Everything You Need to Know to Grow Onions

Did you know that onions are one of the most popular vegetables for home gardeners? True story! In the 2014 National Gardening Association Report onions were actually ranked at #7 out of 10 (National Gardening Association). And it makes total sense – onions are an extremely versatile food crop, are relatively easy to store to have on hand in the future, and are consumed on a widespread global basis as a staple of nearly every major cuisine. In fact it is estimated that at least 175 countries grow onions – that’s more than twice the amount that grow wheat! (BBC)

As I’ve been reading and researching more about onions I have become fascinated with this incredible vegetable, and how to grow onions successfully. (Read: there is a LOT of information in this post!) But before we can dive into the ‘how to grow’ portion, we need to first understand the WHY behind the way we grow. And don’t forget to check out our Garden Reference Cards! These pocket-sized cards have all of the key information you need to know, from sowing to harvest.

How Onions Grow

Onions are a biennial crop, and like many other plants, are classified according to the amount of light and dark needed to create bulbs – called ‘photoperiodism’. This means that the shift between when they grow leaves and then switch to start forming bulbs is determined based on the amount of light that the plant receives – the light acts as a trigger to signal the plant that it’s time to start producing its crop. Why does that matter? First, each leaf that is able to grow before bulbing begins is another layer for your onion - more layers equals larger onions! If the day length of the onion isn’t right for your area you end up with either failed bulbing, or a plant bolting (going to flower) before it even begins bulb formation. Neither is good for the home gardener. The best way to ensure you get good onion bulb production is by choosing a variety that is suited for your growing area and daylight.

The 3 groups you’ll see are called “short day”, “long day” and “intermediate day” or “day neutral”. 

Onions and Day Length

Day length is a major determining factor in onion bulb formation.

Short day plants need 10-12 hours of light to create the onion bulb. These are usually grown in the southern United States, in zones 7 and warmer - below the 36th latitude parallel to be precise. In order for the onions to have sufficient growth before they begin bulbing they are usually planted in the fall (when day length is shorter and they can focus on producing luscious leaves), allowed to overwinter (which they can do well due to milder winters in these zones), and then begin forming bulbs as the days begin to lengthen in late Spring – in my growing zone in Georgia the days reach 11 hours in mid-February. (And as an aside here, I didn’t just happen to know this information (although that would be a pretty cool garden party trick, right?!) – you can look it up through sites such as Weather Underground or Time and Date.)

Long day plants need 14-16 hours of light in order to form onion bulbs. These are best grown in the northern US where the days are naturally longer because of the tilt of the Earth’s axis, in zones 6 and colder. Because the winters are so cold in these areas you’ll want to wait to plant your onions until the spring, as soon as the ground can be worked. The onion will then begin growing, putting on its leaves, and then begin bulbing as the days hit 14 hours of light – in Augusta, Maine this is around late April. 

Besides short and long day varieties, there’s also intermediate or day neutral varieties, which begin bulb formation when days reach between 12 and 15 hours. While they can be grown just about anywhere, they really thrive in zones 5 and 6, across the middle of the US. If you have a mild winter you’ll want to plant in the fall, like you would for a short day variety, or in early spring if you have more frigid weather, like for a long day variety. 

If you’re like me, at this point you might be wondering “well what if I want to grow one of the other varieties in my area?” Hey, I get it, I like to color outside the lines sometimes, too! So let’s look at my garden for an example. If I tried to grow-long day onions in my Georgia garden I would get beautiful leaf growth, but never get any bulbs because my days never really hit that threshold of daylight to trigger bulb formation. I could still get some good onions greens that I could harvest fresh, but wouldn’t get any of the desired large bulbs. On the opposite hand, if a grower in the north tried to grow short-day onions in the spring they wouldn’t have enough time for leaf growth before the days hit the 10-12 hour threshold for bulb formation, resulting in super scrawny bulbs (if any at all).

Once you’ve decided which day length onion is best for you it’s time to start choosing how you want to grow your onions - do you want to grow from seed or sets? Let’s take a look at each!

If you choose to start onions from seed be sure to use fresh each year - they do not store well for extended lengths of time.

Getting Started with Onions

Starting onions from seed gives you the best selection - there are TONS of different options for onion seed. But this is the slowest way to grow onions - seeds need to be started 8-10 weeks before planting out, and it takes usually 4-5 months before onions are ready for harvest. If you decide to grow your own from seed be sure to keep in mind that unlike many other vegetables started indoors from seed you want to limit the amount of light they are exposed to to under 10 hours - remember they are daylight sensitive so keeping them under prolonged light makes them think it’s time to stop growing leaves and switch to bulb production. You can also skip the seed starting step and purchase onion transplants from growers, but the variety options will be more limited. But planting purchased transplants definitely shortens your time to harvest to around 60-70 days. Whether you start your own seeds, or purchase transplants, make sure to harden off your transplants before planting them outside by gradually exposing them to the outdoors over 1-2 weeks. 

