How to grow onions from seeds, sets, or transplants

Onions grow best when the type matches where you live. Gardeners in Canada and the northern US grow long-day onions. Gardeners in the southern US grow short-day onions. Plant in early spring, two to four weeks before your last frost, 4 to 6 inches (10 to 15 cm) apart in full sun. Keep the bed weeded and watered. Harvest when about half the tops have fallen over. Then cure the bulbs in a dry, airy spot for two to three weeks before you store them.

Get the day length right and onions are one of the simplest crops in the garden. Get it wrong and you end up with a row of green onions that never form bulbs.

Pick the right onion for your day length

Onions start to form a bulb when the days reach a certain length. Each type has its own trigger.

Type	Where it grows best	Example varieties
Long-day	Canada and the northern half of the US	‘Copra’, ‘Yellow Sweet Spanish’, ‘Walla Walla’, ‘Red Wing’
Intermediate or day-neutral	The middle of the US	‘Candy’, ‘Red Candy Apple’
Short-day	The southern US	‘Texas Grano 1015Y’, ‘Red Creole’

Plant a short-day onion in the north and it forms a bulb too early, while the plant is still small. Plant a long-day onion in the south and the days never get long enough, so it never bulbs at all. Seed catalogs and transplant bundles list the type, so check before you buy.

Seeds, sets, or transplants

You can start onions three ways. Each one has a trade-off.

Onion seeds

Seeds cost the least and give you the widest choice of varieties, including the best storage onions. They also take the most time. Start them indoors 10 to 12 weeks before you plan to plant out, which in cold areas means January or February.

Sow seeds 1/4 inch (6 mm) deep in a tray of seed starting mix. You can sow them thick, about half an inch (1 cm) apart. They sprout in 7 to 10 days at room temperature.

Give them strong light for 14 to 16 hours a day. When the tops reach about 5 inches (13 cm), trim them back to 3 inches (8 cm) with scissors. These haircuts keep the seedlings sturdy and stop them from flopping. You can trim them several times before planting out. My guides on starting seeds indoors and the best grow lights for seedlings cover the setup.

Onion sets

Sets are small, dry onion bulbs grown the year before. They are the easiest way to grow onions, and you can plant them the day you buy them. The trade-off is a smaller choice of varieties, and sets bolt more often than seeds or transplants.

Choose sets about 1/2 to 3/4 inch (1.3 to 2 cm) across. Larger sets are more likely to bolt. Use the big ones for green onions instead.

Onion transplants

Transplants are bundles of young onion plants sold in spring. They give you a head start without the indoor work, and the bundles are often matched to your region. Plant them as soon as you can after you buy them.

When to plant onions

Onions handle cold. Plant them outside as soon as the soil thaws and dries enough to work, two to four weeks before your last frost. A light frost will not hurt them.

In mild winter areas, gardeners often plant short-day onions in fall for a spring harvest.

How to plant onions

Choose a spot in full sun. Onions need loose, well-drained soil with plenty of compost and a pH between 6.0 and 7.0. Heavy clay makes small, misshapen bulbs. If your soil is heavy, grow onions in raised garden beds.

Space plants 4 to 6 inches (10 to 15 cm) apart, in rows 12 to 18 inches (30 to 45 cm) apart. Closer spacing gives you smaller onions. Wider spacing gives you bigger ones.

Plant sets and transplants about 1 inch (2.5 cm) deep. Set onion sets pointed end up. Do not bury them deeper. Onion bulbs grow on top of the soil, not under it.

Keep weeds out

Weeds are the biggest threat to an onion crop. Onions have shallow roots and thin, upright leaves that cast almost no shade, so weeds take over fast and steal water and food.

Pull or hoe weeds every week. Hoe shallow, so you do not cut the onion roots. A thin layer of straw or dry grass clippings between rows helps keep weeds down and holds moisture in.

Water and fertilizer

Give onions about 1 inch (2.5 cm) of water a week. They need the most water while the bulbs are swelling.

Each leaf on an onion plant forms a layer in the bulb. More big, healthy leaves early in the season mean a bigger onion later. Feed with a nitrogen-rich fertilizer every two to three weeks in spring. Stop feeding once the bulbs start to swell and push the soil aside. My post on how to add nitrogen to soil lists organic options.

Do not pile soil or mulch on top of the bulbs as they grow. They need to sit up on the surface.

What to do if onions bolt

A bolting onion sends up a thick, hollow flower stalk. Cold snaps after the plants are a good size can trigger it, and so can large sets.

Snap off the flower stalk as soon as you see it. The bulb will still be good to eat, but it has a hard core where the stalk grew and it will not store well. Pull it and use it first.

When to harvest onions

Onions show you when they are ready. The necks soften and the tops fall over. When about half the tops in your row have flopped, stop watering. Wait a week or so, then pull the onions on a dry day.

You can also pull onions any time for fresh eating. Green onions are young onions picked early.

How to cure and store onions

Curing dries the outer skins and seals the neck so the onion keeps for months.

1. Lay the onions out in a single layer in a warm, dry, airy place out of direct sun. A garage, covered porch, or barn works.
2. Leave them for two to three weeks, until the necks are dry and tight and the skins are papery.
3. Trim the roots, and cut the tops to about 1 inch (2.5 cm). Or leave the tops long and braid them.
4. Store them in mesh bags or crates in a cool, dark, dry place.

Sweet onions like ‘Walla Walla’ keep only a few weeks. Pungent storage onions like ‘Copra’ can keep for months. Eat any onions with thick necks first, since they spoil sooner.

Onions and other alliums

Onions are part of the allium family, along with garlic and leeks. If you enjoy growing them, plant garlic next fall. My guides on how to grow garlic and when to plant garlic in fall by zone show how. For good neighbors in the vegetable bed, see my list of companion plants for onions.