How To Start Onions From Seed

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My first year of starting onions from seed produced subpar results. Unfortunately, I find a lot of gardening advice is directed more towards northern gardeners and I currently live in the south.

This holds true for my first time growing onions from seed. I started my onions from seed in January because that was what I read online, and then I ended up with very small onions when it came time to harvest.

I’ve definitely learned from my mistake, and I’m here to help make it easier for you to start your onions from seed.

Growing Onions From Seeds vs Sets

Some gardeners prefer to grow from sets because it is an easier process. Onion sets are onions that were partially grown before being pulled and sold. With sets, you don’t have to plan farther ahead and they can go directly into the garden.

One of the main reasons many gardeners grow onions from seed is because it gives you greater selection of the variety grown. Some people like to grow rare or heirloom varieties which typically are only available in seed form. But most sets come in just a couple variates which may or may not grow well in your area. Growing from seed allows you to pick a variety that will grow the most optimally.

How to select onion seeds

It’s important to note the day length requirements for the specific onion you are purchasing (see more below) so you get beautiful, large bulbs. Also, pay attention to the storage length of mature bulbs. If you plan to grow a lot of onions and store them, choose a variety that stores well. Also, make sure to buy new onion seeds each year as they don’t germinate as well after 1-2 years.

What type of onions grow best in your area?

Onions are slightly different than other garden plants, like tomatoes, in that their bulb size is determined by the amount of daylight they receive. The type should be listed on the seed packet. To figure out which type of onion grows best in your area, check out this map.

• Short-day onions: Short-day onions grow best in the southern states and they are what I grow. When the daylight hours reach 10-12 hours in length, they begin to form bulbs.
• Long-day onions: Long-day onions grow the best in the northern states or Canada. They won’t form large bulbs if you try to grow them in the south. They require 14-15 hours of daylight before they start forming bulbs.
• Intermediate or day-neutral onions: Intermediate or day-neutral onions require 12-14 hours before they start forming bulbs. If you live closer to the 35th parallel then these onions will grow the best in your area. However, intermediate or day-neutral onions grow well in most of the United States unless you live in the far north or far south.

When to start onions from seed?

The best time to start onions from seed is going to depend on where you are located. If you live in the south, the best time to start onions is in the fall. I start mine just after Halloween and let them grow until early February when I transplant them outside to the garden.

If you live in the northern US or Canada, you’ll want to start onions from seed in January or February. Typically this will be about 8-12 weeks before your last frost date, but of course, follow the planting directions on your seed packet.

What you will need to start onions indoors

• Potting soil or potting soil mixed with a seed starting mix
• Clean water
• Spray bottle
• Onion seeds
• Container with holes drilled into the bottom. I use old, clean salad or leafy green containers from the grocery store and drill holes into the bottom. Larger berry containers will work too.
• Seed starting trays
• Heat mat (optional)
• Grow lights or a sunny window (south facing is best)

How To start onions from seed

On the side of the container, label the type of onion you will be planting. I write on the side with a sharpie.

Fill the container with at least 4 inches of soil.

Using your finger tip, make two parallel trenches lengthwise in the soil about an ⅛ inch deep.

Sprinkle your seeds in the trenches (you can thin them later) and then lightly cover them with soil.

Using a spray bottle filled with water, lightly mist the top of the soil. Be careful when you first start spraying to spray extra lightly so you do not disturb the soil too much.

Place your container on top of the seed starting tray and fill the bottom of the tray with a centimeter of water. This is known as bottom watering. After the initial planting, you should continue to bottom water when the top layer of soil dries out.

Close the lid of the container and place the tray in a warm spot (like the top of the refrigerator) or on a heating mat designed for germination.

Once the onions begin to sprout, open the lid of the container and place the container under grow lights or near a sunny window.

While the onion seedlings are growing, you can fertilize them with seaweed or fish fertilizer every 2 weeks until they are transplanted outside. Once the seedlings reach 4 inches tall, you can trim the tops off. Repeat this process 2-3 times until the seedlings move outside to the garden. Once they are planted outside, do not trim the tops as these are what give the onions energy to form bulbs. The onion tops are edible and can be used like green onions.

Make sure to harden off onion seedlings 2 weeks before planting outside. They can tolerate a little cold but not hard freezes. If needed, you can cover young transplants.

Transplanting Onion Seedlings

Make sure the site you are planting in has loose, well-drained soil. The bulbs need the loose soil so they can easily expand as they grow. Make sure the site gets full sun, and amend the soil with compost and fertilizer (if needed) before planting.

Keep in mind the spacing when planting onions. Make sure to follow the guidelines on the seed packet, but typically onions should be planted at least 4 inches apart.
Take your seedling container and gently squeeze it to loosen the soil. Tip the container on its side and gently slide the onions out onto a flat surface.

Separate the onions one at a time, carefully loosening them from the group. It’s important to separate them one at a time so the roots don’t dry out during planting.
Create a small hole just slightly bigger than the bulb portion of the onion seedling. Place the white or bulb into the hole and lightly fill with dirt. Repeat this process for each of your onion seedlings.

Water the new onion transplants well around the base of the plant and mulch.
Once established, onions are a low maintenance crop. They will be ready for harvest when the majority of the tops begin to fall over.

Stay tuned for a more in depth onion grow guide, but this will give you a solid roadmap on how to start onions from seed!