Onion sets are another option for growing onions. These are immature bulbs that actually started growing the previous year, but then were pulled before they reached maturity. They really speed up the time to harvest, and in the north (long-day areas) will provide a much earlier crop. In the south (short-day) you can get much larger bulbs, because they are starting from a small bulb already, so they grow even bigger than you would normally expect if growing from seed. There is a risk, though, that the sets could be more likely to bolt, or go to flower, leaving you with a pretty bloom but not your desired onion harvest (because onions are biennial, so it’s only natural for them to want to go to seed in their second year). Choose smaller sets to help reduce this risk. Follow the same general planting guidelines for onion seeds and transplants - late fall in southern areas of the US, and in the spring in colder/northern areas. 

Growing Needs

Regardless of how you decide to start your onions, their growing needs are the same. 

Make sure you have loose, rich soil for good bulb formation

Planting location and spacing needs: Like most vegetables onions need full sun to grow best. And like other root crops, such as carrots (check out the blog post HERE if you’d like to read more about those), onions need loose soil and plenty of space between them - at least 6” for good bulb production. If you’re growing bunching onions for their tasty, leafy tops, then plant 2-3” apart. Keep your growing area free of weeds by using a good mulch such as straw, but just be sure to move the straw back when the plants begin bulb formation, so that their tops can grow and cure properly.

Fertility: Although onions are technically a root vegetable, they are usually grouped as a leaf crop in crop rotation recommendations because of their high nitrogen requirement. If you follow crop rotation practices these are a great vegetable to rotate and plant after legumes, such as peas and beans, or cover crops such as alfalfa and clover, as those plants add nitrogen into the soil so there’s a good supply for your onions and other nitrogen-hungry vegetables. Every 3 weeks during the season, starting about 3-4 weeks after transplanting, side-dress your crop with a nitrogen-rich fertilizer. Good organic sources for nitrogen are blood meal, feather meal, or fish meal, or if you are looking for a fertilizer free from animal sources then consider alfalfa meal, cotton seed meal, or soybean meal. Just remember that nitrogen feeds plants for leaf formation, so when your days hit the right length for bulb formation you’ll want to stop adding fertilizer so your plants can focus their energy on bulbing. 

Water: Onions do best with consistent moisture during the growing season - at least 1” per week. Because they have very shallow roots consider using drip irrigation to be sure you get the water right where it’s needed most. 

Pests and Companion Planting

Onions make great companion plants in the garden!

As far as pests go onions are pretty easy keepers - there’s not many pests who choose onions as their prime target in your garden. One that is perhaps more common to keep an eye out for is the onion maggot, the larval form of a small fly (about one-quarter the size of a house fly), which likes to snack on onion bulbs. An easy way to limit this is to use crop rotation, clean up any onion debris (such as from thinning plants, or any leftovers from the year before), and inter-plant your onions throughout the garden, making them harder for the fly to target to lay its eggs. Mixing onions throughout your other plants is also a good thing for your garden - onions make great companion plants, deterring many garden pests from your other crops. Other vegetables which benefit from having onions close by are plants in the brassica family, such as broccoli and cabbage, as well as nightshade vegetables like tomatoes, potatoes and peppers, and even other root vegetables like beets and carrots. But while there are many vegetables that love to call onions friend, be sure to avoid onions near beans, peas and other legumes - traditional garden wisdom reports that allium plants (onions but also garlic, chives and shallots, among others) inhibit their growth. 

Diseases

Onions are a pretty tough crop, but diseases, such as black mold, leaf blight or bulb rot are possible. The majority of the diseases to be mindful of are fungal, and are best reduced by providing adequate spacing between plants, using crop rotation to avoid planting onions in the same area in subsequent growing seasons (at least 3-4 years is recommended between rotations), and handling bulbs gently when harvesting and allowing to dry, to avoid damage that will allow pathogens to enter. 

Time to Get Growing!

So there you have it! Everything to get started growing onions. They are such an important and versatile vegetable, and with the right information and growing conditions you can have a great harvest! Remember, it’s all about the amount of light, starting your seeds/transplants or sets at the right time, in the right conditions, and with the right neighbors. To help you keep it all straight be sure to check out the Garden Reference Cards - pocket-sized and garden-proof cards, with everything you need to know from sowing to harvest!

Will you be growing onions this year? I can’t wait to hear all about your experiences!