Guide to Growing Onions

Did you know that onions are one of the top-selling vegetables in the United States, coming in just after tomatoes and potatoes? If you’re like our family and you have meals made with onions 3 or 4 times a week, that’s probably not too shocking. If you do eat onions regularly, it can be well worth the time it takes to grow them in your own garden. Depending on the variety of onion, it can take between 3 and 5 months before they’re ready to eat or to be cured and stored. With onions, though, you have a few options for how to grow them. Here’s a look at the choices you have and what to know before you start growing onions in your garden.

Onion Seeds, Sets, and Plants

There are 3 ways to start growing onions. You can start them from their tiny black seeds, plant sets, which are small, dry, immature bulbs, or plant partially grown live plants that you can purchase from garden retailers and some online suppliers.

There are advantages and disadvantages to each of these methods. Growing onions from seeds gives you access to the most varieties, and onion seeds cost less than sets or plants (ex. $3-4 for a packet of 300 seeds vs. $12.50 for a bunch of plants, which is approximately 50 onion plants). However, in most climates, onion seeds need to be started indoors because the soil temperature needs to be above 50 degrees F to germinate, and that means you’ll need a seed starting medium, a container, and grow lights, and you’ll be putting in more time to care for the seedlings. Onion sets and plants on the other hand, while more expensive and limited in terms of choice, can be directly planted in the garden and will produce onions that are ready to harvest earlier in the season.

Long-Day, Short-Day, and Day-Neutral Onions

Onions start to form their bulbs when the number of daylight hours reaches a certain point, and different onion varieties require different daylight hours. Onions are categorized into 3 groups, which makes it easier to choose onion varieties for your specific growing zone.

Long-day onions require the longest day lengths, typically 14 to 15 hours, which happens in Northern regions around the summer solstice. Long-day onions typically grow well in Zone 6 and colder. Long-day onions include varieties such as Walla Walla, White Sweet Spanish, Yellow Sweet Spanish, Ailsa Craig, and Ruby Red.

Short-day onions will start to form bulbs when the day length hits 10 hours per day. These onions grow best in Southern regions where daylight hours do not reach 14 hours per day. Planting these onions in the fall gives them enough time to produce the top growth necessary to form large bulbs when the daylight reaches 10 hours per day. Short-day onion varieties include Georgia Sweet, Texas Super Sweet, Red Burgundy, and more.

Day-neutral (or intermediate) onions require around 12 hours of light per day. These onions will grow in most regions, but may not be the best fit for the most Northern or Southern regions of the United States. Sierra Blanca, Candy, and Cabernet are examples of day-neutral onions.

For detailed information regarding the right zones for each onion category, I suggest going to Johnny’s Selected Seeds to view their map.

Choosing the Right Onion Variety

Once you’ve determined the type of onion that grows best in your climate, you’ll want to consider other factors, such as flavor and storage length. Some onions are sweeter than others, while some are more pungent. There are some onions that store well for 6 months or more, and there are others that are best when eaten fresh. Sweet onions tend to not store well because they have high moisture content.

One thing to note is that if you’re planning on growing from sets, you may not have that many options to consider. Many suppliers simply offer red, white, and yellow sets without giving information on the variety. If you’re considering specific varieties, you’ll find them as seeds or possibly as bunches of plants.

Basic Requirements for Growing Onions

  • Onions like well-drained, nutrient-rich soil with a slightly acidic to neutral pH (6.0 to 7.0).
  • It’s a good idea to work in about 3 inches of compost before planting your onions.
  • Onions need full sun to produce nice big bulbs.
  • Onions have shallow roots, which means weeds must be controlled. I can’t emphasize this enough. Most plants don’t like to compete with weeds, but with onions, if you let the weeds get out of control, you will not have onions to eat.
  • Onions are typically frost tolerant, which means, in the North, they can be planted in the garden in spring as soon as the soil can be worked. And in the South, they can be planted in the fall and grown over the winter. To provide a bit of protection to newly planted seedlings, you can add a layer of straw around them until the threat of frost has passed.
  • Depending on the variety of the onion, you’ll need to space them between 3 and 6 inches apart.

Starting Onions from Seeds

Onion seedlings
Onion seedlings. In this image, the growing medium has been covered with a light layer of vermiculite to help prevent damping off disease.

