Though they are pretty and bees love them, I’d rather my onions not make flowers. Why? Because the flower stalk ruins the core of the bulb.
Once the onion plant sends up a flower stalk there is nothing you can do to stop the process, and the center of the bulb eventually dries and becomes hollow. Onions that have flowered do not store as long either.
This spring, many of my onions are sending up flower stalks. I don’t like it. I need to learn more about why some onions flower. Join me in the investigation.
Three contributing factors
According to my California Master Gardener Handbook there are three potential causes of flowering. The book states that “seedstalks develop” because of “cold weather after plants are 6-10 weeks old,” and that a gardener must “plant [the] right variety at [the] proper time.”
So we have: (1) cold weather; (2) variety; and (3) planting time.
For the past nine years, I’ve kept records of my onion plantings so I looked at my sowing and planting dates to see if I could find a correlation with flowering. I found one. I found that, in general, the earlier the sowing date, the more flowering.
Specifically, my onion crops sown between September 14 and October 12 have flowered a lot whereas my onion crops sown on October 17 or later have flowered only a little or not at all.
Sowing onions later than November resulted in little flowering but it also resulted in smaller bulbs.
My records show that only one variety has produced excellent bulbs and never flowered: Gabriella. But it’s not clear that the variety is key here because I only grew it two years and both years I sowed after mid-October. So was it the variety or the later sowing date?
A variety named Madalyn had zero bolting one year but had some flowering in the two other years I’ve grown it, including this year.
The year that Madalyn had zero flowering had the latest sowing date (October 24, 2020). Here is the harvest of that sowing:
That same year, I sowed Red Rock onions on the same date. Very few of those plants bolted.
There have been some varieties that have always bolted a lot on me, such as White Castle.
So from my experience, it does seem that varieties have different tendencies toward bolting. And combining a later sowing date with a variety that resists bolting might find best success.
The California Master Gardener Handbook did not say that cold weather causes or prevents onions from flowering. It said that “cold weather after plants are 6-10 weeks old” causes onions to flower.
Did you know that there are farms in Southern California that grow onions for seed production — not for bulbs, but for seeds? These farms are in the Imperial Valley. These farmers actually want their onions to flower, for that is how they get seed.
And look at how they achieve this flowering: “Onion seed are planted in the field in August or September. Planting must be early enough for the plants to attain sufficient size to be vernalized.”
Vernalized? “Seed stalk formation requires a period of cooling known as vernalization. Induction of flowering occurs when plants or bulbs are subjected to temperatures of 45-55 degrees F for approximately one month or longer.”
What does this mean? In order to achieve onion flowering, the seed farmers plant early enough (August or September) such that the plants experience cold weather when the plants have grown for a couple of months already. Experiencing the cold while they are younger and smaller will not cause the plants to flower reliably. Hence a later planting date (say, October or November) is not advised because the plants would be too small during the cold weather of winter and would not be vernalized.
Aha! It sounds like if we don’t want our onions to flower, we should plant later than the farmers who grow for seed production.
To further support this is another University of California publication called “Fresh-market bulb onion production in California” which says, “Bolting is primarily driven by temperature when plants with a leaf base greater than approximately 3/8 inch in diameter are subjected to temperatures of 45 to 50 degrees F.”
I’ve been planting too early, sometimes. My new rule of thumb will be to sow only after mid-October and through November. Planting dates can be in November through December, for best bulb size and least flowering.
As for varieties, I’ve had the best production and least flowering from Gabriella. I should stick with it as my main variety.
This winter was unusually cold and long. That likely vernalized some onions that wouldn’t have flowered as much in warmer, shorter winters. So I’ll cut myself a little slack and blame part of my flowering on the weather.
But this is what I’d like to see more of (sorry, bees):
You might like to read my general post, “Growing onions in Southern California.”
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