Step-by-Step Guide for Planting and Growing Onions

Onions play a significant role in most of the food our family eats. We use them raw, cooked, pickled, and “kimchi-fied” at breakfast, lunch, and dinner; and we love to grow them in the garden. Growing our own gives us a chance to experience all the different kinds of onions, including some unavailable in grocery stores. And, as with other homegrown veggies, our backyard onions add a layer of fresh flavor that cannot be replicated with commodity crops from the store. If you’re interested in growing onions at home, keep reading. In this onion growing guide we’ll also share onion care and maintenance tips to help you grow big flavorful onions.

Types of Onions

Before getting into the nitty gritty of onion growing, it’s important to establish a few terminology basics. We grow scallions, green onions, and onion bulbs. Scallions, which many people casually call “green onions,” are special types of onions that never develop a bulb. True green onions, also known as spring onions, are the immature or “green” stage of bulbing onions that come from thinning the onion patch – more on that later. Scallions and green onions are used interchangeably in recipes. Onion bulbs are the red, yellow, or white globe shaped roots we simply call “onions.” 

Bulbing onions are day-length sensitive. At first, the plants grow up straight and thin like scallions, but then at a certain point in the season, top growth stops, and the root quickly takes on a globe shape. Bulb growth is triggered by the number of hours of sunlight. 

If you want bulbs, buy onion seeds, sets, or transplants adapted to your growing season and day length.

• Long day onions grow best in the north. They begin bulbing when day-length is 14-16 hours. Some popular long day varieties are Walla Walla, Red River, and Highlander.
• Short day onions grow best in the south. They begin bulbing when day-length is 10-12 hours. Popular varieties include Vidalia, Yellow Granex, Texas Sweet, and Red Creole.
• Intermediate or day-length neutral onions grow best in between. These onions begin bulbing when day-length is 12-14 hours. Popular varieties include Yellow Candy and Super Star.

Types of Scallions

For a steady supply of scallions, plant scallions such as Tokyo Long White or Italian Red of Florence, instead of bulbing onions. Scallions grow into clusters or “bunches” of pencil thin onions without ever producing a round bulb. If your priority is to harvest a crop of big round onion bulbs, but you’d like to also use a few green onions along the way, then go for a bulbing type like Yellow Granex or Red Amposta.

Use Onion Plant, Not Sets

You can grow a beautiful and delicious onion crop from onion seeds, onion sets, or onion transplants. All types of onions grow well and easily from seeds, but this method takes the longest and may require some growing time indoors. Seed sellers also offer onion sets, which are small dormant onion bulbs, and transplants, or green onion seedlings. 

We prefer to start with seeds for scallions and buy transplants for our bulbing onions. Sets can produce good green onions, or bulbs for immediate use but they don’t store as well over the long term as those grown from transplants or seed.

Another problem with onions grown from sets is that they are more likely to bolt. Onions are biennials, meaning their entire life cycle is completed in two growing seasons. The first season, they sprout from seed, produce foliage, and expand their roots. Then they go dormant. The second season, they bolt, or use the energy stored in the bulb to produce a flower stalk that then goes to seed. As essentially second-season onions, set-grown onions often bolt instead of producing a bulb. Onion plants are seedlings that are much less likely to bolt. 

When to Plant Onions

Start onion seeds indoors 2 to 3 months (8-10 weeks) before your last frost date. Follow the directions on the seed packet. Be sure to keep them moist and provide 12 hours of bright light. Supplement with a grow light if needed. Under ideal conditions, the seeds will sprout in 4 to 10 days. Transplant seedlings to the garden when they are about 6 inches tall. 

If you purchase onion transplants or sets, plant them in the garden 4 to 6 weeks before the last frost date. These dormant plants are less sensitive to cold and will begin growing as the soil and air temperatures permit. Plant them 1 inch deep, spaced every 4 inches for mature bulbs or every 2 inches for green onions, in rows 12 inches apart.

