How to Grow Onions

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Growing onions in your vegetable garden can sometimes be tricky. But with a few hints and a bit of practice, you can learn how to grow onions and successfully harvest and store almost a year’s worth of these flavorful bulbs!

Onions can be grown from seed, from transplants (little seedlings that you transfer into your garden), or from “sets” (small onion bulbs ready to do their final bit of growing in your garden).

Choosing which option to grow depends on your space, determination, and experience. Below, we discuss the pros and cons of each method and how to grow onions each way.

How to Grow Onions from Seed

The most “from scratch” way to grow onions is to buy and grow a packet of seeds from the beginning. Onion seed packets are available in a wide variety, so this method gives you the most selection and choice.

Because onions have a long season, they must be planted among the first of your seeds – about 8-10 weeks before your average date of last frost. Plant them in a good seed starting mix with a direct light source and a heat mat under the pots.

As your onions grow taller, move the light source further to encourage growth. You should not need to separate them until they go into the ground, but if they start to overcrowd their pot they can be spread out to allow more space.

We have had decent luck growing onions from seed, but the rate of success from germination to transplanting is lower than that of some of our other vegetables. I believe this is because onions spend so long indoors; we just can’t always keep them healthy. This has led us to explore other options, such as the transplanting and “set” options below.

If you live in a climate where your winters are not as harsh, you can actually plant onion seeds in the fall. They will overwinter in the soil and get an early start the next spring.

How to Grow Onions from Transplants (or Seedlings)

Growing onions from transplants simply means letting someone else start the seeds for you. Onion seedlings can be purchased from many gardening centers as well as online.

They’re typically about 6-8″ tall and look like large circular blades of grass. When you order transplants shipped by mail, they will come in a bunch that seems dried out.

You simply store this bunch of plants in a cool, dry area and try to plant them within a week. You may also find onion seedlings in the soil at your local garden center. In this case, you simply transplant them into your garden.

Onion transplants can be planted in your garden about four weeks before the last frost date. Plant onions in a location where they will get full sunlight and good drainage.

Follow the package or seller’s instructions—onions should normally be planted about four inches apart in rows that are about 12-18″ apart. If you want to harvest onions smaller, you can plant them as close as two inches apart.

We have had great luck with onion transplants, as the success rate seems to be higher than with seeds. We love Renees Garden’s mixes, which are specific to your region and offer a few unique varieties. We ordered three varieties – Walla Walla, Redwing, and Ringmaster.

If you are just learning how to grow onions and want to be able to choose from a number of options, transplants would be our highest recommendation.

How to Grow Onions from Sets

Onion sets are onions that have already had one season of growth. When planted, they will enter their second season and come to full size in your garden. They basically look like small onion bulbs that are dried out and somewhat flaky.

The best sets should be about the size of a marble; if they are larger than that, they tend to bolt early (set flowers above ground), and the bulb beneath the ground will not get as large.

Onion sets are planted much like transplants but won’t show the green part above ground until they grow. Plant them about 4″ apart, about 1-2″ deep, in 12-18″ between rows.

Onion sets help to guarantee an onion harvest because you are getting a head start on the size and scale of your plant.

However, the downside is that your choices will be limited. There will not be as many onion varieties available to you as sets and you may have to simply choose between “yellow, white, or red” varieties. For this reason, we sometimes use sets when looking for a large quantity of storage onions and don’t care too much about their unique flavor.

Planting and Caring for Onions

When planting your onions, be sure that you are starting with high-quality soil. Amend your soil with compost and a high-quality organic fertilizer at planting time to give your onions the best shot at growth. Onions love high nitrogen fertilizers like blood meal, bat guano, composted chicken manure, or cottonseed meal.

Once your onions are growing in your garden, you’ll care for them like other vegetables. Make sure they have plenty of water and sunshine and are free from invasive weeds.

You can side-dress with additional fertilizer every few weeks – until the bulbs start pushing up from the soil (stop fertilizing when the soil cracks from emerging). This is also when regular watering is essential, as it will help the bulb reach its full potential.

Harvesting and Storing Onions

Onions are ready to harvest when the top of the plant turns yellow or brown and starts to fall over. This means that they have reached their full potential for long-lasting storage. That said, you can harvest an onion as soon as you see the bulb if you plan to use it right away, but do this sparingly, as those bulbs will be smaller and won’t last as long.

The best time to harvest onions is in the morning on a sunny day when the soil is still slightly moist and will release them readily. Then, lay your onions in the sun to dry for about two days.

Once the onions are thoroughly dry, you can cut off the tops and store them in a cool, dark place for up to a few months.

The process of harvesting and curing onions is similar to that of drying and curing garlic, and care must be taken to ensure that onions are thoroughly dry before storing. The entire neck should be dry down to where it meets the bulb before cutting and storing.

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