Coriander

Coriander is an annual from the eastern Mediterranean and western Asia, and it lives fast. It germinates in a week, gives leaves for a month or two, flowers, sets seed and dies, and no amount of care changes that plan. What care does change is how long the leafy part lasts, and the difference between three weeks and three months is entirely down to sowing at the right time of year, sowing where it is to grow, and never letting it dry out.

Sowing, and the two hemispheres

Sow directly into the ground or a deep pot, one centimetre deep, and thin the seedlings to about fifteen centimetres apart. Coriander makes a taproot and hates being moved; a transplanted plant very often bolts within a fortnight of the disturbance, which is why the supermarket pot never lasts.

In the northern hemisphere sow in spring and again in late summer, roughly March to May and August to September, avoiding the hottest weeks of midsummer.

In the southern hemisphere the pattern is different, and it is not the northern one shifted six months. Coriander is at its best there in autumn and early spring, sown from February through May and again in August and September, and the summer months are avoided for the same reason they are avoided in the north. The autumn sowing is the important one and has no real northern equivalent, because a northern autumn sowing runs into a hard winter while a southern one runs into a mild wet season the plant enjoys. In frost free parts of South Africa and Australia coriander can be grown right through winter.

Light and warmth

Full sun in cool weather. In hot weather, afternoon shade, which buys weeks of extra leaf. The trigger for flowering is a combination of heat and lengthening days, and above about 24 degrees the plant begins bolting within days regardless of what else you do.

Water

Keep the compost evenly moist, which in warm weather means watering every two or three days. This is not a plant to let dry out between waterings. A single dry spell is read by the plant as the end of its season, and it responds by throwing up a flower stem.

Harvesting

Cut whole outer stems at the base rather than picking individual leaves, starting when the plant is about fifteen centimetres tall. Take no more than a third at a time. In the northern hemisphere that is May to July and again in September and October; in the southern, April to July and again in September and October.

The real technique is successional sowing. A small batch every three weeks gives continuous leaf, where one big sowing gives a glut and then nothing.

When it flowers anyway

Let one plant go. The flowers are pretty, hoverflies and bees work them hard, and the green seed is worth tasting fresh. Left to ripen and dry on the plant, the seed is the spice, and it also sows the next generation, which in a mild garden means coriander appears by itself every year.

The taste argument

A proportion of people, varying from a few per cent in some populations to a fifth in others, find coriander leaf tastes of soap. This is not fussiness. It is a real difference in how a cluster of smell receptor genes reads the aldehydes in the leaf, and those same aldehydes are found in soap. If someone in the house has it, coriander seed is unaffected and tastes warm and citrusy to everyone, so the plant is still worth growing for the spice.

Ailments

Early flowering is the whole story of this plant and the ledger below sets out the four things that cause it. Thin pale seedlings mean crowding or poor light, and crowded coriander bolts sooner as well. Yellow lower leaves on a plant that has flowered are simply the end, and the right response is another sowing rather than a rescue.