How To Start Onions From Seed (Tips For A Big Harvest)

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Want to know how to start onions from seed? Learn the simple steps to grow your very own onions from a tiny seed to a full-sized bulb ready to harvest and enjoy.

Starting onions from seed is not just a great way to save money, but it can also yield better, healthier onions than those you start from sets. It has many advantages including more variety, less bolting, and the ability to adapt to your specific garden conditions.

Growing up we often started onions from sets that were available in the local farm and garden centers. There were normally 3 or 4 types available yellow onions, sweet onions, red onions, and sometimes white onion sets.

Some years we had a good harvest but we always had issues with onions bolting or just producing small bulbs.

Once we started growing onions from seed we had much more consistency in our harvest, less bolting, and larger onions. Yes, onions can take a long time to grow before you can harvest them, but it’s well worth the wait! Home grown onions have a wonderful flavor and are great for long-term storage.

Why Start From Seed

Growing onions from seed has become my favorite way to grow onions. While starting onions from sets is quick and easy, using seeds has many advantages.

Many Varieties

When you buy onion sets from your local nursery or farm store you are limited to what varieties are available.

Normally what I find in local stores is one type of Spanish onions, one red, one white, and one or two types of cooking onions.

But when you start to look at the large number of onion varieties available when you start your own from seed, it feels like a whole new world opens up.

You can pick from onions that mature faster, store better, or have other characteristics that are important to you.

Larger Bulbs

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One problem we always ran into when starting onions from sets was waiting on the stores to have them available. In our area, we can often plant onion seedlings in the garden a month before sets are in the stores. Yes we could mail order them and we did that for years, but nurseries won’t ship live plants or bulbs until after certain dates because they could freeze in shipping.

Buying onion seeds also makes it easier to know if you are getting long day onions or short day onions. There are different types of onions you want to grow depending on where you live. Some varieties are triggered to start forming bulbs when the daylight hours decrease and others when the daylight hours increase.

Less Bolting

Another reason to start onions from seed is that the plants have fewer problems with bolting.

When you buy onion bulbs in the spring, these are immature bulbs that were started late in the year last garden season. Then harvested young and dried so they can be sold in the early spring.

The problem with this is onions are biannual plants. This means they will grow one season, over winter and start to grow again. But that second year they want to flower and produce seed.

Once an onion sends up a flower stock, the bulb will no longer grow larger even if you cut the flower off.

Plants started from seed in the spring have a lower chance of bolting in your garden.

Starting Onion Seeds Indoors

Onions are best started from seed in the late winter or early spring. For our zone 5 growing area the best time for sowing onion seeds in January or February works well. But you can go as late as March.

Fill your cell packs nearly to the top with moist seed starting mix. Make a small hole in the center of each cell.

Then sprinkle 5 onion seeds in the center of each cell. Cover them lightly with your soil mix.

Gently water the tray and label it with the onion variety.

Place them under your grow light set or in a bright sunny window. The seeds should germinate in 6 to 12 days. If you have a poor germination rate make sure you check the date on your seed package. Onion seeds don’t stay viable for long and it’s best to buy new seeds every year for the best germination.

We use the multi-sowing method of growing onions so we don’t need to thin seedlings. This means we let 4 to 5 onion plants grow in the same place together. They will grow in a nice clump in the garden and make it very easy to harvest.

Trimming Onion Seedlings

When your onion seedlings have grown to 5 to 7 inches tall cut them back to about 4 inches. Some people like to cut the plants back to 2 inches, but we find 3 to 4 inches works well for us.

This will help the plants grow thicker and stronger.

Don’t forget to save the onion trimmings. You can use them just like green onions or chives in recipes.

Soil Preparation

Select an area of your garden that gets full sun. Remove any weeds from your planting area, then add some organic matter to help build up the soil. A 1-inch layer of good-quality compost is a great way to build fertile soil.

Like most root crops onions are prone to root rot so make sure your garden has good drainage. If you have heavy clay soil adding compost and sand can help improve the drainage or try growing in raised beds or containers.

Transplanting Into Your Garden

Once the snow has melted and the ground is workable it’s time to transplant your onion seedlings into the garden.

This is normally in April or early May. Don’t worry, onions are very cold-tolerant plants you don’t need to wait until after your last frost date to grow onions in your garden. But it is important that you harden them off first.

If you missed harvesting any onions last year, you’ll find they start growing very early in the spring. This is a good sign that it’s time to transplant your onion seedlings outside.

Make holes in your garden bed to the same depth the onions are growing in the cell packs. Space them 6 to 8 inches apart.

Gently remove the seedling from the cells and place it into the hole. If needed pack the dirt around to fill in any gaps.

FAQ

How Long Do Onion Seeds Take To Germinate?

Onion seeds can take 6 to 12 days to germinate on average. It really depends on the growing conditions. The cooler the soil temperature the longer the seeds can take to germinate.

I’ve seen them take as long as 14 days in cool conditions. But in warm soil they can germinate as fast as 4 to 5 days.

How Far Apart Should I Plant Onions?

To encourage large-sized onions transplanting your clumps of onion seedlings 8 inches apart (2 per square foot) is best. But if your goal is to produce more medium-sized onions in a smaller space you can reduce the spacing to 6 inches apart (4 clumps per square foot).

Do I Have To Trim Onions?

No, you don’t have to trim your onion seedlings. But I have found much better results when I do. Onion seedlings get very tall and can easily get tangled and flop over.

Trimming helps them to fit well under your grow lights for the 3 months they will be indoors. It also encourages the plants to grow stockier thicker stems.

Now you know how easy it is to grow larger bulbs! Just start your favorite onion varieties from seed and place onion transplants in your garden to harvest in the late summer.

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