Q How do I avoid coriander bolting?

A Coriander tends to bolt in hot or dry weather or if its roots are disturbed. This is good if you want to harvest the seed, but not if you’re using the leaves in cooking.
Suggested solutions include growing it in pots to avoid root disturbance; harvesting the leaves regularly; keeping the plants cool by putting them in shade during the hottest part of the day; or mulching and/or growing them closely together.
Which? Gardening magazine tried variations on sowing time and pot position, number of plants per pot and height when cut using coriander ‘Leisure’. This was sown in 25cm pots in mid-May, and thinned to seven plants per pot when they reached 5-7.5cm.
We recorded each technique’s effect on how many times we could harvest the leaves – cut when the plant reached 15cm tall. We also tried growing the slow-to-bolt Best Buy variety ‘Calypso’.
Our control pots, which had no special treatment, bolted after just one harvest, as did the pots that were cut at 10cm to harvest them more regularly. Those that were mulched with grass clippings also bolted after the first cut and grew a lovely selection of weeds to boot.
Growing the plants more densely (20 plants per pot) gave us two harvests. Plants kept in the shade were harvested three times, but were disappointingly sparse and leggy.
Sowing in July produced fewer plants, but we harvested leaves from them on four occasions.
The most successful technique was growing the Best Buy variety ‘Calypso’, which we harvested five times before it bolted in August.
WHICH? GARDENING RECOMMENDATION
Make successional sowings of coriander ‘Calypso’ in pots.