Onion Plant Care & Growing Tips

Here are some FAQ’s about onion growing. Please also check out this article “How To Grow Bigger & Sweeter Onions” for an in depth look at what it takes to grow bigger, sweeter onions. I think you’ll enjoy it.

What Temperature Is Too Cold For Onion Plants?

Onion plants are cold resistant and can survive light freezes. A hard freeze, temperatures below 25 degrees, of 4 hours or more can kill the foliage of an onion plant and possibly damage the root system.

Where I live in southwest Michigan onion plants usually do fine over our cold winters and come back in the spring.

I think the main problem with cold temperatures over the winter is the chance of the onion bulb freezing and then unthawing and becoming rotten. But this hasn’t been a problem for me.

Using a thick layer of mulch and row covers can help young onion plants do better in the early part of the season.

If you’re direct sowing onion seeds into the garden wait until soil temperatures are at least 40 degrees. Like most seeds, onion seeds will germinate faster with warmer temperatures than cool temperatures with the optimum soil temperature for onion seed germination being 75 degrees.

Use a soil thermometer to test the soil temperatures. Along with the last expected frost date it’s a great way to know when to plant what outside.

Why Weeding Around Onion Plants Is Important

Onion plants are especially vulnerable to weeds. They have shallow roots and sparse foliage above the ground allowing other plants to move right in next to onion plants. Weeds can steal nutrients and water from an onion causing stunted growth or even killing off small onion plants.

Can You Grow Onions From Store Bought Onions?

You can grow green onions from store bought onions that sprout. What you’ll do is split the sprouted onion into 2-3 starter onion plants and plant those just like they are onion seedlings.

It is also possible to get harvestable onions from sprouted onions but they will be smaller and more spherical than onion bulbs grown from seed. Also there’s a good chance that planted sprouted onions wil go to seed.

It’s also possible to plant just the bottom of an onion which will grow green onions out of the top but won’t regrow an onion!

Do Onion Plants Need Full Sun?

Yes! All varieties of onions need full sun. Without enough sunlight, the onion will be stunted and develop a smaller bulb. Depending on the variety (long, short, or day-neutral) they may require more sunlight.

  • Long-Day Onions require 14-16 hours of sunlight.
  • Short-Day Onions require 10-12 hours of sunlight.
  • Day-Neutral Onions require 12-14 hours of sunlight.

Check out “Long Day vs. Short Day Onions” for more on that.

Is It Better To Plant Large Onions Sets or Small Sets?

Smaller onion sets are usually better than larger sets as larger sets tend to “bolt” early, develop a flower and go to seed. If you’re looking to harvest green onions however you should use large onion sets as they will be ready faster and you can harvest them before any bolt.

Onions that produce a flower end up being about half the weight of normal onions and they don’t store well.

Why Are My Onions So Small?

Probably the biggest reason onions are small is that the plant didn’t get enough growing time before day length hours triggered bulb growth.

At a minimum, an onion plant should have 4-6 leaves that are at least the size of a pencil before daylight hours trigger bulb growth.

In northern states it’s almost necessary to start onions indoors to ensure they get that size before day lengths reach the 14-16 hour range.

You can use this sunrise/sunset calendar to see when day lengths reach this period in your area. If you go to the settings there is an option for day lengths that you need to check and then it will display the day length for each day on the calendar.

Other reasons onions might be small is tight plant spacing, onion variety, not enough sun, irregular watering, or soil nutrition levels.

Check out “How Often To Water Onions” for the right schedule and other tips on watering onion plants.

Are Onion Plants Perennials?

Onion plants are biennials that take two years to complete their life cycle. In the first year a bulb, roots, and foliage above the ground grow. Then the plant goes dormant when temperatures cool and wakes back up in the spring when temperatures warm. In their second growing season onion plants will flower and produce seeds.

You might also be interested in “Onion Plant Growth Stages & Life Cycle (With Pics & Tips)” for a more detailed account of an onion plants life cycle.

Please comment below with your own tips for growing onions.

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