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Growing Onions

Updated December 18, 2018

David Graper

Professor Emeritus of Horticulture Science

Onions have been a commonly grown vegetable for thousands of years. They are easy to grow, nutritious and can be stored for months until they are needed as part of a meal. There are many different kinds of edible members of the Allium genus but bulbing onions are the most commonly grown. Onions may be white, yellow or red – referring to the color of the outer scales of the onion bulb. Onion plants are photo-periodic meaning that changes in day length, like the long days that we experience here during the summer and shorter days of winter affect how onions grow and when the onion bulb develops. In the northern Great Plains, we should mostly grow long-day onion varieties because we plant in the spring and the onion bulb develops during the long days of summer. There are also day-neutral types of onions; these can be grown here too. If you plant short-day onions, they will not likely develop a very large bulb because bulb enlargement won’t really start until the day length shortens in the fall.

The most common method of planting onions in this area is to plant onion sets in the spring of the year. An onion set is actually a small onion that was planted late in the season then harvested when it slows growth in the fall. The onion sets are dried and packaged for spring planting. Onion sets then begin to grow again when we replant them in our own gardens in the spring. Onion sets are a convenient way to plant onions but growing them from seed will usually yield much larger onions that will also keep longer when harvested in the fall. Onions act like a biennial plant that puts on vegetative growth the first season, goes dormant, then resumes growth the second season, mainly to produce a flower stalk. Since onion sets are actually starting their second season of growth when you plant them, they have a tendency to produce a flower stalk, also known as bolting. Bolting can also be brought on by stressful weather conditions, like the heat and drought many gardeners have been experiencing late this summer. Once an onion bolts, it is best to cut off the hollow flower stalk which will help to stop the onion from utilizing its stored food reserves to make the flower and seed. Harvest onions that have bolted first because they will not store well after bolting.

Growing onions from seed is also quite easy. Check with your local garden center to see if they sell onion plants in the spring. These may be young plants that are actively growing in a pack or they may be slightly older onion plants that have been bundled and dried somewhat before they are ready for sale. If you cannot find locally available onion plants, you can grow your own too. Purchase some onion seed from a local garden center or via a catalog or online source. Be sure to choose varieties that are labeled either as long day or “northern” varieties or listed as day neutral. Plant the seed in a small flat or pot about 6-10 weeks before you want to plant your garden. Grow them on a sunny windowsill, or under artificial lights. Fluorescent lights work well but you need to have the little sprouting onion seedlings just a few inches below the tubes. If the baby onions do not get enough light, they will be weak and spindly and not transplant well into the garden.

Onions can tolerate cold temperatures and a light frost but it is best to wait to plant them until you are close to your last frost date. Onion seedlings that have each developed at least 2-3 leaves are large enough to plant. Use a hoe to make a shallow V-shaped trench. Carefully pull apart the seedlings into individual plants. Space the plants about 3-5” apart. Hold the plant up so that the roots are spread out in the trench then cover the roots, gently firming the soil over the roots. Gently water the row of young plants to firm the soil over the roots. If the soil dries out, water again to avoid water stress to the plants. You can start harvesting the young onions from your garden for fresh green onions in a few weeks. Just watch for the small bulb to begin developing.

onions still in the ground with their tops bending over
Tops of onions flopping over in a garden.

You can continue to harvest fresh onions the rest of the summer as you need them. But, if you want to harvest and store the onions, it is best to wait until the onion plant quits growing for the summer. You will know this has happened when the onion tops begin to dry and fall over at the top of the onion bulb.

a hand holding an onion and twisting the top off
Twisting off the stem off an onion.

At this point you should notice that the onion bulbs are beginning to develop dry papery outer scales. Pull up the onion, remove any soil that is still adhering to the bulb and remove the top. I prefer to twist the top on the bulb until it tears off.

This will help the inner parts of the onion from drying excessively and seems to provide better storage of the bulbs. However, some people will braid the tops together to make a string of onions that they can then hang up for drying and longer term storage. Just cut off a bulb when you need it. One last comment on this topic, some people have asked me if they should break over the tops on their onions to hasten maturity. I don’t think this is a good idea because you will be sacrificing additional onion bulb growth by doing this.

Let the newly harvested bulbs dry in the sun for a few days. You should see that the outer bulb scales are getting dryer and turning brown, tan or a reddish color, in the case of the red onions. You can carefully remove a few of the outer scales if they still are coated in dirt but do not remove all the outer scales because these act as a wrapper for the bulb. This wrapper of leaves protects the fleshier inner bulb scales and also helps to protect the bulb from excessive drying.

a bunch of onions in a rectangle basket
A crate of onions without the tops.

When the onions seem quite dry, usually in a few days, you can transfer them to mesh sacks, a plastic ventilated crate or a box with holes in it. Store the dried bulbs in a cool location, ideally around 50°F, until you are ready to use them. Many gardeners will store their onions in a garage. This is fine as long as it does not get too cold. If the onion bulbs freeze they will likely rot when they thaw out again.

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