How To Grow Onions From Seed

Read on to learn how to grow onions from seed in your garden!

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Onions are one of those crops that we use a lot in the kitchen but can be a bit challenging to grow. When planning your garden for onions, think about how many you use as a family. We would easily use an onion a day in the kitchen for cooking so at least 365 onions would be needed! If that’s a scary number, consider growing some onions and some leeks. Leeks take up less space and are great for a smaller garden!

Types of bulbing onions

The bulbing of onions is triggered by day length. Short-day onions generally require at least 10-hour days to begin the formation of the bulb, intermediate-day onions are around 12 to 14 hours and long-day onions will form a bulb with daylight hours of 14 hours or more.

Long-day onions are varieties like Ailsa craig, New York Early, Rossa di Milano, and Brunswick.
Intermediate-day onions are varieties like Walla Walla, Flat of Italy, and Valencia, these are also known as day neutral onions.
Short-day onion varieties are like Texas Early Grano, White Creole, and Red Burgundy.

Here in the US, short-day onions are typically grown in the southern states, and long-day onions are grown in the northern states.

You might be able to sow your onion seeds directly in the ground in April or early May depending on where you live. In short-season areas like mine, you want to sow onions in February indoors. If sowing directly in the ground, thin your onions so each plant is about 3 to 4 inches apart for bigger onions or, 1 to 2 inches apart for smaller onions.

When starting onions indoors you can either start them by broadcast sowing in a large container and thin them later when transplanting into a bigger container. Or you can sow 5 seeds into a module and keep the best 3 plants per cell.

I’ve used the winter sowing method to sow onion seeds successfully in milk or water jugs so if you are running low on space starting your seeds because the warm weather crops are taking over then winter sowing might be a good option for you!

Onion growing tip!

When your indoor seedlings are about 5 inches tall use scissors to trim the leaves to 2 inches. I know it seems quite drastic to do this but it helps to encourage strong growth in your onion seedlings! And the good news is you can use the trimmings as onion seasoning in the kitchen!

Planting out onions

If you are sowing the multiple onion seedlings together, transplant them so they are about 6 inches between each group in your row. If transplanting single onion plants for bigger onions, transplant them about 4 inches apart.

You will want to transplant your onions after hardening them off for 2 weeks. Generally, onions are transplanted into the garden about 2 to 4 weeks before your last frost date. You can transplant by digging a 4-inch hole with a hand trowel and planting your onions in there and giving them good water or, like I learned you can make a furrow 4 inches deep with a garden hoe and place your seedlings in there then push the soil around your onion seedlings then water them in.

Where to plant onions

Onions need a sunny location and you want to keep the weeds at bay. The plants are shallow-rooted so you will need to be careful if weeding around your plants with a hoe. Mulching is a good option and clover is a great living mulch option that provides nitrogen. Some onion growers say that they grow better without mulch so the plants can get as much light as possible to help the onions get bigger.

Adding some well-rotted compost into the planting space before transplanting is going to help your onion plants grow and improve the soil. Onions need regular watering of about an inch of water every week.

Onion companions

Carrots and beets are great root crop companions for onions cabbage family crops like cabbage, kale, early Asian greens, mustards, broccoli, or kohl rabi also work well. Avoid planting beans or peas near your onions though, onions tend to make beans and peas stunted in growth.

Harvesting onions

When the necks are soft and the leaves are falling over it’s time to lift up the onions with a garden fork or pull them by hand and allow them to dry in the sun for a couple of days (depending on the weather!).

If it is rainy move the onions to a sheltered area where they can dry. A rack made of chicken wire can be super handy so the soil can fall through. Braiding onion tops is a traditional way to have them for storage. You can hang them somewhere cool and dry or you can trim the tops and the roots and store them in mesh bags.

Seed saving

Here at Mossy Bottom, we grow both leeks and onions. Both are part of the Allium family. Common onions are part of the Allium cepa species along with shallots and multiplier onions. Leeks are the Allium ampeloprasm species so not something that will cross-pollinate but other Allium cepa plants will. So spring onions, multiplier onions, or green onions can cross-pollinate with your common onions. This isn’t too much of a worry if you are just growing the onions to eat but might be something you want to be aware of if you are wanting to save seeds to grow a resilient and self-sufficient garden.

Bulbing onions are biennial, so they flower in their second year of growth. The bulbs tend to rot if they are left out in the garden over winter. Onion plants need a period of cooler weather to trigger the plant into entering the reproductive cycle of growth. This is called vernalization.

Most onion seed savers dig up the onions at the same time as harvesting. The bulbs are cured in a warm, well-ventilated location out of the direct sun. When the tops and skins are dry they are stored until spring.

Some onions will start sprouting in storage but they can be planted out in spring for seed as long as they don’t show signs of rotting. You want at least 5 plants for seed saving and bees love visiting onion plants in flower!

Dig In and Learn More

If this post has started ideas for your garden, be sure to check out these related posts and helpful books that have inspired my homestead garden:

What bulbing onions are you growing in your garden?

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Types of bulbing onions