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Onions – Bolting – What Causes It and What We Can Do

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 prematurely setting seed heads.

Borettana Cippolini onions prematurely setting seed heads (bolting.)

Life Cycle

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!

Final Thoughts

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.

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Related Posts:

Growing Onions

Onions – Starting from Seed is Easy and Economical

Onions – Why Grow A Lot?

How to Have Garden Onions April thru January

Onions Tip – What to do with the Small Ones

How to Grow Onions Especially Bigger Ones

Onions – Seed, Sets, and Transplants
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20 comments to Onions – Bolting – What Causes It and What We Can Do

  • Patti

    I didn’t order from Dixondale this year and as difficult as it was, I ordered very few seeds, deciding to try and use what I have, so I didn’t have great success with my onion seed germination, but the ones that did germinate are growing slow and steady. We just moved to TN from MA and my husband built me 15 raised beds, but the soil is going to take a lot of amending.

  • Theresa

    Hi Patti,
    I know what you mean about trying to use the seed you have. That’s always a good policy.
    Just keep in mind that prime time for onion germination is the first year. After that it’s all downhill and
    germination will be spotty at best. Same goes for peppers.
    Many seeds like tomatoes, lettuce, cukes, squash, keep for many years if stored in cool, dry place.
    Glad to hear that the ones that did germinate are growing well.
    Hope you will have a great first season at your new home!
    Thanks for commenting.
    Theresa

  • MB

    Thank you for this great article. I bought sets from a local garden supplier – some kind of red onion- maybe a spring onion? I thought they were going to bulb, but they didn’t so I’m not sure what they are. I planted them early – last fall/winter(we live in Sonoma County, CA). We do have some cold nights – freezing temps, and about 20% stared bolting in May. Maybe more. I didn’t even know that onions are tough if bolted, I assumed ALL the onions are tough since I’ve been eating the seeded ones to get rid of them – so I’ll try an unbolted one today an compare. Thanks again.

  • Toni Brock

    Hi Theresa,
    I am sorry about your specialty onion bolting. How dissapointing. I am very excited about your seeds doing so well. I have planted 2 varieties of seed: purple scallion, and red ambrosia. The scallions are doing great. The red ambrosia, however, are still very tiny. I am going to keep learning, and hopefully, each year have a steadily improving crop. Thank you for your tips. They help immensely.

  • Sandra

    Yes, they were from Dixondale, Theresa. And all the ones that I planted from seed are not bolting.

    Have you ever sown seeds in the Fall, or has it always been via the wintersown method? What are your thoughts about sowing several smaller staggered plantings, in Fall? I think I’d have to do it in jugs for more control, but then transplant them into their spring place in the garden for overwintering once the roots outgrew the jugs. Another experiment.

  • Is this true of elephant garlic also? that if it bolts, it’s done growing? I’ve been trying to grow it the past 2 years and just get bulbs, no cloves. Isn’t it supposed to make cloves? I know that it’s not a true garlic but I guess I don’t know what it’s supposed to do normally. Any ideas?

  • Ed Aycock

    Good Morning from N. Idaho Theresa,
    Well, we have had a late cool start to our very short growing season.
    Our last heavy frost was middle of May with several light frost up to Mothers
    Day. Put out 100 Vadalia onion sets on Mothers Day, all are up an growing nicely.
    I like to try odd things, so when one of our local Master Gardeners told me she
    doesn’t buy sets but uses organic white baby onions in the bag from WALMART! Well,
    sounds weird but gave it a try putting in 50 of them. ALL of them came up and are
    almost a foot high with no bolting…mystery variety but that’s part of the fun.
    Hope the Lord blesses us all with a long late growing season.
    Ed

  • Theresa

    Hi MB,
    Onions are not too tough when that seed head just starts forming. If you allow them to continue they get really tough. When you see that seed head forming, pull the onion and use it as soon as you can.

    Hope you have already tried and enjoyed your “unbolted” onion. You should have found it sweet, tender and delicious! Nothing like onions fresh from the garden.

    Hi Toni,
    Glad the tips were helpful. Yes, growing onions from seed is definitely a learning experience, and my hope is what your is: to each year have a steadily improving crop!
    Always great to have you join the conversation Toni. I appreciate your time!

    SANDRA
    ! You told me you had no telepathic communication skills, but now I see YOU DO!
    All of your ideas are on my list to experiment with. Definitely want to sow seeds this fall and winter over. Want to do several staggered planting in the fall and January thru March. I’m going to try some direct seeding, but will do jugs also for more control.

    Now I know I don’t have to tell you my ideas. I’ll just think them and you’ll catch ’em! 🙂

    Betty, I don’t know a lot about elephant garlic because I’ve not grown it. I’ve heard that sometimes cloves form a single “round” rather than a segmented bulb. If replanted, the bulb will make a regular segments bulb the next year. In general though, I think most of them should have segments cloves even if not many.
    Also elephant garlic makes a tall flower stalk that can grow 5 feet. I think those should be clipped when they’re about 8 inches.
    Sorry, I don’t know enough about the elephant garlic to be of help.
    Know this is a busy time for you Betty. Thanks for taking time to join in.

    Hey Ed! Good to hear from you.
    Sounds like those onions are doing great! Let me know “the rest of the story” when all is said and done.
    And yes, I hope we all will be blessed with a long late growing season!

