Onions are one of the easiest vegetables to grow, but they don’t always produce the big bulbs we want.
Their top growth may look impressive, but when you pull the onions up, you stare in disbelief at the tiny onions and ask yourself, Why did I even bother? and How do I get my onions to grow bigger next time?
After all of your hard work, instead of harvesting big onions like the ones you see at the supermarket, you end up with onions about the size of an egg (or smaller).
What happened? And what can you do to make sure you get bigger onions in the future?
Here’s 9 common reasons onions fail to produce large bulbs:
- Growing from onion sets instead of onion seed.
- Growing small-to-medium varieties.
- Onion variety unsuitable for day length.
- Incorrect soil conditions.
- Onions planted too late.
- Lack of nutrients.
- Onions planted too close together.
- Lack of watering.
- Insufficient weeding.
To grow bigger onions, you need to make sure you take care of everything on that list.
Let’s run through each problem.
To Get Big Onions Grow From Seed Instead Of Sets
Onions sets or bulbs are easy to plant and they don’t need weeks of preparation beforehand, which is great for busy gardeners. But sets don’t always produce big onions.
When you start onions from sets, you’ll usually get small or medium-size onions.
Why is that?
Onions sets are immature onions that grew from seed planted the previous summer. The small onions are harvested in the fall and stored over winter.
They’ve already gone through their first growing season, and when you plant those sets in the spring, they enter their second growing season.
How Does That Affect Onion Size?
Onions are biennial. That means they set seed in their second year.
Under natural conditions, where they’re not harvested in the first year, the top growth dies back and the onion bulbs lie dormant in the ground over the winter months.
In spring the onions begin to grow again, but because they’re in their second season of growth, they’ll send up a bloom (or a flower stalk) which eventually produces seed. This process is called bolting.
Onions sets are prone to early bolting because they’re already in their second growing season.
A cold snap in the spring, or a hot dry spell in early summer, can both trigger bolting. And once that process starts, the onion won’t grow any bigger because it’s putting all of its energy into seed production.
To grow bigger onions, your best option is growing from seed. This way, the onions you grow are in their first year, and since there’s no risk of bolting, they’ll grow to their full size.
If you don’t want to grow from seed, then pick up transplants at your local nursery. You’ll be limited to whatever varieties they have available, though.
Grow A Variety Capable Of Producing Big Onions
When you grow onion from seed, you get to choose from a wide variety of onion types. But you can’t just pick willy-nilly from the seed catalog or online store.
You need to choose varieties that can produce large onion bulbs, and varieties that match your day length.
To Get The Biggest Onions These Are The Best Varieties To Grow In The U.S.A.
Texas Grano – Heirloom variety. Sweet, Vidalia style, yellow skinned, short day onion. Softball size onions store for up to 4 months.
Kelsae – This is a giant onion and a world record holder. Heirloom, sweet, long day onion, which stores well all winter.
Walla Walla – Another Vidalia type onion with a sweet flavor. This long day onion can reach 6 inches in diameter, however it has a shorter storage life.
Sierra Blanca – Large white onion, neutral day. This onion is mild with a fairly short shelf life.
Ailsa Craig – Another giant variety! Mild and sweet, yellow heirloom onion. Grows up to 8 inches in diameter and keeps fairly well. Long day.
Sweet Jumbo – Long day Spanish onion. Sweet and mild and keeps for several months.
To Get Big Onions You Must Choose Onions Suited To Your Growing Zone
Notice in the list above that some onions are short day, some are long day, and some are neutral day?
What does that mean and why does day length matter?
Onions bulb up based on day length.
- Short day onions begin to form bulbs when they receive 11 – 12 hours of daylight.
- Neutral or intermediate day onions will bulb with 12 – 14 hours of daylight.
- Long day onions need at least 14 hours of daylight to form bulbs.
Choosing onions with the right day length for your growing zone is absolutely essential because if you grow the wrong type of onion, your onions won’t bulb up or the bulbs will be small.
- If your growing zone is above 37 degrees latitude, you should grow long day onions.
- Between 32 degrees to 42 degrees latitude, you can grow neutral day onions.
- For growing zones between 25 degrees to 35 degrees latitude, choose short day onions.
If you try to grow a long day onion in a short day zone, your onions won’t form bulbs because they’ll never receive the 14 hours of daylight they need.
And if you try to grow a short day onion in a long day zone, it will bulb up too early before it’s had time to develop good top growth (leaves).
I’m in growing zone 9, at 48 degrees latitude. By April, we already get 12 hours of daylight. If I planted short day onions here, they would fail miserably because they wouldn’t have time to grow many leaves before the day length triggered bulb formation.
Each leaf on an onion grows one layer of the onion. More leaves grow as the season progresses. When onions don’t have enough leaves, it’s impossible to get big onions.
