How to Grow Mint in Australia

Mint (Mentha species) is a hardy perennial herb in the Lamiaceae family, grown for leaves that go into tea, sauces, salads and cold drinks. One pot picks for years anywhere from Darwin to Hobart. The mistake most gardeners make is planting it in the ground, where its runners take over the bed.

Three mistakes that matter

1. Planting mint in the ground. Shallow runners cross a bed and root wherever a stem touches soil. Grow it in a pot on paving, or sink a bottomless pot with its rim above the soil (see Site, soil and setup).
2. Expecting seed to give a named mint. Seed-raised mint varies, and peppermint, a sterile hybrid, rarely sets seed. Sow spearmint for the kitchen, then multiply the best-smelling seedling from cuttings (see Sowing Seeds).
3. Leaving it uncut. An unpicked clump runs to flower, then turns sparse and woody. Cut it hard after flowering, and split the clump every couple of years (see Cutting Back).

When to plant

Mint seed germinates in soil of 18°C to 24°C: a spring sowing over most of Australia, a dry-season one in the tropics. One sowing is enough for a perennial; add plants from cuttings, not fresh packets (see Sowing Seeds).

Where soil is still cold in the window, start trays indoors on a seedling heat mat and plant out after the last frost. From the second year, pick from the spring flush until autumn dieback, or through winter where frosts are light.

Site, soil and setup

Mint wants rich, moist soil and 4 to 6 hours of sun, though it manages on 3. Full sun grows the most leaf where the soil never dries; afternoon shade suits inland gardens and the hot north.

The other honest way is to let it run where you can pull the surplus out by the armful, for the compost or the chooks.

Container growing

Give one plant a pot at least 30 cm across, about 13 L, of premium potting mix with a few handfuls of compost. Pennyroyal (Mentha pulegium), a 5 to 15 cm mat, suits a wide, shallow bowl instead.

• Water: check daily in warm weather and water when the surface feels dry.
• Feed: seaweed solution at half strength, 15 mL per 9 L, once a month in the growing season.
• Position: morning sun with afternoon shade in summer, full sun in the cool months.

Sowing Seeds

Start with spearmint seed, the mint that grows true enough from seed, or with a cutting from a plant you already like.

Peppermint (Mentha x piperita) is a cross between spearmint (Mentha spicata) and water mint (Mentha aquatica). Seed sold under its name gives plants that vary in scent, flavour and habit. Wild mint seed is the European horse mint (Mentha longifolia), which runs like the rest. Korean mint (Agastache rugosa) and mountain mint (Micromeria thymifolia) share the name, not the genus, so follow their packets.

Cutting propagation

A tip from a non-flowering shoot, cut in spring, roots readily in a jar of water, then goes into a 7.5 cm pot. A piece of runner with a node roots at any time of year. Both copy the parent exactly, which seed cannot. See how to take cuttings and rooting cuttings in water.

Starting under cover

• Press the seed onto moist seed-raising mix and leave it uncovered, because it needs light.
• Mix the dust-fine seed with dry sand to spread it thinly (see sowing tiny seeds).
• Keep the tray warm (see When to plant) and evenly moist; seedlings show in 10 to 21 days.
• Prick out into 7.5 cm pots once each seedling has several true leaves.

Sowing direct

Once the soil is warm (see When to plant), scatter seed on a raked, moist surface, press it in and keep it damp.

Transplanting

• Harden off (accustom seedlings gradually to outdoor sun and wind) for a week first.
• Plant out at 5 to 10 cm tall, 30 to 45 cm apart, or one to a 30 cm pot.
• Water in with seaweed solution at half strength, 15 mL per 9 L.

Care by growth stage

Moisture decides this crop: mint grows where the soil never dries, and a pot dries fastest. Feed lightly, because very fertile soil grows lush leaves with little flavour.

Mulch beds and pots with 5 cm of compost or pea straw, clear of the crowns. If growth stalls in poor soil, a light dose of all-purpose controlled release granular (NPK around 14-5-10) scratched into the soil can help.

