In plants, “bolting” refers to the development of flower stalks, which will eventually develop into seeds. Since all plants have the goal of reproducing themselves, bolting will happen eventually in all cases, but sometimes it is undesirable, especially in crops. Gardeners struggle the most with bolting in plants which are cultivated for their leaves, such as spinach, chard, bok choy, kale, cabbage, cilantro, and so forth. There are a number of ways in which premature bolting can be avoided.
The reason bolting is undesirable is that it changes the flavor of the leaves of the plant. As the plant starts to sink energy into producing flowers and seeds, it allows the leaves to die, and they start to turn woody and bitter. If a plant is not caught soon enough, the leaves will simply wither away, leaving only the flower stalk behind. In plants cultivated for their flowers, like cauliflower and broccoli, premature bolting can also cause the heads to be relatively small and tasteless, which is also not usually desired.
Plants bolt prematurely because they are exposed to cool temperatures in an early period of their development. As soon as the weather warms up, the plants bolt to seed quickly, with the goal of getting as many seeds out as possible. Gardeners typically notice bolting in the first warm spell of the year, as their plants suddenly put out flower shoots which may grow at an astoundingly rapid rate. Once a plant starts to bolt, it needs to be harvested or allowed to go entirely to seed.
To prevent bolting, it is important to keep plants in a temperate environment in the early stages of development. In cool climates, plants should be started in a greenhouse, where the controlled temperatures will prevent exposure to cold. If plants are already in the ground and a cold snap is predicted, they should be protected with covers and mulch in the hopes of staving off a bolting episode. In warm weather, ample watering can also prevent bolting, by assuring the plant that the end is not nigh.
If a plant has bolted to seed and one still wants to use the leaves, all is not lost. The leaves will need to be cooked so that they are more tender, and the bitter flavor will need to be tempered in the cooking process. The best way to do this is to boil the leaves in several changes of water, which will help flush out the bitter flavor and make the greens palatable again.