When to sow coriander

Coriander germinates once the soil has reached about 50°F, so outdoor sowing starts in April in most of Central Europe and the UK. From then until August you can keep sowing. The trick is to sow a short row every three to four weeks rather than one big patch: each sowing gives you a few weeks of good leaves before the plants run to flower.

Midsummer is the hardest time. Long days, heat and dry soil all push coriander to flower early, so sowings in June and July often bolt fast. Either skip them or sow in a spot that gets shade at midday. Sowings in August and September, under a cloche, fleece or in a cold greenhouse, give tender leaves well into autumn.

If you want coriander seeds, sow as early as the soil allows, from late March to April. The plants then have the whole summer to flower and ripen their seeds by August or September.

Sow coriander where it is going to grow. It forms a long taproot early on and sulks when moved, so direct sowing in the bed, or straight into the pot it will stay in, works far better than raising seedlings to transplant.

Pick a spot with free-draining soil that doesn't sit wet. For leaves, sun to light shade is fine, and in summer some shade at midday helps. For seeds, give it full sun and a little shelter from wind, because the flowering stems are thin and flop over easily. Coriander is a light feeder: a thin layer of compost is plenty, and a rich bed gives lots of leaf with less aroma.

Draw a drill about 0.4 in deep, a little shallower on heavy soil, and sow the round seeds thinly. Keep the soil evenly moist. Germination is slow and uneven: expect one to three weeks, depending on warmth.

For leaves, thin the seedlings to 2 in apart in rows 8–12 in apart. A 120 × 80 cm raised bed filled that way holds about 96 plants, far more than most kitchens need, so one row across the short side, 16 plants, is a good sowing every few weeks. For seeds, thin to 8 in apart; three rows along the bed then hold 18 plants.

Coriander's natural urge is to flower. As the days lengthen and the weather warms, it sends up a tall stem with fine, feathery leaves and white umbels, and the broad leaves at the base stop coming. That is bolting, and in hot, dry summers it can happen within a few weeks of sowing.

You can't stop it, but you can delay it. Water in dry spells so the soil never dries out completely, give summer sowings some shade at midday, and choose a slow-bolting leaf variety. Keep picking regularly: plants that are cut often stay leafy a little longer.

Once a plant has bolted, the leaves taste harsher. Either pull it and sow again, or leave it to flower: hoverflies and other insects visit the umbels, and a few weeks later you can harvest the seeds.

Start picking leaves once the plants are about 4–6 in tall, typically six to ten weeks after sowing, sooner in warm weather. Cut the outer stems a little above the base and let the centre keep growing. Young leaves have the best flavour; once the feathery flowering leaves appear, the harvest is winding down.

Coriander seeds ripen about three to four months after sowing, usually in August or September. They turn from green to beige-brown, and they ripen quickly and all at once, so check often. Cut the stems when most seeds have coloured, ideally in the morning when fewer drop, and put them straight into a paper bag.

Spread the seed heads on paper in a warm, dry room for about a week, rub the seeds off and store them in an airtight jar. The dried seeds taste warm and citrusy, nothing like the leaves. Keep a handful back: they are next year's sowing.

All coriander gives both leaves and seeds, but varieties are bred for one or the other. Leaf varieties such as 'Calypso', 'Cruiser' and 'Confetti' are selected to bolt slowly and stay leafy for longer; 'Confetti' has fine, feathery leaves with a milder taste. For leaves, these make a real difference in summer.

Seed varieties such as 'Jantar' flower readily and set plenty of seed. They are the better choice if coriander seed is what you're after, and a poor one for leaves.

In pots on a windowsill, coriander grows all year. Use a deep pot for the taproot, sow directly into it, and sow a fresh pot every six to eight weeks.

Slugs and snails are the biggest threat to young seedlings, especially in damp weather. Check the rows in the evening in the first weeks after germination.

Aphids gather on the undersides of leaves and on flower stems. A sharp jet of water knocks them off, and hoverflies, ladybirds and lacewings usually catch up with them.

Leaf spots and downy mildew show as yellow-brown patches, sometimes with grey-white fuzz underneath. Several of these diseases travel on seed, so buy fresh seed from a good source, water the soil rather than the leaves, and don't crowd the plants.

Coriander is an umbellifer, in the same family as carrots, parsnips, parsley, dill, celery and fennel, and shares some diseases with them, parsley in particular. Leave three to four years before coriander or another umbellifer grows in the same bed again.

Coriander is often said to keep aphids and carrot fly away with its scent. We found no trial showing that; it's gardening tradition. Its flowers do often draw in hoverflies, whose larvae eat aphids, so letting a few plants bloom near your vegetables does no harm.

The one rule with a real reason: don't crowd coriander together with other umbellifers such as parsley, dill or carrots, and don't follow them in the same bed, because they share diseases.