Cutting Back

Leaf oil peaks just before flowering and falls once the spikes open, so cutting back follows the flowers. Nectar-rich mint flowers feed bees, a fair reason to let one pot bloom.

• Kitchen pots: nip off flower spikes as they appear, so the plant keeps making leaf.
• After flowering: shear the whole plant to about 5 cm above the base, then water and feed it (see Care by growth stage).
• Late autumn: in cool districts the clump dies down; cut the old stems to the ground before new shoots push in spring.
• Every couple of years: tip the pot out, quarter the clump and repot the best pieces in fresh mix, as dividing perennials describes.

Companion planting

Mint's companions share its bench, not its pot: the pairings below are about matching conditions.

Grows well with

• Parsley, which likes the same moist, fertile soil and light shade.
• Coriander in spring, which needs the same afternoon shade and daily water.

Keep apart

• Basil, which mint's runners crowd out of a shared bed or pot.
• Thyme, whose roots rot in the damp soil mint needs.

Harvesting Sprigs

Start picking 8 to 12 weeks after sowing, once the plant has several stems; young shoot tips carry the best flavour.

• Cut sprigs with scissors just above a leaf pair, so two shoots follow from the joint.
• Pick often: frequent cutting brings new growth, and new growth has the most flavour.
• For drying, cut whole stems as the first flowers show, when leaf oil is highest.

A bunch keeps 3 to 7 days in water on the bench, or about a week wrapped in the fridge. Freeze rather than dry, since freezing holds more flavour; dried mint keeps a year airtight.

Use the leaves in tea, in mint sauce for lamb, in tabbouleh and in a jug of something cold in summer.

Common problems

Seed that never comes up. Mint seed is dust-fine and needs light, so buried seed, or seed sown into cold mix, rots. Read more

Runners in the lawn or the next bed. Stems root where they touch soil and runners creep under a shallow barrier; cut strays off and lift them whole.

Weak flavour. Very fertile soil and heavy feeding grow lush leaves with little oil, and flavour fades again once the plant flowers (see Cutting Back).

Pests and diseases

Mint's pests are the usual sap-suckers, and its one serious disease is rust.

Mint rust (Puccinia menthae). Pale, distorted spring shoots, then dusty orange pustules under the leaves, later yellow or black, and leaves drop. The fungus winters in the runners, so remove infected shoots before the black spores form and restart from clean cuttings elsewhere.

Powdery mildew (Golovinomyces biocellatus). A white, floury coat spreads over leaves in crowded pots and still, humid air; cut out badly affected stems, and space the pots further apart.

Green peach aphid (Myzus persicae). Small green, yellow or pink insects cluster on soft tips, curling new leaves and leaving honeydew; hose them off or spray insecticidal soap. Read more

Two-spotted mite (Tetranychus urticae). Fine pale speckling, then bronzed, dry leaves, shows mites feeding underneath, breeding fastest in warm weather. Read more

Greenhouse whitefly (Trialeurodes vaporariorum). A cloud of tiny white insects lifts when the plant is brushed, with honeydew and sooty mould below. Silverleaf whitefly (Bemisia tabaci) replaces it in warm regions. Yellow sticky traps and insecticidal soap under the leaves control both. Read more

Prevention first

• Check the leaf undersides on any plant you bring home, because rust travels in cuttings and divisions.
• Space pots so air moves between them, and water the mix rather than the leaves.
• Turn a few leaves each week to catch aphids, mites and whitefly while a hosing still works.
• Skip broad-spectrum sprays, which kill the ladybirds and lacewings that eat aphids and let mites flare.

Tips

1. Keep in-ground mint a little drier than a potted one, which slows its spread.
2. Sow pennyroyal only between pavers or in a bowl, where the scent releases underfoot and the mat cannot run.
3. Native pennyroyal (Mentha satureioides) is a low native mint for damp ground, and it spreads less. Some populations carry the same pulegone as pennyroyal, so keep it out of the kitchen.