How to Grow Onions for Big Bulbs

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Onions are always a “must-have” vegetable in my garden. They take up little space, I harvest them in early June (thus making room for another summer crop), and they boast a long shelf-life.

Though easy to plant, it’s not always a sure bet that they’ll grow into huge bulbs each season. In fact, onions can be a bit finicky. Since I’ve had both awesome harvests and disappointing ones, I’ve set out to discover what I can do to give my onion plants the best shot at huge, healthy bulb growth.

What I’ve found, more than a list of “how-to’s,” is that learning about onions’ particular growth habit and preferences helps me to take steps to optimize the best conditions for growth.

In this episode of the Beginner’s Garden Podcast, I go over the “long and short” (pun intended — you’ll understand after you listen to the episode) of growing onions. Listen here or read the highlights below.

How to Grow Onions for Big Bulbs

In my experience, mother nature affects onions more than other crops, so understand — you can’t control everything. BUT, hopefully by understanding them better and learning from my mistakes, you will have a satisfactory harvest yourself!

Which Onions to Plant for Bulbs

First, what to plant can get confusing. You have three options: seeds, sets, or transplants.

Seeds. If you plant seeds, you’ll need to plant three or more months before your last spring frost. I recommend this only if you’ve had some experience with onions.

Sets. Onion sets are little “mini-onion” bulbs, but contrary to what you may think, these are not the ones to grow if you want large bulbs. Grow these instead if you want leafy green onion tops.

Update: I’ve recently discovered that some gardeners CAN plant these onion sets for bulb onions. From what I understand, though, these onions are long-day onions, which explains why I wasn’t able to get a bulb from them in the southern US. Northern gardeners who grow long-day onions could plant onion sets for bulbs. (See below for why the differences in onion types matter.)

Transplants. Transplants are bundles of onions you find in your garden center. These are what I plant.

onion transplants

How to Plant Onions

Plant onions about 6″ apart in all directions. For transplants, bury about an inch into the ground, until the leafy part protrudes from the soil. Bulbs should be planted just below soil level.

You can plant in rows, in a square foot gardening method, or stick them around existing plants. I usually do a combination of all of them!

In my experience, they have grown best in raised beds, but that’s because my native ground soil is heavy clay.

I think success with growing onions will take time for you to try out different things — location of planting, timing of planting, etc. — to see what works best in your climate.

onion harvest

Basic Growth Habit of Onions

Onions are a cool-season crop that take a long time to grow. Their growth habit can be split into two distinct stages: leafy growth and bulb growth.

Whether grown from seed or planted as transplants (the green onion-like bunches you purchase at the garden center or feed store), the onion will pour all of its energy into producing tall, vibrant green leaves. At some point in the growth, the onion plant will discontinue its focus on leafy growth and switch to bulb growth. The timing of this switch is determined by day length.

In the spring and summer, the further you live from the equator, the longer your days are. So, those of you in the North will have longer summer days than those of us in the South. 

Because day length triggers bulb formation, you need to purchase the type of onion suitable for your area.

  • long-day onions form bulbs when daylight is 14 to 16 hours 
  • short-day onions form bulbs when daylight is 10 to 12 hours 
  • intermediate day onions form bulbs when daylight is 13 to 15 hours

Click here to view a map of US and which onions to grow where. 

Factors that Can Interrupt Proper Bulb Formation

Even if you plant the type of onion idea for your area, other factors play into an onion’s growth. Ideally, the transition from leafy growth to bulb growth will occur naturally, but sometimes environmental stressors will interrupt that transition.

If this happens, the onions will stop focusing on bulb growth and will instead start to bolt, or form a seed stalk, instead.

The main stressor I have found is inconsistent temperatures.

Ideally, leaf and bulb development occur when the temperature is around 55 – 75 degrees. Onions that have grown to thicker than a pencil width are frost tolerant and in general can withstand occasional dips in temperature as low as 20 degrees (as long as that cold is not sustained), but on the other end, once they’ve started developing bulbs they can withstand temperatures above 75 degrees.

But, if temperatures rise above 75 degrees and bulbing hasn’t started yet, or if temperatures dip below 40 after bulbing has started, onions are susceptible to bolting. These are generalities, but you get the idea:

  • before bulb growth starts, onions prefer cooler temperatures and can tolerate frosts and freezes to some point, but they don’t like a hot environment
  • after bulb growth begins, onions are more sensitive to cold temperatures and prefer warmer weather, though they still don’t prefer hot temperatures

If the onions experience extremes in temperature, they may begin bolting. Bolting is when the onion stops sending its energy to either leaf growth or bulb growth and instead spends its energy producing a seed stalk. You’ll notice this when one of the onion leaves grows taller and sturdier than the others and you’ll see a tear-drop shaped tip on the end. Inside the translucent covering, you can spot the seed pod, which, when left alone, will burst out with a beautiful round flower.

What Happens When Onions Bolt

If your onions start to bolt, here’s what you need to know.

  • The bulb formation has stopped. You can either harvest them now for the bulb that has already formed, or you can leave them in the garden to set seed and attract pollinators and beneficial insects.
  • If you harvest the bulb, you’ll need to use it within a few days. Bulbs from onion plants that have bolted will not store.

How to Prevent Bolting (as much as possible)

Onions are a little picky when it comes to temperature and day length. Day length is a little easier to manage because you just have to purchase the right onions for your latitude. So then, there are two factors I’ve found to getting the best bulbs to form: one you can control and one you can’t.

  • Timing. In my area (Zone 7b with a last frost date of March 31st), onions planted in early March do the best. I recommend planting transplants 4-6 weeks before your last spring frost.
  • Spring Weather. Many times our springs seem like we’re on a roller coaster — hot then cold then hot. But when it comes to temperature, it’s the soil temperature that will have more of an effect on the plants than air temperature. Therefore, mulch well after planting. 

Onions, to me, are one of the most fun and frustrating crop to grow. With a good season, harvests are amazing! But when an unpredictable spring gets thrown in, it’s hard to predict what kind of harvest I’ll get. But when we take proactive measures such as planting the right type of onion at the right time and mulch to keep soil temperature consistent, the rest is up to Mother Nature!

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  1. I just listened to your podcast on this , I also live in a climate where the spring is a constant brutal battle between winter and summer 😂 (I live in a high altitude desert Colorado , size 7a and very hot summers )

    I wonder if you issues with bolting come from using dried transplants and or sets. One year I ordered from Dixondale farms and the years I’ve used sets have almost all bolted. Fresh seedlings and home sown seedlings don’t do this *at all*.

    I think it’s because the drying process, for sets especially which is bulbing and drying, plus the cold storage after that is like “winter “ to these plants. So once you plant them out it’s the second year and they do what biennials do, bolt. At least that was my theory 😂

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