Bolting

also called Running to seed, Premature flowering

A plant deciding it is time to flower rather than time to feed you — the lettuce shoots up, the leaves turn bitter, and there is no way back.

Disorder Disfiguring Doesn't spread

What you'll see

A sudden vertical stem shooting up from the middle of a lettuce, spinach or brassica, sometimes ten centimeters in a few days.
Leaves turning bitter, tough and milky when cut, and the heart opening out.
Onions throwing a hard round flower stalk up the middle of the neck, so the bulb will not store.
Coriander and rocket flowering within weeks of sowing, with the leaves narrowing to feathery strips.
Beetroot, carrots and Florence fennel putting up a flower stem instead of swelling their root or bulb.

Why it happens

Flowering is what the plant is for; the leaf or the root you wanted is only the stage before it. What triggers the switch differs by crop — heat and long days for lettuce and spinach, a cold spell for onions, beetroot and brassicas, and any sharp check for coriander and rocket. Once the plant has read the signal and switched, the decision is chemical and irreversible.

A hot spell in early summer for the salads, a cold snap after planting out for onion sets and brassicas, and drought or root disturbance for almost everything. Long days from midsummer bring on spinach and rocket whatever the temperature.

Treating it

There is no reversing it. Pull the bolted plant and sow a replacement, or leave it to flower deliberately.
Pick and use the leaves at once — they are edible for a few days more, only bitter, and better cooked than in a salad.
Use bolted onions immediately in the kitchen; they will not keep, but they are perfectly good now.
Let a few bolted coriander, rocket and dill plants flower for the hoverflies and then collect the seed.
Cut a bolting lettuce off at the base — a few cultivars will send up a modest second flush of leaves.

Keeping it away

Sow bolt-resistant cultivars, which exist for lettuce, spinach, beetroot, Florence fennel and onion.
Sow little and often rather than one large sowing, so there is always a young plant coming.
Water steadily and mulch; drought stress is the commonest trigger of all.
Give summer salads afternoon shade, or grow them in the shadow of taller crops.
Plant onion sets and brassicas after the cold snaps, and do not leave transplants pot-bound and checked in their trays.

Who gets it

Lettuce, spinach, rocket, coriander, brassicas, onions, beetroot and Florence fennel — most of the cool-season crops.