Onion Grow Guide

People often ask me what my favorite vegetable to grow is, and I always struggle to answer the question, often forgetting what I’ve told other people. Mentally scanning the summer garden, I always land on onions, because growing them from seed is the singular food we grow that ties all the seasons together. And then some.

They are some of the first seeds we sow, and some of the most storage friendly crops that we are still enjoying from summer — all while this year’s seeds are about to be sowed. They take a good 180 days from seed to harvest, and go on to last more than another 180 days, encircling the seasons in overlapping, pungent wafts of culinary utility. They are a part of our everyday lives, year after year, and I think that makes them both rather mundane and invaluably unique to grow.

I also believe growing them from seed is both rewarding and easy. Admittedly, I’ve never grown onions from sets, so I don’t know how they compare. I imagine you’d get earlier harvests with all that bulb energy ready to set roots and shoots, but there’s something so simple about a 4” square pot chock full of onion seedlings under my grow lights that makes me so happy.

And I definitely want all of you who’ve never tried them to consider growing them from seed.

Types of Onions

There are so many different kinds of onions. Scallions (green onions), storage/bulbing onions (yellow, red, white, sweet, for example) and even more delicate culinary onions that bulb but are for fresh eating. Let’s not forget, while they are not onions, I’ll lump shallots in here too because I treat them exactly the same as onions in our garden. And let’s be inclusive and apply this knowledge to leeks as well, because I don’t give them any different treatment than my onions.

It hopefully comes as no surprise that I like to grow some of each that grow well in northern latitudes, as onions are daylight sensitive. They will start to bulb when they are exposed to the correct day length, their internal clocks set to mature based on their geographical location. Another reason I am fascinated and in awe of an onion seed!

Here in Minnesota, I’ve not tried growing any sweet onions as they are a short-day variety that is suited to growing in more southern latitudes. Being 45 North, our days are filled with long sun, and thus long-day onions are the only types that thrive here. I could probably pull them off, but because of the added day length, the bulbs won’t form as robustly as they will in more southern latitudes.

The key to any crop is to grow a variety. I strongly recommend picking a storage onion and a fresh eating onion to grow from seed (if choose from my recommended onion varieties). When timed right, you’ll have amazing fresh onions for dinner while the storage onions slowly mature and flop over. If simple is your thing, just sow more bulbing storage onions and plan to start eating them fresh as soon as they are a harvestable size; I am usually reaching for a bulbing onion around the 4th of July here.

This is both for succession planting, to spread out the harvests, but also to use that diversity as insurance against any unforeseen issues. Thankfully for us, we have few (read: none) pest issues with onions, though I know that leek moths are an issue in some areas of New England, upstate New York, and even the mid-Atlantic states. And the notion of my onions not being a shoe-in is a terribly daunting concept I try not to think about lest I conjure the moths to take up residence in Minnesota.

To Indoor Sow or Direct Sow

Onions can be started indoors from mid-February through mid-March here. This puts me starting them 8-10 weeks before my last frost. It’s pretty common to recommend indoor sowing them 6-8 weeks before your last spring frost for cold climates with short growing seasons. And that’s exactly what I do: sow some in mid-February AND then sow another round in early March too. It’s sort of like insurance for me, but also I like to try and see if sowing early really makes a huge difference. And, honestly, it hasn’t made a visible difference come summer, but I can’t resist waiting until march to break the seal on my indoor seed starting, so I do both.

Last spring for the first time ever, I direct seeded spring onions uncovered in early April interplanted on the northside of our spring shelling peas. It worked out really great and honestly has turned my “must sow indoors in February” method on its head. I will try direct seeding some bulbing onions in early April to further test this approach.

Because onions are cold hardy, they can go out a bit early and thus can even germinate in cooler soils. I always. transplant mine 2 weeks before last frost or sometime in late April here.

Starting Indoors

In mid-February, I sow an indiscriminate number of onion seeds in 4” plastic pots — I usually aim for 25 or so. This puts them about 1/4” apart spacing in this pot. I cover them lightly with soil, burying the seeds 1/4” or so, and put a germination dome on top. I lightly water the seeds in and set them atop a seedling mat.

With new onion seeds, I usually see germination in less than a week, but total germination can take 1-2 weeks. I will remove the heat mat once all the pots have a majority of seeds germinated.

