How Big Do Onions Get?


If you are planning on growing onions in your garden this year, you might be wondering how big they will get.  That way, you can plan the number of plants and the amount of space you will need for your crop of onions.

So, how big do onions get?  Onion plants grow 10 to 30 inches (25 to 76 centimeters) tall, 4 to 24 inches (10 to 61 centimeters) wide, and produce bulbs that are 1 to 6 inches (2.5 to 15 centimeters) in diameter.

Of course, the quality of your onions (if you get any at all!) depends on the care that you give your plants.  Let’s take a closer look at onions, including size, growing conditions, and time to maturity.

How Big Do Onions Get?

Onion greens (the long, thin leaves that grow up from the onion bulb itself) can grow to a height of 30 inches (76 centimeters) above the soil, with a width of 24 inches (61 centimeters).

onions
Onion greens can easily grow to 12 inches or taller.

The average size of an onion bulb is 3 to 4 inches (7.5 to 10 centimeters) in diameter.  However, there are long, thin varieties with a diameter of only 1 inch (2.5 centimeters), and also large varieties with a diameter of up to 6 inches (15 centimeters).

How Long Does It Take Onions to Grow?

Some onions can mature in as little as 75 days (2.5 months), while others can take up to 160 days (over 5 months!) to mature.  However, most onions mature somewhere in the 90 to 110 day range.

yellow onions
Onions can take anywhere from 75 to 160 days to mature.

Keep in mind that there are multiple types of onions, including short day, intermediate day, long day, and day neutral.

Short day onions need 10 hours of daylight to produce bulbs.  They perform best in the southern United States (Zone 7 and warmer).  If the day length reaches 10 hours too soon, the bulbs form too soon, and the leaves will not be large enough to form large bulbs.

Long day onions need 14 to 15 hours of daylight to produce bulbs.  They perform best in the northern United States (Zone 6 and colder).  If the day length does not get long enough, you will get green leaves with no bulbs.

Day neutral (intermediate day) onions produce bulbs regardless of day length, and can perform well anywhere.  They should be planted in early spring in colder (northern) regions, and in fall in warmer (southern) regions.  They are especially well-suited to Zones 5 and 6.

For more information on how day length affects onion growth, check out this article on onions from the NC State Cooperative Extension.

According to the University of California, onion seeds will germinate in as little as 3 to 6 days with optimal soil moisture and a soil temperature of 65 to 85 degrees Fahrenheit (18.3 to 29.4 degrees Celsius).

How Do You Know When to Pick Onions?

According to the University of Minnesota, you should harvest onions when about half the tops (long, thin green leaves above the bulb) fall over and dry out.

Onions are ready to harvest when about half of the leaves have fallen over, and start to dry out (you can see that some of the leaves are brown here).

Remember that you can chop up the greens and eat them as well (my favorite way is to add them to a hot bowl of chicken broth after the weather cools down).

Why Are My Onions So Small?

The most common reason for small onions in your garden is due to planting the “wrong” variety for your region.  As mentioned above, onions may grow small if:

  • You plant short day onions in the northern United States and the bulb forms before the leaves can support a large bulb
  • You plant long day onions in the southern United States and the day length does not get long enough, so the bulbs never form

Choose the right onions for your area, or choose day-neutral onion varieties that can grow well anywhere.

red onions
Small onions are often the result of planting the wrong type of onion for your location. Try short day onions for the southern U.S., and long day onions for the northern U.S. Day-neutral (intermediate day onions) can do well anywhere.

Also, be sure to provide your onions with the proper care so that they can grow large – more on this later.

What Do Onions Look Like?

Onions grow into large bulbs in the ground, with small, thin roots below and long, thin, green leaves above.

Onion bulbs can be white, yellow, red, or purple, with paper-like outer layers to protect the crisp inner flesh.

Are Onions Hard to Grow?

Onions like full sun, so be sure to plant them in an area where they get 8 or more hours of sunlight per day.  Avoid planting onions in a place where they will be completely shaded by a tree or tall neighboring plants (such as tomatoes).

Onions grow best in well-drained soil with a pH of about 6.5 (slightly acidic).  However, there are many other factors that affect onion growth, including temperature, watering, fertilizing, and spacing.  Let’s start with temperature.