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If you’ve weighed your options for starting onions, and you’ve decided you want to start them from seeds, here’s how to do it.

  • When do you start onion seeds indoors? This depends on where you live and how long your growing season is. In the South, you start them so they’ll be ready for planting in the fall. In the North, you start them so they’ll be ready for planting 2 to 4 weeks prior to your last frost date or whenever the ground can be worked up. In general, they should be started so they spend at least 8 to 10 weeks growing indoors.
  • Option 1: Prepare your growing medium (premade seed starting mix or a DIY seed starting mix of compost, perlite, vermiculite, and Sphagnum peat moss) by adding it to your container and thoroughly wetting it. Make sure the container you use provides adequate drainage. Old berry containers that are at least 4 inches deep are a good, budget-friendly choice. Create a shallow trench in your growing medium and scatter your seeds into the trench, then cover them with an 1/8 of an inch to a 1/4 of an inch of growing medium. Gently water in. A spray bottle can come in handy when watering them in as you don’t want the seeds to move around too much, which can happen if you flood them with water. The advantage of this method is that it is fairly simple, and less time-consuming than the next option.
  • Option 2: Pre-sprout (or chit) your seeds by scattering them on a wet paper towel. Place the paper towel in a resealable storage bag and place it in a warm area. Check it once a day to make sure the paper towel does not dry out. In 3 to 7 days, you should start to see that the seeds have sprouted. Take these sprouted seeds, and using tweezers, gently plant them in your growing medium with the white tip (root) down and the green shoot (if you see any) up. Do not wait too long or the roots will start to grow into the paper towel. This process takes more patience than the scatter method above, but it’s a good option if you want to determine the seed germination rate or have older seeds that may not germinate as well. This method also allows you to space the seedlings to give them adequate room to grow, which is important if they’ll be spending a long time growing under the lights before being transplanted.
Chitting (pre-sprouting) onion seeds
Pre-sprouting (chitting) onion seeds.
  • You may want to add a light layer of vermiculite on top of your growing medium to help prevent damping off. Damping off is when seedlings die from a soil-borne disease that’s most commonly caused by fungi.
  • Place your seedlings under grow lights. A Southern facing window may work for some, but in the Northeast, there’s no getting around the need for grow lights when it comes to onions. We have to start them too early when there isn’t enough light, even with Southern exposure, to make them grow well. They become too leggy and weak.
  • Onion seedlings will need a “haircut” when they grow taller than 5 to 6 inches. Cut them back down so they stand at about 3 inches tall. The first time you do this will feel odd, but this trimming is important for stimulating growth and keeping the seedlings untangled and healthy. You can also eat the trimmings, so there’s no waste.
  • Use a fan to gently blow air across your seedlings. This will help them toughen up and get ready for windy conditions outdoors.
  • About 10 days before planting the seedlings outdoors, you’ll need to harden them off. The young plants will need time to acclimate to the more intense sunshine and wind they will experience outside. Begin by setting the plants outdoors in a protected, shaded area for a couple of hours. Even the shaded sunlight outside will be more intense than the direct light from a grow lamp. Each day, increase the time the seedlings spend outside by a couple of hours. After a week, you should be able to set them out in direct sunlight. A few days later, they should be ready to transplant into your garden bed.

When and How to Harvest Onions

You can harvest onions to eat fresh whenever you’re satisfied with the bulb size, but to store onions, you should wait until the tops turn yellow and fall over. You don’t have to wait for them to all do this on their own, though. Once they start turning yellow, you can bend over the tops to stop bulb formation so that all of the onions can be harvested together.

If storing the onions, you’ll need to cure them. To cure them, pull the onions out of the ground and gently brush off the dirt. Lay or hang the onions in the sun until the greens dry and the skin of the bulb toughens. You’ll need to protect them from rain while they cure. Once your onions are cured, you can braid them together and hang them in a cool (40 to 60 degrees F), dry place. You can also remove the dried green tops and hang them in a mesh bag.

The Onion Growing Experience

Growing onions offers a unique experience compared to other well-known vegetables that you can grow in your backyard garden. They’re not the easiest plants to grow from seeds (I’d give that award to tomatoes), but they’re not hard to grow either. If you’re growing onions for the first time, let me know how it goes in the comments section below. I’d love to hear about it, whether it goes really well or you run into trouble and don’t know what went wrong.

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