Bolting is triggered either by cold spells or by the changes in day length through the seasons. Although

When a vegetable plant starts flowering and forming seeds, often prematurely, making the crop unusable – salad leaves may turn bitter and root crops fail to swell. Adverse weather or changes in day length can cause bolting in a wide range of vegetables, including lettuces, onions , carrots and otheer root crops.

Annual crops will flower naturally in the first year, whereas

Biennials are plants that complete their life cycle over the course of two years. In the first year, biennials typically produce leaves and roots but no flowers. In the second year, they flower and produce seeds before dying. Some common biennials include foxgloves (Digitalis), honesty (Lunaria annua) and viper's bugloss (Echium vulgare).

Annual crops

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 are plants that complete their life cycle in one growing season. They are generally easy to grow from seed or can be bought as young plants from garden centres. Annuals are ideal for growing in summer containers and filling gaps in borders. Some examples of annuals include sunflowers, cosmos, sweet peas and zinnia.

Biennial crops

Some

Biennials are plants that complete their life cycle over the course of two years. In the first year, biennials typically produce leaves and roots but no flowers. In the second year, they flower and produce seeds before dying. Some common biennials include foxgloves (Digitalis), honesty (Lunaria annua) and viper's bugloss (Echium vulgare).

Sowing times

• With cold-sensitive plants, sowings can be delayed until temperatures are more stable. This strategy is advisable for endive and Swiss chard
• Alternatively, for early crops of vegetables such as onions, beetroot and kohl rabi, plants can be raised in modules in a greenhouse and planted out when temperatures are warmer, or they can be directly sown under cloches or horticultural fleece to provide additional warmth
• Spring cabbages, which are always quick to bolt in spring, should be sown around 20 July (one week earlier in the north and one week later in the south). Although such crops will still run to seed in spring, they will bolt later than crops sown earlier, while later-sown crops may be too small to survive winter
• Successional sowings will also help to achieve a constant harvestable supply if the season is changeable
• To prevent bolting in Chinese cabbage and other oriental brassicas, these crops should be sown from July onwards
• Vegetables such as radicchio, Florence fennel, and oriental greens bolt when the nights become warm – on average above 10-13°C (50-55°F)

Soil conditions

• Annuals will inevitably flower but good growing conditions will encourage rapid growth and formation of a usable portion (lettuce hearts, for example) and so an adequate crop should develop before flower production 
• Dry soil can also encourage bolting, particularly with cauliflower, rocket and spinach. Careful watering can avoid this
• For over-wintered onions, bolting can be suppressed by topdressing with 70-100g per sq m (2-3oz per sq yd) of nitrogen rich fertiliser in January

‘Bolt-resistant’ cultivars

• Gardeners can grow specially-bred cultivars that are resistant to bolting, such as ‘Boltardy’ beetroot. These are useful for early sowings of annuals, such as spinach, and for sowings of biennials such as onions, carrots and turnips in autumn or early spring
• Red onions seem to be more prone to bolting than white or brown types, so home trials are worthwhile. If bolting in onions is a recurring problem, plant heat-treated sets in early spring (exposure to high temperatures suppresses flower-bud formation)
• Florence fennel is particularly prone to bolting so try AGM winners such as ‘Amigo’, ‘Victorio’ and ‘Pronto’ F₁ and sow in mid-summer