Date: Aug 2007 Author: Doug Hocking
The mints belong to the genus Mentha in the family Labiatae (Lamiaceae) which includes other commonly grown oil-yielding plants such as basil, sage, rosemary, marjoram, lavender, pennyroyal and thyme. Within the genus Mentha there are several different species, varying in their appearance, aroma and end use. The most common ones are spearmint (M. spicata), peppermint (M. × piperita), eau-de-cologne mint (M. × piperita var. citrata) and apple mint (M. rotundifolia). All are low-growing plants, readily sending out runners, or stolons, which develop new roots and shoots at the nodes. Under good growing conditions, stems will generally reach 1 m in height.
Spearmint (M. spicata). This is the most common mint grown commercially in New South Wales as well as in home gardens. Leaves are smooth, bright green and elongated with a pointed end. Flowers are a pink to lilac colour and grow in clusters on the ends of the stems.
Peppermint (M. × piperita). This is a low-growing plant that has small, pointed, dark green leaves with a purplish tinge. Peppermint is the most commonly grown species for oil production.
Eau-de-cologne (M. × piperita var. citrata). This mint has a very strong, sharp perfume. It has smooth green, oval-shaped leaves that are tinged with purple.
Apple mint (M. rotundifolia). Not a commonly grown mint in New South Wales, it is very flavoursome and characterised by its strong apple taste and perfume. The leaves are light green, soft and downy, with a rounded shape.
When identifying mints, remember that all varieties will cross-pollinate, making sorting them out difficult. If varietal purity is to be maintained, each one must be grown in isolation.
The mints will grow in a wide range of climates as shown by their popularity in home gardens all over Australia. Ideally, they require plenty of sun, growing best in the long midsummer days of the higher latitudes. For this reason, the Australian mint industry has developed mostly in Tasmania, particularly for oil production. Ideal growing temperatures for mint are warm sunny days (25°C) and cool nights (15°C). This is why, in the hotter climates, mint generally grows better in the more shaded areas of the garden.
Mints do best in deep, rich soils of friable texture high in organic matter. The preferred pH range is from 6.0–7.5. A high water requirement means that soils must be deep and well drained while holding plenty of water.
Mint can be propagated either vegetatively or by seed. Vegetative propagation is achieved by digging up plants in late winter–early spring and dividing them into runners with roots, then replanting. This will prevent the plants from becoming root-bound and prone to disease, ensuring strong, healthy plants for the new season.
Planting distance will vary with the type of mechanical equipment used to cultivate and manage the crop. A suitable row spacing is 50 cm with runners planted 10 cm apart within the rows. Using this system, three rows can be planted to a raised bed. As plantings develop, rows will become a continuous mass of mint.
Mint requires a well-balanced nutrition program. Experience has shown that an annual dressing of animal manure will supply a good balance of major and minor elements. Care should be taken not to supply excessive amounts of nitrogen. Approximately 10 t/ha of good quality fowl manure applied midwinter will provide a reasonable nutrient program. If soil pH drops below 6.0 it may be necessary to apply dolomite or lime to raise the pH to the desired level.
For maximum production, mint requires large amounts of water compared with other crops. To keep soil moist during periods of high evaporation, plantings should be irrigated at least twice a week. During the growth period in summer, plants can require up to 1500 mm of water.
Weed control in mint crops is important to ensure that there is no contamination by foreign plant material at harvest. The selection of planting areas with low weed populations and a good kill of weeds prior to planting is important. Due to the lack of herbicides registered for use in mint in New South Wales, hand weeding is the most effective form of weed control.
Mint rust is a serious disease that attacks the common mint species. The use of disease-free planting material and a sound rotation with other crops will help control it. Rust affects the leaves and shoots and if not controlled will quickly defoliate the plant. Plants can be flamed with a propane gas burner in spring to destroy diseased material before new shoots appear.
Mint can be attacked by a wide range of pests. The main ones are loopers, leafrollers, slugs, snails and aphids.
The intensity of flavour and aroma in the mint plant is dependent on the level of essential oil in the plant. Oil content is at its maximum at the commencement of flowering. Harvesting is best done early in the morning when the plants are turgid and before any temporary wilting occurs.
If mint is to be fresh it is best cut with shears or a sickle bar mower and bunched. It should then be kept moist, and cooled prior to marketing.