    Theresa

  • Beverly

    Theresa, I also, for some years, ordered from Dixondale, but not last fall. I would be thrilled to grow as many as you. Some day maybe. Got inspired late last fall and tried my hand at starting onion seed, first time for me. Sad, it didn’t work for me , I have only multipliers, this year. I will keep trying. Onions are also on the leafcutter ants menu. I learn a little something each I read a post. Thanks.

  • Theresa

    Keep trying Beverly! There’s got to be a way to avoid or get rid of those leafcutter ants! Awful to have to deal with that.
    Theresa

  • Dixondale transplants here are doing quite well this year; out of 150 plants, I have only had three put up seed stalks. This year doing Copra, Red Candy, Texas Legend. My worst stalks were with the Candy, more than 50% sent up stalks. My Copras seem to be done early – they are already laying down, have picked a few up off the ground and they are mild and delicious. Red Candy and Texas Legend still growing.

    Have had the same experience with elephant garlic another reader noted, most of mine head up to cloves, but a couple will just produce a single bulb. I love the stuff and grow more each year. Unfortunately, something has been running amok in my garlic bed, knocking down plants so not sure how my harvest will do this year.

  • Theresa

    Great report Kate! Thanks.
    Hope your garlic makes it ok in spite of the intruder.
    Theresa

  • Stephanie

    Our onions are doing great. But, we live in the Midwest and didn’t plant until mid April. They have seen much fluctuation in temperatures. We planted about 675 onions and so far no bolts, but like I said, we planted late. It has been a strange year here…We had winter and then it jumped to Summer. So, will see how things go.

    As for elephant garlic, did you plant it in the spring or in the Fall? Garlic needs a “freeze” so to speak for the bulb to split into cloves. Also, if you have a mild winter, you will get one bulb. And, after further research trying to find the name for when it doesn’t split, I also read that if you start with too small a clove of elephant garlic, you will get one head. This is due to the fact that elephant garlic actually has a 2 to 3 year span until maturity. If you plant that one large head of elephant garlic, you will get “extra large cloves the next year” according to what I read. I hope this helps.

  • millard

    This year our weather in northwestern Europe has been like a “roller coaster”, with a warm winter, much rain and two exceptionally hot days in May. We plant small onions as bulbs rather than the living plants, as well as numerous varieties of onion seeds. This year the onions planted from bulbs are in a terrible condition, whereas the seed-grown plants look quite healthy and vigorous. I am considering digging under the rows of onions planted from bulb. It’s very sad.

  • Theresa

    Stephanie,
    Good hearing from you. It’s been a while.
    Sounds like your onions are doing great.
    Thanks for the additional input on elephant garlic. I’m sure Betty will find that helpful.

    Hey Millard. Any particular reason you plant sets (bulbs) rather than transplants (living plants)?
    Also, if you have a minute, please elaborate on “bulbs are in a terrible condition.”
    Thanks in advance, Millard.

    Theresa

  • Megan

    Hi Theresa,
    Thanks for the informative post. This year we tried direct sowing onion seeds into the ground. Some of them came up close to each other. I want to thin some of them out. I also don’t want to lose any of them. So, I was thinking of digging the whole bunch up and separating them. Then replanting them. It would disturb the roots though. What would you do?

  • Theresa

    Good to hear from you Megan!
    It won’t hurt small onions seedlings to be separated. It’s done all the time.
    Take up the whole bunch or as many as you think you can transplant in one session. As you go along, gently pull each seedling from the bunch and plant.
    They’ll be just fine!
    Hope you are having a great season.
    Let me know how things go.
    Theresa

  • Janet

    I just found your website while searching for onion seeds. Lots of great information! I garden just south of Charlotte, NC, and we had a rollercoaster spring, weather wise, as well. Extra cold winter, unusually warm spring days followed by unusually cold spring days, etc…

    I usually get my onion transplants from Dixondale, and have had lots of success, but this year I noticed a lot of bolting, which I hadn’t seen in previous years. Glad to know it was the weather and not me.

    Another thing, I decided to try fall planting this year, and bought some Candy transplants from my local garden center, planted them in October, covered with row cover, and not only did not one of them bolt, but the tops began falling last week, and they are big beautiful bulbs. Any thoughts?

  • Megan

    Thanks Theresa. I’ll do just that. I look forward to your next post.
    Have fun gardening, Megan

  • Theresa

    Welcome to TMG, Janet!

    My thoughts? : Congratulations on a job well done! Excellent that you are thinking outside the box and trying new things. You really hit on it with this one!

    If you’re asking why your bulbs are falling over, it’s because your beautiful onions are finished and ready to cure. After proper curing (see my post http://tendingmygarden.com/how-to-have-garden-onions-april-thru-january/ Candys should last about 2 to 3 months.

    And just for your information (and anyone else reading), I grew Candy for years. The variety was owned by the company Seminis. Monsanto bought out Seminis and I will no longer buy anything that supports Monsanto – no matter how little. Thus, I no longer grow Candy. And when I buy from Dixondale, I make sure that the varieties I’ve chosen are not owned by Monsanto. For more information you may want to read this post and this one.

    Again, congratulations on a great job with your onions!
    Theresa

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