Temperature for Onions

The minimum temperature for onion seed germination is 32 degrees Fahrenheit (0 degrees Celsius).  If the soil is any colder than this, you will see low germination rates – that is, if you can get any seeds at all to germinate!

This is nature’s way of protecting onion seeds from sprouting at a time when they will be unable to survive.  This is why it is suggested that you start onion seeds indoors to avoid cold soil temperatures in early spring.

The maximum temperature for onion seed germination is 95 degrees Fahrenheit (35 degrees Celsius).  If the soil is any warmer than this, germination rates will decrease.

Combined with high humidity, high temperatures can encourage the growth of mold, which is another threat to your plants.  So, don’t wait too long to plant your onion seeds and transplant your established plants outside!

You should start onion seeds indoors 6 weeks before transplanting into the garden.  Onions should be transplanted as soon as the ground can be worked (i.e. when the soil thaws).

For more information, check out this article on onions from the Old Farmer’s Almanac.

The ideal (optimal) temperature for onion seed germination is between 65 degrees Fahrenheit (18.3 degrees Celsius) and 85 degrees Fahrenheit (29.4 degrees Celsius).

Keep in mind that these temperatures refer to soil temperature, not air temperature.  If you want to find out the soil temperature, use a probe-type thermometer to check.

If the thermometer reads a temperature that is too low, then you have some options.  One option is to wait until the sun warms up the soil.

To speed up this process, clear away any debris, such as leaves or grass clippings, from the soil surface.  Also make sure to choose a location for planting that gets plenty of sun, so that it can warm up the soil faster.

If you are worried about a short growing season, you can also use a cloche (a plastic or glass cover) to trap some heat and warm up the air and soil near your onion seeds.

A cloche can be made from a plastic water bottle to retain warmth and humidity in the soil for seeds or seedlings as they grow.

For more information, check out the table below, and check out this article from the University of California on ideal seed germination temperatures.

Seed
Germination
Temperature
Temperature
(degrees
Fahrenheit)
Temperature
(degrees
Celsius)
Minimum 32 0
Ideal 65 to 85 18.3 to 29.4
Maximum 95 35

Watering for Onions

Onions need regular watering, and they will need more water as they get closer to harvest time.

garden hose
Onions will need more water as it gets closer to harvest time.

Putting mulch on top of your soil will help to retain moisture, especially during periods of hot, dry weather.  If you find that you have a problem with dry soil, check out my article on how to treat dry soil.

On the other hand, over watering your onion plants (or any plants for that matter) can lead to root rot and eventual death.  The best way to decide when to water is to feel the soil with your fingers.

If the soil feels dry 2 or 3 inches below the surface, then go ahead and water.  For more information, check out my article on over watering your plants.

Try to water early in the morning, rather than at night, to allow water to soak into the soil before evaporating.

Fertilizing for Onions

Adding compost to your soil before planting onions is a good way to improve drainage for clay soil, improve water retention for sandy soil, and add nutrients to your garden.

compost bin
Compost adds nutrients and organic matter to your soil, and can be made from kitchen scraps and yard waste.

For more information, check out my article on making compost.

Onions require plenty of nitrogen, so consider using a nitrogen-based fertilizer (such as ammonium nitrate or ammonium sulfate) at a rate of 1 cup per 20 feet of onions.  Stop fertilizing once the bulbing process has begun.

For more information, check out this article on growing onions from Texas A&M University.

Spacing for Onions

Onion transplants (sets) should be planted no more than 1 inch (2.5 centimeters) deep in the soil.  Onions should be 4 to 5 inches (10 to 13 centimeters) apart, with 12 to 18 inches (30 to 46 centimeters) between rows.  This will allow space for watering, weeding, fertilizing, and harvesting.

Conclusion

By now, you have a much better idea of how big onions get, in terms of both the bulb and the leaves above ground.  You also know a bit more about the care that is necessary to ensure a healthy crop of onions in this year’s garden.

I hope you found this article helpful – if so, please share it with someone else who can use the information.  If you have any questions or advice about onions, please leave a comment below.

jonathon.david.madore

Hi, I'm Jon. Let's solve your gardening problems, spend more time growing, and get the best harvest every year!

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