Mint (Mentha spp)

How well established is this crop?

Initial trial plantings of these crops may have been done. There is little information on plant suitability to the region and no current information on markets, commercial returns or current research. This is a new crop.

Background

Although there are many different types of mint, the two covered in this datasheet are peppermint and spearmint.

Peppermint is a sterile hybrid, grown vegetatively from root pieces. It grows to about 1m tall and has a purplish stem and some purple tinges on the leaves, particularly if it is water stressed. The leaves are attached to the stem with a petiole. Peppermint grows a lilac-pink flower spike that opens in the middle of summer.

Spearmint is a very similar looking plant to peppermint. It is also grown vegetatively from root pieces and has a purplish stem. The difference however is that the leaves are attached directly to the stem with no petiole. It also flowers in mid summer but the flowers are a more lilac colour.

Both peppermint and spearmint can be grown for their essential oil. Peppermint is also dried and used in teas and for some medicinal uses. Spearmint is a common ingredient as a fresh herb in cooking.

Climate

Most mint production in the Northern Hemisphere occurs at a similar latitude to Otago. Research has shown that warm days and cool nights are ideal conditions from which to harvest plant material for good quality oils.

Winter Chilling and Frosts

Mints are relatively tolerant of frosts during the growing season. However American research suggests that if frosts get much over -5°C for any period then damage to the crop is likely.

Rainfall and Irrigation

Water is essential during the establishment phase of a mint block. Regular rainfall or irrigation will ensure the crop keeps growing. Water stress during any phase of the plant's life can lead to infection from fungal diseases.

Wind

Even though mint is a relatively low growing plant, shelter from the wind is advisable. It will reduce the amount of damage to the plants and subsequent disease problems. Be careful with design to allow the plant's to get the maximum amount of sunshine during the growing season.

Soil

The key physical requirement of the soil for mint growing is a free draining root zone. Wet roots can lead to fungal diseases.

Fertility

Mints do not need high levels of soil fertility to grow well. Targeted applications based on soil and foliage tests are recommended. Aim for a soil pH of between 5.5 and 6.5.

Weed Control

Having a clean bed at planting is advisable as this crop is grown on a broad acre basis. Weed control is very difficult after planting because no chemicals are registered. Weeds to watch out for include thistles, couch, yarrow and docks.

Varieties

The three man varieties of peppermint include Black Mitcham, Todds’s Mitcham and Murray Mitcham.

Spearmint does not have the same selection of bred cultivars, but a variety Mentha spicata var crispata has curly leaves and is ideal for the fresh culinary herb market.

Pests and Diseases

Mints suffer from few insect pests because of their repellent smell. The main problem with the crop is the susceptibility to the fungal disease rust (Puccinia menthae). Another fungal wilt disease Verticillium dahliae is a major problem on mint crops in the USA, but has not been recorded on New Zealand crops at this time. Both these diseases can be controlled using crop rotation and fungicides.

Layout

Mints are planted on a broad acre basis from vegetative root pieces. These are collected from a previous crop by ‘discing’ the ground to cut the roots into 10-15cm lengths, then using a potato type harvester to lift them. They are then kept moist before being distributed over the ground and ‘turned in’ to plant. Approximately 600kg of stolon root pieces are needed to plant 1 hectare.

General Management

Once planted, watching for water stress and fungal infection are the main tasks. Weed control can be done by hand. When the crop begins to die down in the autumn a desiccant is advised to kill all crop residues and reduce the disease load from rust spores from one season to the next.

Harvest

Harvesting tends to be done by machinery.

If harvesting for dried herb production, the crop is cut just before the flower buds appear. Two-three cuts should be possible per season, especially in climates such as Central Otago. Drying of the material should occur as soon as possible after this time at temperatures no greater than 40°C. If the temperature gets higher, the volatile nature of the flavour can be lost.

If harvesting for oil the crop should be cut when approximately 20% of the flowers on the head have opened. The crop is left in the field to dry, then gathered up and steam distilled to extract the oil. International standards have been developed to grade the oil. Class 1 peppermint oil must contain 1.0% essential oil, while spearmint must contain 0.4% essential oil.

Equipment

The size of the block will have a large impact on the machinery requirements. A tractor and spray equipment will be needed as well as bins for harvested material, a drier steam distillation unit etc.

Returns

It is very difficult to estimate returns for New Zealand grown mint. It is grown as a commodity product in the USA so finding niche markets will be the key to success.

Contacts

Crop and Food Research Ltd Broadsheet # 18: www.crop.cri.nz