Advice
RHS Help & Advice
Bolting in vegetables
‘Bolting’ is when a vegetable crop runs to seed. Some vegetables can become unusable if they bolt but careful selection of cultivars and sowing dates can help deter the problem.
Cause
Bolting is triggered either by a cold spell (vernalisation) or by changes in day-length (photoperiod). Although bolting is only seen on crops that are fairly mature it is initiated much earlier. Annual crops will bolt in the first year, whereas biennials do not usually bolt until the second.
Annual crops
Annual crops sensitive to photoperiod 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.
Biennial crops
Some biennial crops (e.g. onions, leeks, carrot and beetroot) produce a large storage organ the first year in order to flower the next. All of these crops produce flowers after a period of cold. This would normally be during winter, but unsettled weather conditions early in the season can trigger flowers in the first year. This usually occurs after a prolonged cold spell but cold nights, hot days and late frosts may also contribute to vernalisation.
Different biennial crops have varying reactions to cold. Turnips are cold sensitive as soon as germination begins, whereas onions and some brassicas are only sensitive to vernalization after they reach a certain growth stage. Celery and celeriac, on the other hand, do not have distinct phases, instead there is a gradual increase in cold sensitivity as the plants develop.
Control
Sowing Times
With cold-sensitive plants sowings can be delayed until temperatures are more stable. This strategy is advisable for turnips, endive and Swiss chard. Only sow vegetables such as radicchio, Florence fennel, and Oriental greens when the nights become warm (on average above 10-13°C/50-55°F). In cold districts use horticultural fleece or cloches to reduce the minimum temperatures. Alternatively, for early crops of vegetables such as onions, beetroot and kohlrabi, plants can be raised in modules in a greenhouse and planted out when temperatures are warmer, or they can be directly sown under cloches.
Annuals will inevitably bolt if sown early but good growing conditions will encourage rapid growth and so an adequate crop should develop before flower production. Dry soil can also encourage bolting, particularly with cauliflower, rocket and spinach so soil moisture levels need to be constant. Successional sowings will also help to achieve a constant harvestable supply if the season is changeable.
Chinese cabbage is complex in that it will bolt if sown too early in the year due to cold temperatures, with longer days enhancing the effect of the cold. To prevent bolting this crop should be sown from July onwards.
Crops which are only cold-sensitive after they reach a certain stage can also be manipulated. For example, if onions bred for autumn drilling are sown in August they should not get to their cold sensitive stage before winter begins and consequently will be less likely to bolt the following spring. Research has shown that for autumn sowings of onions, a lack of nitrogen in early spring can also induce bolting, so topdressing such crops with 70-100g/m² (2-3oz/y²) of nitrochalk in January is helpful.
Similarly brassicas which, as with onions, are only cold-sensitive after reaching a certain growth stage can be manipulated to postpone bolting. 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.
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 spring cabbages in autumn or early spring. Bolting resistance varies between cultivars of cabbages, and 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 in cool summers or colder parts of the country.
Not all vegetables will spoil if they produce flowers, of course, and for some such as podded vegetables its an essential process if we’re to get any yield. However by following the above advice vegetable flowering can be avoided where necessary.