1978), Cradle of Incense, Society for Growing Australian Plants. -Elliot, R and Jones D (1980-1997), The Encyclopaedia of Australian Plants, all volumes, Lothian Publishing Company Pty Ltd, Melbourne. -Wrigley, J and Fagg, M (1996 -4th ed), Australian Native Plants, Collins Publishers Australia. Several issues of the Society's journal "Australian Plants" are particularly useful for those interested in Australian Lamiaceae, in particular: Vol 10, No.83 June 1980; Entire issue devoted to Lamiaceae. In addition, the article "Smoke Stimulates the Germination of Many Western Australian Plants" by K.Dixon and S. Roche contains useful information on research into the use of smoke to improve germination.

The Society for Growing Australian Plants

is a non-profit organisation dedicated to the growing, conservation, promotion and appreciation of Australian native plants.

ASGAP is made up of seven independent, non-profit, Regional Societies, one in each of the six Australian States and the seventh in the ACT. Individual membership is through the Regional Societies. In total, the combined membership of the seven Societies numbers about 9000. The Society's activities are wide ranging and include special interest Study Groups and support of research through the Australian Flora Foundation. The Society is also involved in the publication of a range of practical and educational books on Australian plants.

For further information, email

The Association of Societies for Growing Australian
Plants: sgap@ozemail.com.au