What Type | When to Plant | How to Plant | Care | When to Harvest | Curing and Storage | Troubleshooting | Suppliers

Know Before You Grow - Essential Information You Need to Have a Shot at Success

Onions are a staple in many global cuisines.  They are outrageously versatile and impart superior flavor to countless dishes.  As a crop, they are extremely pest resistant, take up very little space, and can even lend a hand as perfect companions to countless other crops you may be growing.  They can also have phenomenal storage potential.  Growing your own onions sounds like a no-brainer and can be if you are armed with some essential information.

• Timing: Onions have a long growing season and are cold weather crops.  To accommodate this, they are typically planted very early in the spring and harvested mid-summer or planted late in the fall and harvested early to mid-summer. 

• Day Length Sensitive: Onions are day length sensitive meaning that their growth stages are dependent on the number of daylight hours. They are classified as either short, intermediate, or long day varieties.  Growing the right variety for your area is essential for success!

• Biennial life cycle: The full life cycle of an onion plant is two years.  In year 1, they focus their energy on producing leaves and bulbs.  In year 2, their primary goal is to produce flowers and seeds (also known as bolting) so they can pass on their genetic information before they die.  Onions often recognize cold periods like winter as the transition into year 2.  This has the potential to cause problems though when growing from sets or planting in the fall in colder regions.

• Edible at all stages:  Onions can be harvested early for delicious green onions (essentially interchangeable with scallions) or anytime during their development. This means that if you are impatient or just really need an onion, you don’t need to wait for them to ripen or cure to utilize them in your kitchen. They are never over ripe or under ripe - just onions, always ready to use.

Setting up For Success - The basics of how to get a successful harvest

Start With the Right Type of Onions

Day Length

Onions are “day length sensitive” which means that their growth stages are triggered by the number of daylight hours.  Onion plants start off by developing and growing leaves.  Bulb formation is triggered once a certain number of daylight hours / day are reached.  Different onion varieties have different triggers as to when this happens.  This is so essential to their success, that onion varieties are classified based on the day length. 

Your geographic location determines whether long, intermediate, or short-day onions are best suited for your area.  The map from Dixondale Farms (more on them later) indicates these designations in the US.

Starting Point: Seed, Sets, or Transplants

Who doesn’t like options?  When it comes to growing onions, we have three options to start from. 

1. Seeds: Many seed catalogues have a plethora of onion seed options.  Just note that onion seeds have a short viability period so if going this route, it is recommended that you purchase new seeds every year or two.

2. “Starts” or “Transplants”: Onion starts or transplants are simply onion plants that have been started and then pulled up when they were in their leaf growth stage. 

3. Sets: Sets are likely the most controversial and the most hit-or-miss way of growing onions.  You will often see bags of onion sets, which look like baby onions, at big box stores.  They are essentially that, onions that had JUST started bulbing and then pulled up.  They can be problematic though due to the biennial nature of onions.  Because sets are produced by stopping growth right after the bulb begins to form, this can lead to some of the onions thinking that they are entering year 2 once you plant them.  When that happens, those onions will prioritize their energy into producing flowers and less into producing the actual onion bulb.  The result? Smaller onion bulbs.

When to plant 

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Onions are a cold season crop meaning they do best when the majority of their growth occurs in cooler weather.  They can be planted once soil temperatures have reached 50°F/10°C or as soon as the soil is “workable” (meaning it forms a ball in your hand and falls apart when you push your thumb into it – not too wet or too dry).  These conditions are met at different times based on the zone you are in:

How to Plant 

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    Prep The Soil

Like most vegetable plants, onions will love a fresh 2-3 in / 5-7.5 cm layer of compost in well-draining soil.  They are also heavy feeders so starting them off with a fertilizer high in phosphorus will help the plants develop strong root systems to get established.  Bone meal is a good organic option for this but you can also use any all-purpose fertilizer with a higher middle number of the N-P-K ratios. 

