When you plant onions with the goal of having a supply to last almost until the next growing season, you don’t want to see those hard flower stalks and seed heads that indicate the onion is prematurely bolting. Once that happens the onion is good only for eating immediately and will not store for future use.
Onions are biennial plants. Under normal conditions the onion seed is sown and forms a bulb the first growing season. (This is when most gardeners harvest, unless they want seed.) If left in the ground to winter over, the onion starts growth again with warming temperatures and the plant will send up a stalk and produce the seed heads.
What Causes Premature Bolting?
Unusual weather conditions (hot or cold) and severe temperature fluctuations can throw off this normal cycle and fool onions into setting the seed head (bolting) earlier than normal.
Even in “normal” years a gardener can expect a certain percentage of onions to bolt in their first year. This is especially true for onions planted from sets (already in their 2nd year when planted in your garden) or purchased transplants.
(Just so you’ll know, overall, transplants give you better onions than sets.)
A Hint to the Wise
Onions that the gardener raises from seed will acclimate better to temperature changes. Thus, you won’t normally get as much bolting in the first year from those onions. (Another great reason to start from seed!)
A Certain Percentage of Premature Bolting is Normal
For more than 20 years I’ve ordered transplants (a small onion plant grown from seed) from Dixondale. My average percentage of onions that bolted prematurely was about 10%, possible 20% in some years. Out of 2500 onions, about 250 won’t cure or store. No problem for me, since I eat that many as green onions anyway.
Time for Setting Out Your Transplants
A good rule of thumb for planting onion transplants to the garden is 6 to 8 weeks before the average last frost date. Sometimes I plant even earlier. If you’ve read all my posts on onions you know that I like to put in my transplants in late February or the first of March. Because of that, the roots are unbelievable long and strong before daylight hours trigger bulbing.
Other than the usual 10% + that bolt every year, the crop as a whole is not effected by our usual Virginia roller-coaster weather of below freezing for a while and then up into the 40s and 50s again.
Best Size to Transplant
I’ve read various articles that state onion transplants should be pencil size or smaller for best results. My finding is that the pencil size is just a tad too big for best results. Just a bit smaller is perfect.
Becoming Less Dependent on Suppliers and Why
- Not being pleased with the size onion transplants I’ve received the last two years,
- seeing the changing weather conditions,
- chemical companies buying up seed companies,
- and other tell-tale signs of things to come,
I decided to take steps towards breaking my dependency on Dixondale for transplants.
Something IMPORTANT to Keep in Mind to Temper the Information that Comes Next
Keep in mind as you are reading this, onions are cold hardy and take a lot. I’ve planted in February and the first of March for many years and then had temperatures up and down without consequence to my onions. But it was all considered normal weather for our area.
Premature bolting is caused by temperature fluctuations and stress to the plant. The question that we can only guess the answer to is: fluctuations and stress to what degree?
Steps from Seed to Your Garden
- Onions transplants you buy from any source (say Texas for example) are probably started from seed about mid October to mid November.
- They’re dug again to fill your order just before shipping.
- They’re out of the ground for a number of weeks and are dormant.
- You transplant to your garden 8 weeks before your average last frost date.
Example of What’s Possible and Could Have Easily Happened This Past Winter
Let’s assume for a minute that onion plants were growing vigorously for 2 or 3 months at the supplier’s property and were subjected to the stress of severe cold and went dormant. When the temperatures rose, the plants started to grow again. (From what we know about onions that may be enough to fool them into thinking this is their second year.)
Let’s assume also that they’re dug in late February to fill your order. You plant them in March. Temperatures go up to normal and they start growing. All of a sudden, as was this year, your plants are subjected to severe and long periods of cold. When it warms again they start growing. Even if they had not been under stress previously at the supplier’s, I’d say there would be a great chance most of these onions will bolt.
What I’m Doing
This year I only ordered half what I usually order from Dixondale. The other half I’m growing from seed.
I’m not where I want to be (yet) with growing onions from seed. I have lots more ideas and things I want to try to help get me to my goal. (Details will probably appear in future posts.)
% of Bolting this Year
I ordered 4 varieties from Dixondale. (Total of about 1250 onions.) Bolting is at about 15% except for the Borettana Cippolini variety, which is 80%! It’s a great specialty onion that I really enjoy, and fortunately only ordered one bunch. I’m guessing it’s a variety that’s prone to bolting, since I’ve heard that some varieties are more prone than others.
The ones I started from seed – no bolting.
Sandra, friend and reader in Maryland, wrote to me and said that 99% of her onions (I think from Dixondale) are bolting! Horrible!
I take great hope and encouragement in the hint I gave you earlier. I hope you will as well. Here it is again:
“Onions that the gardener raises from seed acclimate better to temperature changes. Thus, you won’t normally get as much bolting in the first year (a/k/a premature bolting) from those onions.”
Please take a minute to let me know how your onions are doing this year.
Onions – Seed, Sets, and Transplants
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