Why Onions Bolt, Failing to Bulb?

Published: November 1, 2010

Many onion plants bolted this summer instead of forming bulbs. This is truly frustrating, but why did it take place?

Onions (Allium cepa) are biennials. Naturally, they grow vegetatively into bulbs before the hot summer, go dormant, and sprout and flower the next spring. The bulb, the edible part to human, protects the growing point from damage in summer and winter. Onion, like other plants, needs to go through vernalization to flower. A simple definition of vernalization would be the required exposure to low temperatures for flowering.

The low-temperature requirement satisfying vernalization varies from species to species. Some vegetable species do not need the low-temperature exposure (sweet corn, tomato, peeper, eggplant, most cucurbits, etc); some species definitely need it (beets, cabbage, carrots, onion, etc); and some species fall in between. While the geminating seeds of most plants would be able to sense and accumulate low temperatures for vernalization, onions need to have about five leaves for capturing the low-temperature effect (celery and cabbage are also in this category).

Generally, onion seedstalk formation may be initiated before the bulb matures if plants are subjected to temperatures below 50°F for 3-4 weeks, and the lower temperatures (above freezing point) contribute more to vernalization. Therefore, we have observed more boltings in onions from sets or transplants this year, thanks to the irregular cool weather. Bolting in onions is not induced by photoperiod, as is sometimes believed. However, bulb formation of onions requires long days (short nights) of 12-16 hours depending on cultivars and types.

The low-temperature effect is accumulative. Once the chilling units are met, bolting is not avoidable. However, devernalization is possible. Exposing previously vernalized sets or transplants to high temperatures could possibly reverse the plant to nonflowering stage. Onion sets that are vernalized by storing at near freezing temperatures could be devernalized by exposing them to temperatures beyond 80°F for two to three weeks before planting.

Removing seedstalks may help with bulb formation but the effect is limited, depending on the stage of plants. If onion sets bolt shortly after planting, formation of bulbs would be next to impossible. Probably the best option if you see onions bolting is to pull those and sell them as bunched green onions, the sooner the better.