I’ve been using the same plastic pots for a decade, and it’s a tried and true method for us. I’ve also done some soil blocking with multi-sown onions but I feel the pots are a more efficient use of indoor space, aka, precious real estate under grow lights,

Onions sprouted in February here will need light and water and occasional fertilizer until you start to harden them off sometime in April. To keep them manageable, it’s common practice to trim onion seedlings to 4-5” tall while growing indoors. It not only makes them tidier under the lights, it will help redirect energy into root development.

Hardening Off

Just like every plant started indoors, your onion seedlings while a simple structure with minimal foliage, will indeed need to be properly hardened off before being transplanted into your garden. You can follow the same rules for this as I’ve highlighted in my Hardening Off blog post. Definitely take the time to do this, otherwise you’ll risk severely damaging the plants and stunting their growth.

To get them in as early as possible, start hardening them off in early April. Not only is planting them early kind of fun, it helps spread out the transplanting marathon that is the month of May and June!

Transplanting

Onions are hungry vegetables and need a lot of nutrition. Good compost, slow release organic fertilizer, and excellent irrigation are all keys to growing big, fat bulbing onions. They also love sunshine and don’t do well in shade or interplanted. I give them their own prominent space and relish the statement a tidy block of onions brings to the summer garden.

Plant them 4” apart in rows 12” apart. I dig a wide trench and generously spread several scoops of the slow release organic fertilizer into the furrow. I then separate out the onion seedlings and carefully lay them out, leaning them against the side of the trench. Then I quickly fill in the soil from the opposite side and using both hands mound the soil up and together and stand them up. It will feel like you are burying them too deep but don’t even worry about that. Inevitably, some will flop over. Others will even die. But the majority, if watered in well and watered by hand daily for the first week will establish quickly.

While that is my preferred method, I have also planted multi-sown onions as an experiment a few years ago. The result was smaller overall onions, but it does save a little space. If smaller onions are your jam, multi-sowing them might be just what you need. This method is also used for beets, and you can harvest one onion at a time, letting the remainder continue to mature.

Multi-sown onions all spaced out and ready for planting. The ease of this is no root disturbance at transplanting. Note that I am not interplanting ANY of my onions. Full sun for these precious veggies. I love this method for scallions but not for bunching onions, as we often need a few scallions at a time.

When to Harvest

You can truly harvest your onions at any time. True, if you pull a bulbing onion in late May to use in lieu of a green onion for a meal, it’s one less bulbing onion you’ll have come August, but it is one less trip to your grocery store for a bunch of scallions when you only needed one. (Just me, or does this happen to you too?)

My point is that they are “mature” in every stage of development, unlike potatoes, winter squash, corn, or tomatoes. They are more like brassicas in this way to me, and again reminds me of their limitless utility in our home gardens.

Now if you are growing for bulbs, you want to be sure you wait to harvest until they are as big as they will get. The other major tell is that the foliage will start to flop over and bend; this is crucial to cut off flow from the bulb to the leaves, otherwise they will slowly rot from the inside out. Truthfully, even with my careful attention to the tops being fully bent and curing them for 3-6 weeks on our onion rack, we still get about 10% that rot by this time of winter.

If weather permits, after you harvest them all and let them lay in sun in the garden bed for a few days to upwards of a week. This obviously requires a good stretch of sunny days, so think about this when you are timing your harvest.

I usually harvest them for curing around mid-August, which happens to coincide with our wedding anniversary and this now annual photo tradition.

We end up with quite a range of bulbs, from softball sized to golf ball sized. It all depends on how strong those roots developed and quite frankly how much we may have damaged them at transplanting. Again, this is why I grow a lot of them — between 250-300 a year I think. I estimate that nearly half are consumed fresh or used for canning — my ketchup recipe, salsas, and bloody mary mix. That leaves our shallots and the rest of the yellow and red onions that will get cured and put into bins for storage.

Storing Onions

Once cured, onions are one of the easiest foods to store. Our first year growing here, I put them in an old milk crate on our mudroom floor. Looking back, it was my achievement that they crossed the threshold from the garden into the home and that was a much as I could accomplish at that time. We dug for onions for many months, and they stored just fine on the dusty floor. It was probably a bit too humid and warmer than other areas of the house, but it worked and reminded me that sometimes we make things harder on ourselves than necessary.

We enjoy fresh onions until usually sometime in April. We tend to save the shallots for very late in the winter as they do store better for me than onions, so we prioritize onions first.

Here’s a link to our root cellar blog post that shows how I created a “dry root cellar” in one of our closets.