If you've been frustrated when growing onions because the plants produce flower stalks instead of bulbs, it is time to try again. In our Contra Costa County climate, when onions are started in the fall, it is not unusual for them to flower the following spring. After flowering, the onion plants cannot produce a good bulb for harvest. To avoid this result, plant your onions in February.
Onions are biennial plants meaning that they should grow for two years before flowers develop to produce seeds. To understand why premature flowering occurs for onions planted in the fall in our climate, it is helpful to understand two aspects of onion growth—the requirement for long periods of daylight for bulb production and vernalization which is exposure to cold temperatures that triggers flower production.
Depending on the variety, onions need between twelve and sixteen hours of daylight to produce bulbs. Contra Costa County has twelve hours of daylight by early April and about fourteen hours and forty-five minutes of daylight by late June. Consequently, depending on the onion variety, bulbs will not start to develop until late spring or early summer.
Vernalization is a botanical process that induces a plant's flowering production to occur following exposure to cold temperatures. When onions are planted in the garden in the fall in our climate, they can mature quickly. If onion stalks have grown to at least the size of a pencil while fall temperatures remain warm and are then exposed to cold nighttime temperatures in December or January, vernalization occurs. When temperatures begin to warm in early spring, flower production is triggered. Since daylight hours are still too short to produce bulbs before flowers form, the result is an onion plant that grows flowers but will never produce a bulb.
Try planting your onions in February to avoid vernalization. In our climate, if seedlings are planted in February, they are unlikely to encounter nighttime temperatures cold enough to induce vernalization. Without vernalization, flowers will not develop prematurely and good size bulbs can develop as soon as there are sufficient daylight hours.
When starting onions in February, it is best to transplant seedlings into the garden bed rather than placing seeds directly into bed. Using seedlings makes it more likely that the plants will mature sufficiently to allow bulbs to form as soon as daylight hours are sufficiently long.
If you want to plant your own seeds, start them indoors in December so that the seedlings are ready for transplant by February. Keep in mind that onion seeds only remain fully viable for one or two years so always use fresh seeds. If you haven't yet started your own onion seedlings, look for them in local nurseries.
Avoid using onion sets if you want onion bulbs. Onion sets are small onion bulbs that grew the prior season and were harvested and allowed to dry. When you plant them, the plant that grows will be in its second year of growth and will be ready to produce flowers even without vernalization from cold temperatures.
This video produced by the UC Master Gardeners Program in Santa Clara County shows how to transplant onion seedlings into the garden and explains how to harvest the mature onion bulbs: https://www.youtube.com/watch?v=Gb1WhqtsdMI.
For additional cultural tips for onion production, see this UC website: https://ipm.ucanr.edu/home-and-landscape/onions-and-garlic/cultural-tips/index.html?src=307-pageViewHLS
This UC website will help you manage onion pests and diseases: https://ipm.ucanr.edu/home-and-landscape/onions-and-garlic/index.html
Help Desk of the UC Master Gardeners of Contra Costa County (TKL)