Leaves don’t only translate into onion layers. Onion leaves are a carbohydrate store and when onions begin to bulb, it’s the transfer of carbohydrate from the leaves to the bulb that allows the onion to fill out.
Use the map below to figure out your approximate latitude (US)
Onion Soil Conditions & Sunlight Requirements
Onions like a freely draining, loamy soil. They’ll manage in heavier soil, but it’s a good idea to dig in some compost first.
If you’ve got heavy soil, consider making a raised bed to help the soil drain. The bed doesn’t need to be very high, 4 – 6 inches will do the trick. And there’s no need to build a bed with sides. Just take the top few inches of soil from the adjacent area and add it to the growing bed.
Check your soil pH level. A neutral pH is fine, but onions prefer soil to be a touch on the acidic side, and pH 6.0 to 6.8 is ideal.
Soil pH is often overlooked by home growers, even though soil conditions are a crucial factor for plant growth.
When you plant onions or any other vegetable in soil that matches their pH needs, they’ll have easier access to the nutrients in the soil, which results in bigger, healthier plants.
Where you plant your onions is another big factor which determines how big they will grow.
Onions need a sheltered spot in full sunlight to reach their full size.
Green onions (scallions) can tolerate partial shade, but bulb onions need all the sunlight they can get.
Make sure you plan your garden beds in advance, so your onion bed won’t be shaded by any pole beans, peas, or corn that you plant once the weather warms up.
Start Onion Seed In Mid To Late Winter
While it’s possible to direct sow onion seed outdoors in the spring, a spring sowing won’t give your onions the time they need to make bug bulbs.
The trick to growing big onions is to start seed early, so your onions are already growing when spring arrives.
Typically, that means figuring out your last average frost date and starting your onion seed indoors 4 – 6 weeks earlier.
For me in zone 9, that means sowing onions in February so they’re ready to plant out in mid-March.
Find your average last frost date here.
(Remember, last frost dates are just guides, nothing is set in stone when it comes to the weather, and that’s more than true than ever these days).
When it’s time to plant onions out in their growing bed, they’ll tolerate a light frost, even though they look fragile. You can give them some extra protection with a layer of horticultural fleece for the first month to boost their growth.
Big Onions Need Fertilizer!
When you plant your onions, give them an initial feeding of blood and bone meal so they have plenty of nitrogen. Then continue to feed every 3 to 4 weeks throughout the growing season.
Blood and bone meal supplies all the nutrients onions need, and it’s an excellent source of slow release nitrogen.
If you notice yellowing on your onion leaves, that’s a sure indicator your onions need more nitrogen.
Once the bulbs start to swell, don’t apply anymore fertilizer. More nitrogen at this stage of growth will cause onions to develop thick necks; and thick necks make onions difficult to cure which shortens their storage life.
Give Your Onions Room To Grow
To get bigger onions, don’t overcrowd them when you’re planting out.
I know it’s tempting to try to squeeze extra produce out of your garden by planting crops closer than their recommended spacing, but plants need room to spread their roots into the soil to find nutrients and water.
Overcrowding also leads to plants overshadowing one another and blocking sunlight, which leads to lower overall growth.
For big onions you’ll usually be fine leaving 6″ (15 cm) between onions in a row, and 12″ (30 cm) between rows. But if you’re growing giant varieties, then they’ll need wider spacing. Check the spacing recommendations on your seed packet for the exact measurements.
Water Onions Once Or Twice A Week
Unless the weather is hot and dry, onions will be fine with one or two good soakings each week. There’s no need to water every day.
But keep an eye on soil moisture levels by using your finger to check the soil a couple of inches down.
Sometimes, the surface can look damp while the root zone is dry as a bone.
On the flip side, don’t let your onions become waterlogged. If you prepared your growing bed with compost to help with drainage, you shouldn’t have to worry about periods of heavy rain.
Keep On Top Of Weeds
Because onions are shallow rooted, they can’t compete with weeds, especially when they’re small.
Keep your onion bed weed-free to stop fast growing weeds smothering young onions. Weeds also deprive onions of nutrients and water.
Be careful when you weed the seedlings, though! Because onions are shallow rooted, it’s so easy to uproot them when you pull out weeds growing close by.
When onions are small, pinching the tops off weeds is often safer than yanking them out.
To make weed control easier, and help your soil retain moisture, use a light mulch around your onions.
To grow bigger onions, grow from seed or transplants instead of onion sets.
Choose varieties that produce large onions, and make sure they are the right day length for your region.
Add compost to your soil to help with fertility and drainage before you transplant seedlings, and don’t forget to fertilize after planting and throughout the growing season.
Don’t overcrowd your onions, keep the soil moist (no drying out or water logging) and make sure weeds don’t steal the nutrients, water, and sunlight your onions need.
Okay then, that’s the end of this post. I hope I’ve answered your question and given you some practical steps to follow the next time you grow onions.
Thanks for reading: How do I get my onions to grow bigger 🙂