Why Is My Cabbage Running to Seed Before Forming a Head?

Finding that a spring cabbage has sent up a tall central flower stalk before forming any head — bolting straight to seed — is a frustrating experience. The plant's energy is entirely redirected into flowering and seed production, and no usable cabbage head will form from a bolted plant. Understanding what triggers bolting in cabbage and how to prevent it makes the difference between a reliable spring crop and chronic bolting problems.

What causes bolting in cabbage

Cabbage bolting is triggered by vernalisation — exposure to a period of cold temperatures followed by warming and increasing day length. In brassicas, vernalisation is interpreted as the signal that winter has passed and that it is time to flower. The critical factor is the SIZE of the plant when it experiences the cold period. Plants that are very small (fewer than six true leaves) when they experience vernalisation may bolt without ever forming a head. Plants that are large enough when exposed to cold form a head first and then bolt only at the end of the season.

Sowing too early — the main cause

The classic bolting scenario for spring cabbage is sowing too early — in late July or August rather than early August to mid-August for most UK varieties. Early-sown plants are large by the time autumn cold arrives and experience vernalisation as large plants; they then bolt in spring before heading. The counter-intuitive fix is to sow spring cabbages later (late August to early September), so they are still small when the cold arrives, completing their cold requirement without being pushed into early bolting in the following spring. Check the sowing window for your specific variety — this varies.

Cold spells in spring transplants

Summer cabbage transplants set out in May and then subjected to a late cold spell can occasionally bolt if the transplants were grown too large in pots and then experienced prolonged cold after planting. This is less common than spring cabbage bolting but does occur. Use transplants of appropriate size (not pot-bound or over-large) and plant after mid-May when cold spells become less severe.