Bolting, also called going to seed, is one of those arcane horticultural terms that throws beginning gardeners. When a plant “bolts”, that doesn’t mean it is running away, of course, but instead that has started to flower or go to seed prematurely. And by “prematurely”, I really mean before the gardener wanted it to. (It may seem the perfect moment to flower from the plant’s point of view!)
Bolting means the plant has gone beyond the vegetative stage in its life and has begun taking the next step: flowering. This is bad news for many herbs and leaf and root vegetables, like spinach, lettuce, parsley and radishes, as not only do they stop producing more of the edible part the gardener wants, but their leaves or roots become bitter or fibrous and no longer edible.
Hot or dry conditions often stimulate bolting, so you can delay it by keeping the soil cool and moist (a mulch may be helpful) or by sowing the plant in a cooler season. Many leaf vegetables, for example, grow best in spring or fall, even winter in mild climates, but bolt rapidly during the summer. Yet other plants, like onions or carrots, may bolt when they go through a cold snap.
Growing plants well is the best way to prevent bolting… and it can be helpful to grow varieties said to be slow to bolt or resistant to bolting.