Starting Onions From Seeds For a Better Bulb Harvest

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Starting Onions From Seeds For a Better Bulb Harvest

Until recently, I purchased onion sets, not seeds. Why? Because it felt easier. Onion sets are small onion bulbs that were grown from seed the previous season. They’re easy to plant and you start seeing growth very quickly. However, even after years of using them, my onions never had a stellar year. They were always smaller than the ones I saw in the store. I decided to do a little research and based on what I found, I’m now starting onions from seeds, here’s why:

Why onion sets don’t always work

The first thing we need to know is that onions are a bi-annual plant, which means they have a two year life span. The first year all the energy goes into the roots or “bulb”, the second year, the plant flowers and seeds out. Then the plant dies off and the seeds that drop or are collected start the process all over again.

When you purchase an onion set, the onion plant is often confused as to if it’s in it’s first year or second year of life.

Since it was plucked from the ground, had the stems removed, partly dried and then sold and put in the ground again, it’s got a bit of whiplash. You can understand then, why sometimes the plant will think it’s in the second year, and sometimes, in the first. There is then, no guarantee on how big the bulbs get. If the plant thinks it’s in the second year, it will put more effort into flowering out and less into the bulb.

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However, if you’re starting onions from seeds, there is no confusion.

And truthfully, the process is very simple. Why you can direct sow after your last frost date, the growing season (depending on your zone) might not be long enough to produce a large bulb. Often, it’s best to start indoors and give them a bit of a leg up on the season.

I’m also a big fan of secession planting, so I can have a continual harvest all season long. This means I’m planting my first batch indoors, and then I can direct sow the others later into the year.

What you need to start onion seeds

Onions are a cold season crop. This means they don’t need a heat mat to sprout indoors like many other vegetables. You’ll simply need a seed cell tray, good organic seed starting mix, and a set of inexpensive grow lights.

Know your day length when purchasing seeds

In the North, summer days are very long. up to 14-15 hours in some spots! In those areas, look for Long-day onion seeds. In shorter day areas, that only get 10ish hours of daylight, look for Short-day seeds. Then there are also Day-neutral or Intermediate onions that can be used in any zone.

When you go to purchase your seeds, it should say on the website or package if the seeds are long or short day.

Knowing this and planting accordingly is a great way to also insure large bulbs at harvest.

The more you know right?!

I love to purchase my onion seeds at Jonny’s Selected Seeds. This year I decided to grow:

  • Purplette Specialty Onions
  • Hybrid Yellow Onions Patterson F1
  • Brunching Onions Deep Purple

How to start Onion Seedlings

I love to take my seed starting mix, pour it into a bucket or large plastic tote, and mix with enough water to make it spongy where it holds its shape when squeezed in the palm of your hand.

Press this prepared mix into the cell trays, pressing down slightly to remove any larger air pockets. Pour onion seeds into a small bowl so it’s easier to grab them. Plant about three seeds per tray. Onions notoriously have a lower germination rate.

I love to sprinkle a little vermiculate over the top of the seeds.

Onions can take up to 10 to 14 days to germinate so be patient and make sure your dome is in place over your cells and you continue to mist the soil.

Trim and manage

Once your seeds sprout they’ll look like little blades of grass. To keep the seedlings manageable until you transplant outside it’s okay to trim off the stems and keep them at about 2 to 3″ tall. The bulb will continue to develop.

Once starts are about 6 weeks old and the soil outside is 50 degrees or warmer, it’s time to transplant!

Transplant the seedlings 1⁄2 inch deep, 4 to 6 inches apart. They do best in well-drained, slightly acidic, fertile soils in full sun.

11 Companion Plants to Grow With Onions

  • Cabbage
  • Chamomile
  • Summer savory
  • Carrots
  • Leeks
  • Beets
  • Lettuce
  • Strawberries

Do not plant onions with:

  • Beans (both pole beans and bush beans), peas, and other legumes. Onions can kill the helpful bacteria that grows on bean, pea, and other legume roots, stunting the growth. 
  • Asparagus
  • Sage
  • Other onion family plants (garlic, leeks, shallots, chives, scallions)

I hope this is super helpful in starting onions from seeds this year! Keep following along to see how to harvest come fall!

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