Bolting

What you are looking at

A crop you are growing for its leaves or its roots stops doing that and starts flowering. A lettuce lengthens out of its heart into a tower with a bare stem and small pointed leaves. Spinach and rocket send up a stalk within weeks of germinating. An onion pushes a hard round flower stem through the middle of the bulb. Beetroot and radish go woody with pale rings through the flesh.

The leaves turn bitter or coarse as it happens, because the plant is moving its resources into seed and, in lettuce, because the milky sap that carries the bitterness increases.

None of it can be reversed. A plant that has committed to flowering has changed what it is doing with the rest of its life.

Two triggers, and they need different answers

The first is cold. Beetroot, onions, chard, cauliflower and the other biennials are built to grow one year, sit through a winter, and flower the next. A cold spell after sowing is read as that winter, and the plant flowers months early. Beetroot sown in March and caught by a fortnight of frost does this reliably, and so do onion sets planted into cold ground.

The second is day length and heat. Lettuce, spinach and rocket measure how long the day is, and once it passes their threshold they flower whatever the weather. British days in late June are about sixteen and a half hours, which is well past what true spinach will tolerate. Heat pushes the same species in the same direction and a hot dry spell accelerates it further.

Stress sits behind both. Drying out, sitting pot-bound in a module, having the roots torn at transplanting or running short of water in a heatwave all make a plant more likely to flower, because a plant under threat sets seed.

The fixes differ accordingly. Sowing later fixes the cold trigger and makes the day-length one worse. Sowing earlier does the reverse. What crop you have decides which way to move.

What each crop wants

Beetroot wants sowing after the middle of April outdoors, or under fleece before that, and ‘Boltardy’ if you sow early anyway.

Onions from sets want heat-treated sets and planting into ground that has warmed up, and steady water through spring.

Lettuce wants sowing little and often, never drying out, and some shade over midsummer sowings. Soil above about 25 degrees stops the seed germinating in the first place.

Spinach wants an early spring sowing or an August one, with chard or perpetual spinach standing in through midsummer.

Rocket, coriander and radish want sowing direct, in short rows, every few weeks, and cutting hard while young.

Cauliflower and calabrese want to grow on without any check at all. Pot them on before they get pot-bound, plant into moist ground, and keep them watered.

What to do about it

Sow the right cultivar at the right time for the crop you are growing.

Water deeply and regularly, and mulch so the ground stays even.

Sow short rows every fortnight rather than one long row in May.

Sow the crops that resent disturbance direct into the ground.

Shade summer lettuce.

Pull a bolted crop and use it at once, and leave two or three plants to flower for the hoverflies and the bees.

What it gets mistaken for

Drought stress wilts a plant that stands up again after watering, and it is one of the triggers of bolting rather than a separate thing to rule out. The difference is the flower stem.

Nitrogen shortage yellows the older leaves and holds a plant small without producing a stalk, and a feed corrects it within a fortnight.

A cauliflower blowing open in hot weather without a flower stem is a heat response rather than bolting, though the causes overlap and the head is lost either way.

Onions that produce a thick soft neck and no flower are suffering from too much nitrogen or a wet late summer instead, and those will not store either.