Spacing, Depth, and Orientation

When it comes to spacing, you have a few options:

1. To maximize bulb size, space sets, starts or seeds 4 in/10 cm apart.

2. To harvest as green onions, you can plant as closely as 2 in/5 cm apart.

3. To harvest both green onions and larger bulbed onions, plant 2 in/5 cm apart to start and then harvest every other one early in the growing season.  Using this strategy, make sure that once bulb formation starts, all plants are 4 in/10 cm apart to allow full bulb formation of the remaining plants.

Your planting depth will also depend on how you are planting your onions:

• Seeds should be planted .25-.5 in / .6-1.3 cm deep (whether direct sowing or starting indoors).

• Transplants should be planted so that 1 in / 2.5 cm on the stalk is below the soil surface.

• Sets should be planted with the root end down 1-2 in / 2.5-5 cm deep ensuring that the growth tip is visible above the surface of the soil.

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While They are Growing 

    Fertilizing

We’ve all heard the metaphors: “onions have layers”.  For onions, the more layers, the larger the onion.  Each individual layer develops from an individual leaf.  One layer per leaf so, the more leaves, the more layers, the bigger the onion.  With that in mind, your focus should be on setting your onion bulbs up for success by getting your plants to develop as many leaves as possible. Let’s look at how we do that.

To support as much leaf development as possible, we need to provide the growing plants with high levels of nitrogen (N).  So what is possible?  The maximum number of leaves for an onion plant is 13.   So, if we want to aim for that magic number of 13 leaves to get the biggest onions possible, we need to make sure that we are providing plenty of nitrogen.  A side dressing of a high nitrogen fertilizer, like blood meal, every 2-3 weeks is a great way to support large onion bulb development.

Perhaps the most important aspect of fertilizing onions is when nitrogen is applied. Like I said, onions need a steady supply of nitrogen to continuously develop leaves.  Once the proper daylength is reached, however, the plant stops leaf development and transfers its energy into forming the bulb.  Once this happens, no additional new leaves will be produced. This is also your signal to stop adding nitrogen.  Additional nitrogen at this point can negatively impact the onion’s size and storage potential so bulb formation = stop feeding nitrogen.

Watering

Did you know that onions are 89% water?  You’ll need to make sure your plants are getting A LOT of water.  During leaf development, onion plants need about 1.5 in / 3.8 cm of water a week.  Once they start forming bulbs, do your best to double that to 3 in / 7.5 cm per week between rain and hand watering.  As you get ready to harvest though (see below), stop watering all together for about 2 weeks to help maximize their storage potential.

Harvest and Storage

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When to Harvest

You’ve planted, you’ve fertilized, you’ve watered, you’ve stopped fertilizing and stopped watering and now you’re astounded that these once stringy little plants are producing these beautiful bulbs.  You’re so excited to pull them from the ground, pile them in a bin, and use them up for months to come.  Not so fast. Depending on the variety, onions can have a storage potential of up to 12 months!  A little pause and restraint before and after harvest is necessary to maximize this bounty. 

Eventually, you will notice that some of the onions start to flop over at the base of the leaves near the bulb (referred to as the “neck”).  If you pinch this area between your thumb and forefinger at this point and compare those that have flopped with those that have not, you will notice a marked difference in the rigidity.  This area in flopped onions is soft where are those that have not flopped are strong.  Nothing is wrong, this is a natural process and initiates the onions sealing themselves off to cure.  It’s often recommended that once about ½ of the onions flop over to manually bend all other remaining onions in the same way. 

Curing for Maximum Storage Potential

Your objective now is to get any remaining moisture out of the onions to maximize their storage potential.  To do this, the onions need warm, dry conditions for 2-3 weeks.  One option for this is to leave them in the garden and stop watering.  If rain is in the forecast, or you can’t stop watering either because of nearby plants or automated irrigation, dig them up and lay them in a single layer or set up some way to hang them upside down for 2- 3 weeks.  Ideally, keeping them outside in an area protected from weather like in a shed or under a deck is best.  This allows for any excess moisture to escape from the bulbs. After the outer layer is papery and the leaves are completely dried up, cut off the leaves and roots and store in a cool dry place.

