Growing Onions from Seed

As one of the most often used ingredients in my kitchen, I make sure to include onions in my vegetable garden every year. Onions are not particularly difficult to grow but there are a few things you need to know to get started.

Note: If you are new to starting seeds and wondering what sort of supplies you’ll need to get started or how it all works, head over to my three-part indoor seed starting article.

What type of onion should you grow from seed?

See catalogues list dozens of different onions and while it is tempting to try them all, you need to make sure you grow the variety that is appropriate for your area. So what do I mean by that?

It may surprise you to know that bulb formation is contingent on hours of daylight and different areas of the country have different hours of daylight during the growing season. When you read those descriptions in the seed catalogs or on the back of a packet, you’ll notice that somewhere it will say short-day, intermediate day, or long day. Here’s what that means:

Short Day

• Best for areas that fall between latitudes of 25° to 35°
  • Examples of cities in this rage include Austin, Texas and Montgomery, Alabama
• Start to bulb when daylight reaches 10 to12 hours
• Typically started in the fall to maximize growth
• Varieties include White Creole and Texas Grano

Intermediate Day

• Best for areas that fall between latitudes of 32° to 42°
  • Examples of cities in this range include Washington, DC and Nashville, Tennessee
• Start to bulb when daylight reaches 12 to 14 hours
• Typically planted in spring but can be planted in fall where winter is mild
• Varieties include Australian Brown and Cabernet

Long Day

• Best for areas that fall between latitudes 37° and 47°
  • Examples of cities in this range include Chicago, Illinois and Milwaukee, Wisconsin
• Start to bulb when daylight reaches 14 to 16 hours
• Typically planted out in spring as transplants
• Varieties include Ailsa Craig and Walla Walla

In addition to selecting the right variety for your region, also consider what types of onions you typically use. There are some varieties that are great for long term storage, like Talon and Redwing, that will hold for several months with the right conditions. Other varieties, often the sweeter types, like Walla Walla or Ailsa Craig, need to be used within a few weeks of harvest.

Where to find onion seeds?

Here are some of my favorite sources for onion seed:

• Baker Creek Heirloom Seeds
• Territorial Seeds
• Johnny’s Select Seeds
• Botanical Interests
• High Mowing Organic Seeds

No matter where you purchase seeds, you will want to make sure that they are as fresh as possible. After 1-2 years, the germination rate for onion seeds begin to drop. The same goes for leek and shallot seeds.

When should you start onion seeds indoors?

It’s recommended that onions be started indoors 10-12 weeks before the last frost date for your area. There are several calculators out there to help determine that but I like the one from Almanac.com which uses data from NOAA (National Oceanic and Atmospheric Administration) to calculate the most likely last frost date for your zip code.

Here in Cleveland, Ohio, our last frost date will be around April 27th. Counting backward, I can start sowing onion seed indoors around the first week of February. Onions don’t seem to mind a few extra weeks indoors and I have started as early as New Years Day with no issue.

Sowing your onion seeds

If you have your seeds and the time is right, it’s time to get started!

Step 1

Prepare your container with moistened seed starting mix. Many commercially available mediums for seed starting are fairly dry so make sure that whatever you use is fairly moist. Gently tamp down with your fingers to make sure there are no big empty pockets and create a level surface for the seeds.

Step 2

Some folks will also cover their seed starting mix with a very thin layer of vermiculite. It’s not necessary but it can help prevent damping off, a soil born disease.

Step 3

Step 4

Label your container with what variety you’ve planted and when you planted it. This can be helpful if you want to track germination or keep records for next year.

Step 5

Place your container in a tray and cover with a humidity dome or Saran Wrap. Depending on the temperature of the room, a heat mat can be useful. Simply place under your tray.

Step 6

Once the seeds have sprouted, remove the humidity dome or plastic wrap and remove the heat mat (if using). Place the tray under your grow lights.

Water your little baby onions only when the top starts to dry out. In my experience, overwatered onions are quick to rot so I tend to water only when needed.

Fertilizing

Onions are heavy feeders but seedlings in general won’t need much fertilization until they get outside. However, a very diluted feeding every few weeks can help with root development. I use a very dilute liquid seaweed fertilizer.

Trimming your seedlings

Onions have a tendency to grow very fast and will need a bit of a haircut from time to time. Once your seedlings get about 6 inches tall or so, cut back to about 3 inches. Try to not let the trimmings fall back on the surface of the soil. Additionally, trimmings can be used like chives. I like them best in omelets!

You may need to trim every week or so to keep them tidy and to divert energy towards forming roots.

What next?

A few weeks before your anticipated last frost date, you can start hardening off your onion babies. Starting with an hour or so and adding some time each day, this process will gradually acclimate them to life outdoors. You can read more about the hardening off process here.

Once they have been hardened off, you are then ready to transplant into your garden.

Happy seed starting!