How Do Onions Grow?

Whether globe, granex, or cippoline shaped; red, yellow, or white; sweet or pungent; onions of all varieties are cool season vegetables that can be somewhat difficult to grow.

Like most plants, onions grow best when soil nutrients, temperatures, and moisture levels are at the right levels. The outlier with growing onions is that daytime lengths are an important factor in how onions grow.

Onion varieties are separated as short day onions, intermediate day, and long day onions. The bulb growth of the onion does not happen in a specific time frame but begins when daylight hours reach a certain length.

This can make growing onions tricky because onion plants need to reach a certain size, at the least above ground growth of 4-6 leaves the size of a pencil, before they are capable of growing a decent size onion bulb.

Where I’m at in Michigan it’s necessary to start onion plants indoors for them to reach this size before daytime lengths reach 14 hours and trigger bulb growth.

You can use this sunrise/sunset calendar to figure out average daytime lengths in your area. Daytime lengths is a setting you can click and it will display on the calendar already calculated for you.

How Are Onions Grown Commercially?

Onions are grown from the tropics to subarctic regions and in home gardens, small-scale farms, and on larger-scale farming operations. Onions have been a part of the human diet since prehistoric times when hunter and gatherers would find and collect them.

Check out “Where Do Onions Grow” for more on that. 

Over 120,000 acres of onions are commercially grown each year in the United States, producing almost 7 billion pounds of onions.

This onion field is planted in black plastic mulch.

Like any vegetable grown on a large-scale, onions are grown in fields that are properly prepared in advance.

Fields are first deep-turned to bury any residue from previous crops and fertilizer is applied prior to or immediately after planting.

Typically soil tests are done for fertilizer recommendations that are specific to the needs of the field. Check this article out for more on how onions are fertilized.

On larger-scales, onions are most commonly started indoors for 8-12 weeks in a greenhouse before planting into the field.

Plant tops are commonly trimmed down to 4 inches to ensure a sturdy transplant is produced. Transplants are planted outside when they reach about the diameter of a pencil.

Because they are cool season crops that can handle temperatures in the 20’s, onions can be planted as early as mid-March in northern climates.

To transplant, onion plants are pulled and bundled in groups of 50-80, tied together with rubber bands, and then the transplants are hand planted into the field.

Generally, 75,000 to 120,000 onions are planted per acre. Transplants are placed roughly 2 inches apart from one another. Check out “How Far Apart To Plant Onions” to see how spacing affects onions.

Commercially grown onions can be hand or machine harvested.

This is how onions are machine harvested according to the Onion Production Guide produced by the University of Georgia Extension.

Farmers will undercut the onions with a rotating bar or fixed blade. Disks can also help to loosen bulbs from the soil. To help roll onions out of the ground, a rope is often dragged across the top of the field after bulbs are loosened. 

After harvesting, onions are then cured to extend their shelf-life. Curing removes moisture from the outer part of the onion while protecting the high moisture content of the bulb.

Growers can cure onions either in the field or through a batch-curing process. If left in the field to cure, the process can take 3-5 days to as long as 2-4 weeks depending on both the region and the weather conditions. 

If cured using a batch-curing process,  onions are first dried in the field for 3-5 days and the tops and roots are removed.

In batch curing systems, the outside air is heated and then forced through a stack of onions for 24-48 hours. Once cured, onions are graded and packed for consumer sale. 

How Do Onions Grow Step By Step

Onions begin to grow once their primary roots are established. The plant can then focus on leaf growth and eventually produce a bulb. 

Seeds can germinate in temperatures as low as 45℉ and as high as 85℉ with optimal onion seed germination at soil temperatures of 75 degrees.

Onion seeds will germinate in 4-21 days with germination generally happening faster with warmer temperatures and slower in cooler temperatures.

In the first step of germination, the primary root grows, emerging from the seed coat to help anchor the seed to the soil.

The primary root starts absorbing moisture and nutrients from the soil, helping to drive the development of the first shoot. The shoot is pulled upwards through the soil as it grows toward the light.

Once through the soil, plants channel their energy into developing leaves. At this stage onion plants still rely on food stored in the initial seed. Because of their limited access to nutrients, growth of seedlings is relatively slow until the first true leaf appears. 

Once the first true leaf appears, photosynthesis begins and the seedling starts producing glucose for food. Because of its ability to produce glucose for energy, onion plant growth is accelerated, turning new, undifferentiated cells into new leaves.

With more leaves the rate of photosynthesis increases, driving even more growth and the thickening of the seedling stem. 

Once onion seedlings have 5-10 leaves, they should be thinned until each plant is spaced 3-4 inches apart. The more leaves an onion plant has the more layers of the onion bulb that will develop, ultimately creating a bigger bulb at harvest.

Bulb growth will start when the daytime length reaches the amount needed by the variety of onion being grown. At the same time leaves that the plant has already grown begin to widen and lengthen to increase the surface area available for photosynthesis. 

When the onion bulb gets large enough, the plant will begin to push away from the soil and “pop” up out of the ground.

The onion focuses most of its resources on growing the bulb until ready for harvest. Onions are ready for harvest when their tops begin to fall down.

At this point, the remainder of the plant’s resources have moved from its leaves (tops) into the bulb, increasing the bulb size to its maximum point of maturity.

While the exact time to harvest varies with region, weather, and variety, most onions are ready to be harvested when 20-50% of onion tops have fallen over. 

Do Onion Plants Come Back Every Year?

If bulbs are not harvested as annuals, the onion plant will continue to grow for a second year, but not a third.  Because of this, onions are considered biennials.

As biennials onions require two growing seasons to complete their entire life cycle.

During their first growing season, onions grow foliage and a bulb. If left for a second year onions will use most of their energy to produce flowers and seeds.

Bulbs provide the energy for this process and thus will not increase much in size. If harvested at the end of the second year bulbs will be fibrous and woody.

Do Onions Grow Underground?

The bulb of the onion plant grows underground while onion tops grow above ground.  During the first year of their life cycle, bulbs are formed underground.

By fertilizing and watering the soil appropriately, gardeners attempt to maximize underground growth to create a better onion harvest. A major factor in having good sized bulbs is planting at the right time so onion plants can reach a good size before bulb growth is triggered.

How To Grow Bigger & Sweeter Onions” is a very interesting article I recommend to all gardeners!

Onion plants produce bulbs to serve as food sources for the following year of their life cycle, when most of the plant’s energy will go towards producing flowers and eventually seeds. 

While onion tops do grow during the first year of the onion life cycle they grow even bigger during the second growing season.

In the second year of their life cycle, as soon as temperatures warm, onion plants utilize their completely developed root system and small bulb to grow big leaves quickly before sending up a stalk and producing a flower and seeds.

Any remaining nutrients from the bulbs are sent upwards to create the food reserve that seeds will use for germination.

Please comment below with your own thoughts or experiences on how onions grow or are grown.

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