How to Grow Coriander in Australia

Coriander (Coriandrum sativum) is a fast annual herb in the carrot family, Apiaceae. One packet gives three crops: leaves, roots and seed. It grows best from autumn to spring, and the usual failure is a plant that runs to seed before you have picked much.

Three mistakes that matter

1. Sowing into summer heat. Plants can flower within four to six weeks of a midsummer sowing, before they make much leaf. Sow inside your zone's window instead, and keep a pot going in the shade.
2. Planting punnet seedlings. Coriander grows a long taproot that resents being moved, and the setback sends plants straight to flower. Sow where the plants will grow, or into cells that plant out whole.
3. Letting the soil dry out. A dry spell brings on bolting and turns the leaves bitter. Keep the soil evenly moist, which in warm weather can mean watering pots every day, and mulch beds.

When to plant

Coriander germinates in soil between 10°C and 25°C, and grows well in air temperatures of 10°C to 30°C. Sow a short row every 2 to 3 weeks through your window rather than one long row.

Site, soil and setup

Coriander wants free-draining soil and full sun through the cool months. Where a sowing will mature into warm weather, give it morning sun and afternoon shade instead, such as the east side of a fence.

Keep fresh manure and strong fertiliser away from the seed row, because both can burn young roots and thin out germination. Saturated ground brings on leaf disease, so drainage matters more here than richness.

Container growing

A pot at least 30 cm across, holding 10 to 15 L, dries out more slowly than a small one and gives the taproot room. Fill it with quality potting mix blended with compost, up to half and half. Potted plants run to seed sooner than plants in beds, so pick young and sow often.

• Water: check daily and water when the surface feels dry, early morning or evening.
• Feed: fish and seaweed blend at half strength, about 12 mL per 9 L, each fortnight, because nutrients wash out of pots quickly.
• Thin: 5 to 10 cm apart, since potted plants are cut young.
• Varieties: 'Calypso' and 'Santo' hold longest before bolting, and 'Confetti' has fine leaves for cutting young.

Sowing Seeds

Sow direct where slugs, snails and heavy rain allow. Where they do not, start in cells that plant out without disturbing the roots.

Sowing direct

• Sow 6 to 10 mm deep, about 5 cm apart, in drills 20 to 30 cm apart.
• Cover the seed, firm the soil lightly and water gently so the seed stays put.
• Expect seedlings in 7 to 21 days, sooner in mild soil than in cold soil.
• Thin to 15 to 20 cm once seedlings can be handled, snipping the extras at soil level and eating them.

Starting in cells

• Sow into individual cells or small biodegradable pots, at the same depth as in the ground.
• Harden off (gradually accustom plants to outdoor sun and wind) before planting out, as hardening off seedlings describes.
• Plant out at 5 to 10 cm tall, late in the day or under cloud, without breaking the root ball.

Care by growth stage

Watering decides this crop. Feed lightly, keep the bed evenly moist, and expect pots to need water about twice as often as a bed in the same weather.

That 25 mm a week is 25 L per square metre, from rain and watering together. Spread straw mulch between plants once they reach 5 to 10 cm, and top it up as it breaks down.

Bolt Management

Bolting is this crop's clock, and it starts ticking at sowing. Heat above about 25°C to 30°C, long days, dry soil and root damage all bring flowering forward. A bolting plant stretches upward, and its new leaves come through fine and ferny instead of broad and lobed. Leaves can turn strongly aromatic just before the stalk shows. In hot weather, pull the whole plant rather than wait for more leaf.

• Shade: give spring sowings morning sun and afternoon shade.
• Succession: sow a short row or a pot every 2 to 3 weeks, so new plants are ready as older ones bolt.
• Selection: 'Calypso' can bolt about three weeks later than 'Santo', which is slower than standard coriander. Neither holds once days turn hot and long.
• Once a stalk rises: pick the broad leaves, dig the roots for the kitchen, or leave the plant to flower and set seed.

Companion planting

Left to flower, coriander feeds hoverflies and small parasitic wasps, whose young eat aphids on nearby crops.

Grows well with

• Cabbage and broccoli, cool-season crops that aphids find first.
• Sweet alyssum, a low flower that adds nectar without shading the row.

Keep apart

• Fennel, which reaches 2 m and self-seeds readily, crowding and shading low coriander.

Harvesting Leaves

Start picking when plants stand 10 to 15 cm tall with several broad leaves, usually 5 to 10 weeks from sowing. Take the outer stems and leave the centre growing.

• Cut outer stems at the base with scissors, leaving the centre to keep growing.
• Take no more than a third of a plant at a time, then let it recover for about two weeks.
• Pull or dig whole plants for their roots before the flowers open, as the roots toughen afterwards.
• Cut seed heads into a paper bag once most seeds have turned light brown, before they drop.

Leaves keep about a week in the fridge, wrapped loosely in a bag or with the stems standing in water. Dry seed heads on paper in a warm room for about a week, then rub the seed out and store it airtight.

Use the leaves in hot dishes and fresh salads, the roots in Thai and Vietnamese cooking, and the ground seed in curries.

Common problems

Slow or patchy seedlings. Cold soil, seed sown too deep or a dry surface crust all delay or thin out emergence. Read more

Leggy seedlings. Trays kept indoors or on a dim windowsill stretch towards the light and grow long, weak stems. Read more

Stunted plants in salty ground. Coriander is sensitive to salt, and growth slows as soil salinity rises, so salty bore water can hold a crop back.

Pests and diseases

Few pests trouble coriander, so a weekly look at the new growth and around seedlings catches most trouble early.

Insects and molluscs

Green peach aphid (Myzus persicae). Colonies of small green, pink or orange insects suck sap from new growth, curling the leaves and leaving sticky honeydew that turns to sooty mould. Read more

Grey field slug (Deroceras reticulatum). In higher-rainfall southern Australia it feeds on mild, damp nights, shredding leaf edges, and can clear a row of seedlings overnight. Read more

Common garden snail (Cornu aspersum). Widespread outside the tropics and desert country, it breeds for most of the year and chews leaves and seedlings, leaving silvery trails. Read more

Diseases

Bacterial leaf spot (Pseudomonas syringae pv. coriandricola). Water-soaked spots dry to tan patches with purplish or yellow edges, and the seed-borne bacteria spread to nearby plants in rain and sprinkler splash.

Damping off (Pythium and Rhizoctonia species). This soil-borne seedling disease stops seedlings emerging or collapses them soon after, most often where they are crowded, overwatered or short of air.Read more

Prevention first

• Sow clean seed thinly, and bin plants that show leaf spot rather than composting them or saving their seed.
• Water the soil rather than the leaves, and avoid leaving seedlings in soggy soil.
• Hose small aphid colonies off with a fine, sharp spray, or use insecticidal soap on heavier infestations.
• Clear boards, pots and debris where slugs and snails shelter, and check seedlings by torchlight on damp nights.

Tips

1. Pick a few seed heads while still green, as the unripe seed has a milder flavour than the brown spice.
2. Freeze surplus leaves rather than drying them, because dried coriander loses much of its flavour.
3. Sow a pot for the shade in summer and a row in the open bed in autumn, so something is always ready to pick.
4. Where heat and humidity defeat coriander, as in a tropical wet season, grow sawtooth coriander (Eryngium foetidum), a perennial with a similar flavour.