Why Are My Brussels Sprout Plants Going to Seed Early?

Brussels sprout plants that run to seed — producing a flowering stem and small yellow flowers before forming a proper crop of sprouts — have been triggered into their reproductive phase prematurely. This is called bolting, and it is one of the most frustrating problems in the Brussels sprout calendar because once a plant bolts it rarely produces a worthwhile crop of sprouts. Understanding what triggers bolting helps prevent it in future seasons.

Cold vernalisation triggering early bolting

Brussels sprouts, like many brassicas, require a period of cold to flower (a process called vernalisation). This is normally a beneficial mechanism that prevents flowering in autumn, ensuring flowering occurs in spring after winter. However, if young seedlings experience extended cold below about 10°C before they have grown large enough, they interpret this as having experienced winter and begin to bolt prematurely. This is most likely to happen with early-sown plants that are put outdoors too soon, or with transplants that encounter a late cold snap shortly after planting out.

Sowing too early

Sowing Brussels sprout seeds very early — in February or March — and then raising the seedlings in unheated conditions can expose young plants to the cold temperatures that trigger bolting. Plants should be large enough and hardened-off sufficiently before encountering temperatures consistently below 10°C. Sowing in late March or April and transplanting in May or June avoids the risk in most temperate climates. Check the recommended sowing and planting dates for the variety you are growing.

Variety selection

Some older or open-pollinated Brussels sprout varieties are more prone to bolting than modern F1 hybrids, which are bred for greater uniformity and stability. If bolting is a repeated problem, switching to a modern F1 variety may significantly reduce it. Also check that the variety you are growing is appropriate for your climate — early varieties that mature by October are more likely to bolt than late varieties intended for December to February harvest.

What to do with plants that have bolted

Once a Brussels sprout plant has run to flower, there is little you can do to reverse it. Remove the flowering stem promptly — this may slow the bolting temporarily and the plant may still produce some loose or imperfect sprouts in the leaf axils. However, yields from bolted plants are always poor. Remove bolted plants at the end of the season and compost the material, then review your sowing and planting schedule to prevent the issue repeating.