Storage

The storage potential of onions ranges anywhere from 1 - 12 months depending on the variety. Generally speaking, sweet onions have a shorter shelf life than more pungent varieties. I personally grow a few varieties with different storage ranges and use them accordingly throughout the year. If storage potential is important to you, I recommend purchasing from a supplier that provides this information. Regardless of their potential, ideal storage conditions for onions are 32 - 40°F / 0 - 4°C and 65 - 70% humidity. Think unheated basement, shed or garage, not refrigerator as a refrigerator is too humid. If you can’t create those ideal conditions, no worries, do the best that you can and just know that their storage potential may be decreased a bit - no big deal.

If you are not completely sure of the range of storage potential for your crop, take a look at the necks after you cut them. Those with tighter necks, such as the onion pictured on the far right, will generally store longer than those that are more open like the one on the far left.

What Could Possibly Go Wrong?

Problem - Planted the Wrong Type of Onion

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The short, intermediate, and long day classifications are intended to indicate ideal onions for each region.  Planting a variety outside of its designated region will still result in edible onions, they just will not likely reach their full size potential.  No need to panic just expect smaller than advertised bulbs for the variety.

Problem - Harvest Before the Necks Flop Over

As mentioned above, waiting until the necks flop over is ideal state to ensure maximum storage potential.  If you harvest them before this, do all of the steps to dry and cure but know that these may not live up to the maximum storage potential for the variety. If you are overly concerned about the storage potential, remember that onions freeze well!

Pests and Disease

Onions, like many alliums, are not bothered by pests or disease very often.  This is not to say that they don’t exist.  If you are experiencing symptoms of rot or decay, a good resource for identification and treatment is: Quick guide to insects and diseases of onions | UMN Extension

Recommended Suppliers

Hopefully by now you have picked up on the fact that selecting onions that are best suited for your latitude and day length is extremely important to maximize your onions’ size. Because of that, it should be no surprise that my recommendations for onion providers is heavily influenced by how easy they make it to identify this distinction. I am not affiliated with, paid or sponsored by anyone but below are some of my faves and why.

Dixondale Farms

It is hard to talk about onions and not mention Dixondale Farms! Their homepage touts them as the Nation’s largest onion plant grower and I believe it! They are an absolute authority on growing onions and I myself have learned a lot from the resources on their site. I definitely recommend checking it out.

What I love:

• You can easily shop based on day length specification

• They have great selection with over 20 individual onion varieties and day-length specific sampler packs

• Their website is a wealth on onion-growing knowledge

• They have volume discounts (I LOVE a good discount!)

• I’ve been ordering for years and have never had a problem with anything (quantity or quality)

Some gaps:

• They only provide transplants (no seeds)

• If it is important to you as a grower, they do not provide organic options and their resources on how to grow are done so with chemical fertilizers in mind

Johnny’s Selected Seeds

Johnny’s Selected Seeds is an absolute go-to for farmers and home gardeners alike on all things vegetable growing! I don’t know that I would be the gardener I am today had it not been for their catalogues showing up at my door season after season. What sets them apart from other seed providers, in my home-gardener opinion, are the references and resources. The typically table-formatted info provided in their catalogues and website make picking the right variety of any vegetable for me a breeze and onions are no exception! Their Full Size Onion Comparison Chart is a great example and a great resource if you are trying to decide which varieties work best for you.

What I love:

• You can easily shop based on day length specification

• They have great selection with over 2 dozen individual onion varieties and day-length specific sampler packs

• They have great references and information

• I’ve been ordering for years and have never had a problem with anything (quantity or quality)

• They sell seeds, transplants, and sets

• They have both organic and inorganic options

Some gaps:

• I’ll update this article if I find any!