Gardening glossary
Bolting
Bolting is the gardener's word for a plant deciding it is more important to reproduce than to feed you. Once a lettuce, spinach, basil, coriander, brassica, beetroot, or onion has bolted, its leaves go bitter, its roots go woody, and its energy diverts to making seed. You can sometimes slow it down by removing the flowering stalk, but the plant has flipped a hormonal switch that does not flip back easily.
What triggers bolting:
- **Day length** — long summer days are the strongest trigger for spinach, lettuce, coriander, and many onions. These are "long-day" plants that flower as days exceed roughly 14 hours. - **Heat** — sustained temperatures above 24–26 °C accelerate bolting in cool-season crops like lettuce, rocket, and pak choi. - **Root stress** — letting a leafy crop dry out, then watering heavily, can trigger bolting. Smooth, consistent moisture matters. - **Premature vernalisation** — biennials and onions that get vernalised by an unexpected cold spell in spring may bolt in their first year. - **Transplant shock** — coriander in particular hates being moved; direct-sow it.
How to delay bolting in practice:
- **Sow at the right time of year.** Push cool-season crops into spring and autumn; avoid mid-summer sowings unless using bolt-resistant varieties. - **Use "slow-bolt" cultivars.** Coriander "Calypso," lettuce "Cocarde," spinach "Bordeaux F1," and pak choi "Joi Choi F1" hold longer before flowering. - **Provide afternoon shade.** A row of taller crops (tomatoes, sunflowers, sweetcorn) on the south side of a salad bed buys an extra two to four weeks of leaf harvest in midsummer. - **Keep moisture even.** Mulch the bed with leaf mould or compost, water consistently. - **Succession sow.** Replace bolted plants quickly with a fresh batch from a 2–3 weekly sowing cycle.
A plant that has just started to bolt is still salvageable — pinch out the flowering stem and cut leaves aggressively. Once the stem is 10 cm tall and budding, save the seed for next year and clear the plant.