How to Plant Onions

1. Prepare the Garden Soil

Onions are cool weather plants that detest weeds. They must be planted at a time that is often cool and wet, when the soil is difficult to work. If possible, prepare the onion bed in the fall and cover it with mulch through the winter. Remove the mulch during a warm dry spell within a week or two of planting time.

2. Add Fertilizer

A few days before planting, spread a 2-inch layer of compost and organic or granular fertilizer with an NPK ratio of 1-2-1. Use a hoe, rake, or cultivator to work the compost and fertilizer into the upper 2 inches of garden soil. 

3. Plant

Space transplants 4 to 5 inches apart, and rows 12 to 18 inches apart.

Onion Care and Maintenance

The three most important tasks that lead to a bumper onion crop are weeding, feeding, and watering. First and foremost, don’t let weeds grow in the onion bed because they rob water and nutrients from the onions and could harbor pests. As the onions grow, they benefit from subsequent fertilizer applications, especially nitrogen. Apply high nitrogen granular fertilizer every two weeks from planting until the bulbs develop. Water immediately after planting and as often as necessary to keep the soil from drying out. Stop watering about a week before harvest.

How Long Does it Take to Grow Onions?

The type of onion, planting method, and intended use factor into the time it takes to grow onions. Onion seeds take a week or two to germinate. After that point, all parts of the onion are edible and may be harvested. In fact, onion seedlings might be a nice addition to your microgreens mix.

Most of us intend to grow either scallions/green onions or mature onions. Most scallions and green onions are harvested 10 to 12 weeks after sowing seeds, although eight weeks is possible at the peak gardening season in late spring and early summer. Depending on the variety and the climate, it takes three to five months to grow mature onions from seeds.

Because seed-grown onions take so long, most home gardeners plant sets or transplants if they want to harvest mature onions. Doing so reduces the growing time (in your garden) significantly. Sets yield an onion crop within three months, while transplants may produce mature roots in as little as two months.

How to Grow Bigger Onions

If you have tried growing onions in the past, but they don’t “size up” properly, a few issues may be at play. First, be sure to choose an onion type that is suited to your climate, especially in terms of day length, and plant as early as possible in the spring. To grow big onions, the plants must produce many leaves ahead of the critical date when they switch from foliage growth to root expansion. Onion seedlings will easily handle frosty weather, even a light freeze, and there’s no way to make up for lost growing time. 

The care you give your garden also plays into the size of the onions it produces. As noted above, keep the onion patch weed free, apply high-nitrogen fertilizer, and don’t let the onion plants dry out until the last week or two before harvest.

When the onion tops begin to turn yellow and a few starts to fall over, harvest time is near. Stop watering. Dry onions store best and are easier to harvest. Wait until about three-quarters of the onion plants have fallen over and turned yellow/brown. Then, it’s time to harvest. Loosen the soil around the plants before pulling them. Allow the onions to dry in the sun for a day or two, or bring them into a well-ventilated area under cover if wet weather threatens. Clean and cure the roots for storage, and they should last for several months, depending on the variety.

FAQ

Can I plant onions that have sprouted?

If your grocery store onions have sprouted, you can plant them, and they will grow. But don’t plan on getting a high-quality storage onion or a large onion bulb this way. Second-growth onions are more of a curiosity, generally best used as green onions.

Can I plant onions and garlic together?

Onions and garlic are in the same botanical family and share similar growing requirements, so they’ll do well in the same part of the garden. But, in some regions (like ours), garlic grows from fall through spring, while onions grow from late winter into early summer. It’s best to grow these crops according to the best practices in your area.

Can I plant onions with tomatoes?

Onions make a good crop in advance of tomatoes, but they should be planted a month or two earlier, and they could be ready for harvest before you pick the first tomato. Don’t plan to intermingle the two crops, but don’t be afraid to plant them adjacent.

Can I plant onions and potatoes together?

Onions and potatoes grow well when planted adjacent to one another. Both enjoy cool spring growing temperatures and will be harvested around the same time, depending on the varieties planted.