By Conrad Richter

English mint (Mentha spicata)

If ever there were a cure for brown thumbs, mint would be it. The many mint varieties available are among the easiest of herbs to grow. Novice herb gardeners rarely have a problem growing them. And the enduring appeal of mints for tea and for flavour in food make them an easy sell for spring pot production.

If there is a challenge for the commercial pot plant grower it is to find the right propagative material to start with. Like other herbs that have long been misunderstood by the industry, starting with seeds is a recipe for disappointment. Although “peppermint” seeds are widely sold by seed companies, the true peppermint, Mentha x piperita, is a sterile hybrid that cannot produce seeds. Artificial crossing of its putative parents is an exercise in futility because any seeds produced by such crosses result in diverse forms, most of which will not possess the characteristic peppermint aroma and flavour. What is commonly sold as “peppermint” seeds is in fact a rank-smelling form of spearmint (Mentha spicata) which has no value as a kitchen herb.

While commercial spearmint seeds are at least the correct species – Mentha spicata – plants grown from them have all the wrong combinations of essential oils, with, again, an undesirable rank smell and flavour.

In all cases, the most desirable varieties of mint must be grown from cuttings or by root division. For commercial pot plant production, cuttings are the favoured method.

There are dozens of varieties available, but only a few are important for commercial horticulture. They include peppermint, spearmint, orange mint (Mentha aquatica ‘Citrata’), ginger mint (Mentha arvensis ‘Variegata’), and two forms of spearmint, the improved spearmint (also known as ‘Kentucky Colonel’ mint), and english mint. Orange mint is also known as bergamot mint or eau de cologne mint. As its various names suggest, it has a sweet, fruity or floral scent and flavour that is popular in tea and potpourris. Ginger mint is not so notable for its mild, faintly reminiscent ginger flavour and aroma, but more for its attractive gold-flecked leaves. The improved spearmint and the english mint are both superior spearmint cultivars among possibly hundreds of forms found in gardens. The improved form is more disease-resistant than the standard narrow leaf variety, and the nearly round leaved english variety has a milder, more refined aroma and flavour favoured in traditional mint jelly, peas, carrots, potatoes, mint julep, and lamb sauce.

Mints are most commonly grown and sold in 4 inch pots for spring sale. They are not suitable for pack sales because they grow rootbound so quickly in cells. Mint is a quick crop from cuttings or liners, ready for sale in 4-6 weeks or less.

Propagation

Liners are available from specialty herb plug growers. Cuttings are harder to find commercially so most growers maintain their own stock plants. A single five gallon stock plant will produce dozens of cuttings a week, however the cost of maintaining enough stock plants for sizable production cuts into potential profits.

Cuttings. Stem or root cuttings can be used, but stem cuttings are easier and quicker to handle. Cuttings 3-4 inches long are taken from clean stock plants. Soaking soft cuttings in water as they are being made keeps them turgid until ready to plant. Adding a contact miticide such as Safer’s insecticidal soap to the soaking solution helps prevent pest infestation. Rooting hormone is optional as mints root readily without it.

Rooting medium. Lightweight peat-based rooting media with 20% perlite added for drainage is ideal, but mint will root in any well-drained medium. The pH should be between 5 and 7. Watering trays before sticking cuttings ensures even moisture throughout.

Trays and pots. A variety of larger cell trays from 72 trays to 128 trays work well. Growers with sufficient space under mist can root cuttings directly in finished 3 to 4 inch pots.

Temperature, humidity and light. Maintain 22-25 degree Celsius during initial rooting phase. Keep humidity elevated between 90% and 100% under mist. Provide light shade from direct sun for the first 2-3 days until rooting begins. After 7-10 days root development will be well under way and trays can be removed from misting bench and temperature lowered to 18-22 degrees Celsius and humidity to 70-75%.

Fertilizing and watering. When cuttings are removed from mist after 7-10 days, fertilize with one third strength feeding at 35 to 50 ppm. Gradually increase to half strength (50 to 75 ppm) as plugs develop.

Growing On

Mints are very quick crops, becoming saleable in 4 inch pots after only 3-4 weeks from plugs or 4-6 weeks when cuttings are stuck directly in 4 inch pots.

Transplanting. Rooted cuttings are ready for transplanting at 3-4 weeks when root balls are well developed but not to the point of being rootbound. Transplant in 4 inch pots.

Potting soil. Mints prefer a humus-rich soil mix with moisture retention ability and good drainage. Some growers prefer to use compost-based mixes with added peat and perlite.

Temperature, humidity and lighting. Mints require a nighttime minimum of 12 degrees Celsius and a daytime minimum of 16 degrees Celsius for active growth. Above 30 degrees Celsius growth slows. Normal ambient greenhouse humidity (70%) and lighting are adequate for mints.

Fertilization and watering. Raise fertilizer level from half to full strength (100-150 ppm) with successive feedings. Keep mints evenly moist as they do not tolerate drying out. Excessive drying out will cause foliage to turn yellow.

Spacing and pinching. After potting, plants can remain pot-to-pot up to 4 weeks depending on variety and growth conditions. The central stem should be pinched back to encourage bushy growth. If plants become crowded they must be pruned back or spaced. Stolons should be cut back to prevent plants from growing into each other. Ideally plants should more or less fill the pot with foliage. There is a window of 4 weeks when mints must be moved out or they will become tangled and overgrown.

Pests and diseases. Whiteflies, spider mites, aphids and thrips are the main pests. Because herbs are edible plants there are few available controls when outbreaks of pests occur. Close monitoring of pest populations is the first and most important line of defense. Weekly examination of the crop with the aid of yellow and blue sticky traps will catch outbreaks at the earliest stages when acceptible contact pesticides such as Safer’s insecticidal soap are most effective. Other products suitable for application on edible plants introduced in the past 5-10 years such as neem-based (e.g. NeemAzad) and fungus-based (e.g. Naturalis-O) sprays are slower acting and may not provide effective control on fast crops like mints. Beneficials such as whitefly parasites and mite predators are not effective on fast growing crops. Mints are susceptible to a rust for which there is no acceptable control, but symptoms (brown spots or patches on leaves) do not appear until mid summer. If rust appears on greenhouse plants, discard them and try a more resistant variety (the standard spearmint is more susceptible than the improved spearmint or the english mint). Various wilting diseases are known to occur in production fields but are rarely seen in greenhouses.

Mints can be a fast and easy profitable crop. Growers should be able to produce a nice crop on their first try.

Originally published in Grower Talks, June 1999 (Vol. 63, No. 1).