As a former organic market farmer, I’ve grown onions in every possible way. I’ve grown them from onion sets, from nursery-grown transplants, and from their little black seeds. Needless to say, I’ve learned quite a few tricks along the way, but I will tell you without a doubt that my best onion crops always start with planting onion seeds, not by planting onion sets or even by planting nursery-grown transplants. For me, planting onions from seed has always yielded the best results. But here’s the thing – you can’t just grow onions from seed like you do other vegetables. There’s a trick to doing it right.
Why planting onion seeds is better than planting sets
Onion sets are immature bulbs that were grown from seed that was planted in mid-summer of the previous year. The partially-grown bulbs are pulled from the soil in the fall and stored in a dormant state through the winter to be replanted the following spring. Many gardeners plant onions from sets because they’re widely available and it’s easy, but there are a few reasons why this may not be the best way to grow a good onion crop.
First, most gardeners make the mistake of choosing and planting the largest onion sets they can find when they should be picking the smallest sets instead. Texas A&M, Michigan State, and other university Extension Services note that bigger onion sets stop growing and go to flower sooner than smaller sets. When it comes to growing onions from sets, bigger definitely isn’t better; you’ll grow substantially larger onions by planting smaller sets.
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Onion sets are easy to find at garden centers, big box stores, and even in the produce section of the grocery store, but just because they’re easy to find, doesn’t make them the best onions to grow. Typically, only two or three varieties of onions are commonly available as sets, but there are dozens and dozens of onion varieties available from seed that are likely to do better in your garden. Just like growing tomatoes and peppers from seed, growing onions from seed means you’ll have a wider range of varietal options. But, exactly which onion varieties are best for your garden, depends on where your garden is located.
Which type of onion is best for your garden?
There are three different types of onions and picking the right type is key to growing a great crop.
- Short-day onions are varieties that form bulbs as soon the days reach 10 to 12 hours in length. They’re perfect for southern gardeners below the 35th parallel whose days are slightly shorter throughout the growing season. If you grow short-day onions in the north, you’ll end up with tiny bulbs that go to flower early in the season because the bulbs stop growing as the days lengthen. Common short day onions are ‘Southern Belle’, ‘White Bermuda’, and ‘Granex’, to name a few.
- Long-day onions are varieties that form bulbs when the days reach about 14 hours in length. They’re best for gardeners in the northern tier of the U.S. and Canada. Long-day onions won’t form bulbs south of the 35th parallel because the days aren’t long enough to trigger bulb formation. Common long-day onion varieties include ‘Walla Walla’, ‘Ring Master’, ‘Red Zeppelin’, ‘Yellow Sweet Spanish’.
- If you live somewhere across the mid-section of the U.S., grow day-neutral onion varieties (also called intermediate day). Varieties like ‘Red Amposta’, ‘Early Yellow Globe’, ‘Cabernet’, and ‘Superstar’ are a good fit. These varieties begin to set bulbs when days range from 12 to 14 hours in length.
Aside from the ability to grow a wide variety of the right onions for your climate, growing onions from seed also means you’ll grow larger bulbs. But, this is only true if you grow onion seeds the right way.
Two ways to plant onion seeds
Planting onion seeds under lights
Onions are cool-season crops that require 90 days or more to reach maturity. Because of this long growing season requirement and their preference for cooler weather, planting onion seeds directly into the garden in the spring makes it difficult for the bulbs to reach a good size before warm temperatures arrive. This means the seeds have to be started many weeks in advance of moving the plants outside into the garden. To make matters worse, onion seedlings are also slow growing. So, if you want to grow onion seeds indoors under grow lights, you should start them 10 to 12 weeks before it’s time to plant them into the garden in early spring.
But, planting onion seeds indoors under grow lights is a bit more nuanced than growing other vegetables from seed. When growing the seeds of tomatoes, eggplants, and other veggies indoors under grow lights, the lights should be on for 16 to 18 hours per day. But, if you grow onion seeds indoors under grow lights and leave the lights on for that long, it will initiate an early bulb set and result in puny onions. That means that if you want to start onion seeds indoors under grow lights, start very early and only leave the lights on for 10 to 12 hours per day.
To me, all of that seems like an awful lot of work, so I’m now planting onion seeds using a different method that’s far easier and a lot more fun. It’s called winter sowing.
My favorite method: Planting onion seeds via winter sowing
If you want to skip the hassle of grow lights, heating mats, and other seed-starting equipment, growing onion seeds via winter sowing is the way to go. It works like a charm and is super easy. All you need is a packet of onion seeds, a plastic lidded container, and some potting soil formulated for seed starting. I start planting onion seeds via winter sowing anytime between early December and mid-February.
Here are the steps I use to winter sow onion seeds:
- Poke three or four 1/2″ wide drainage holes in the bottom of the plastic container (I use clamshell-type take-out containers or empty plastic lettuce packages). Also make two 1/2″ wide ventilation holes in the top of the lid.
- Open the container and fill it with three inches of potting soil.
- Sprinkle the onion seeds on top of the soil, casually spacing them about 1/4″ to 1/2″ apart.
- Cover the seeds with a sprinkling of potting soil and water them in well.
- Put the lid on the container and label it with a piece of tape and a permanent marker.
Once the seeds are planted, put the container in a protected, shady spot outdoors. I keep mine on a picnic table against the back of our house. It doesn’t matter if it’s freezing cold and snowy outside when you plant the seeds; they’ll just sit dormant until it’s the perfect time for them to sprout (just like Mother Nature intended!). Don’t bother clearing off any snow or protecting the containers from freezing weather. The seeds will be fine.
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When the temperatures and day length are just right, your onion seeds will start to sprout inside the container. At that time, you need to start monitoring the moisture level inside the container, watering your seedlings when necessary. Open the lid on warm days and close it at night. If you get a hard freeze in the spring, after the seedlings have germinated, toss a blanket or towel over the container at night for added insulation.
This video shares more about growing onions from seeds vs sets.
As soon as your garden soil can be worked in the early spring, transplant your onion seedlings out into the garden (that’s usually mid-March in my Pennsylvania garden). Unlike onion seedlings grown indoors under grow lights, there’s no need to harden-off winter sown onion seeds because they’ve been outdoors from the start.
Planting onion seeds by winter sowing means the plants are subjected to the natural day-night cycle right from the time of their germination. This means that bulb set is triggered at the correct time and the plants can form large bulbs before hot temperatures arrive.