Bolting is triggered either by cold spells or by the changes in day length through the seasons. Although bolting is only seen on crops approaching maturity, it is initiated much earlier.
Annual crops will flower naturally in the first year, whereas biennials do not usually flower until the second. In annual crops, bolting occurs before they are ready to gather and, in biennials, when an over-wintering organ (carrot roots for example) flowers before the winter.
Annual crops sensitive to photoperiod (how many hours of daylight received) include lettuce, some radish cultivars and spinach. They are long-day plants, which initiate flowers when day length increases. It is a natural progression for spring-sown annuals to run to seed as summer progresses, but this can happen prematurely under the influence of stress or day-length.
Some biennial crops (which grow in the first year, flower in the second) such as onions, leeks, carrot and beetroot can initiate flowers in the first year. This is due to unsettled weather conditions early in the season and usually occurs after a prolonged cold spell, often during the propagation phase. Cold nights, hot days and late frosts may also contribute to premature initiation of flowering.