How to Grow Onions from Sets

Plant onion sets in early spring, with the pointed end up and the tip just at or barely above the soil surface, usually about 1/2-1 inch deep. Space sets 4-6 inches apart for full-sized bulbs and grow them in full sun with evenly moist, weed-free soil. Many varieties mature in about 90-120 days, but use yellowing, naturally falling tops as your harvest guide. Cure harvested bulbs until their necks are fully dry and their skins are papery before storing.

Onion sets are small, immature bulbs grown the previous season and dried for planting. They offer an easy head start compared with growing onions from seed. One set generally produces one full-sized onion, or it can be harvested early as a green onion.

Choose the Right Onion Sets

Select firm, dry sets without mold or soft spots. For full-sized bulbs, choose small-to-medium sets. Larger sets are more prone to flowering after cold exposure. Heat-treated sets help reduce bolting risk, but growing conditions still matter.

Match the variety to your growing region and day length: short-day onions generally suit southern areas, long-day onions suit northern areas, and intermediate-day onions suit mid-latitudes. Check the variety's growing information rather than choosing by color alone. See our guide to onion set varieties for help choosing.

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When to Plant Onion Sets

For spring planting, set onions out as soon as the soil is workable and well-drained, often 2-4 weeks before the last expected frost. They tolerate light frost, but avoid planting into frozen or waterlogged soil.

Fall planting can work in mild-winter regions with varieties suited to overwintering. It is not a reliable option for every climate. See our spring, fall, and climate planting guide for timing that fits your garden.

Outdoor planting can extend from early spring into early summer depending on your zone and whether you want bulbs or green onions. Late planting may limit full-sized bulb development. For salad onions, plant small batches about two weeks apart. Sets can also be grown indoors year-round for green tops with adequate light.

Prepare the Growing Area

Sun: Choose full sun, ideally 6-8 hours daily.
Soil: Use fertile, well-drained soil rich in organic matter. Raised beds can help where drainage is poor.
Soil pH: Aim for about 6.0-7.0 and use a soil test to guide amendments.
Preparation: Loosen the bed and mix in finished compost or well-rotted manure. Do not use fresh manure.
Weed control: Start with a weed-free bed. Onions have shallow roots and compete poorly with weeds.

How to Plant Onion Sets

Separate the individual sets and discard any that are soft or moldy.
Place each set pointed end up and root end down.
Plant shallowly, usually about 1/2-1 inch deep, with the tip just level with or barely above the soil. Do not bury sets deeply.
For full-sized bulbs, space sets 4-6 inches apart in rows 12-18 inches apart. For green onions, space them 1-2 inches apart and harvest as needed.
Gently firm the soil around each set and water thoroughly to settle it.

Care for Growing Onions

Watering

Keep the soil evenly moist, not soggy. About 1 inch of water per week, including rainfall, is a useful starting point. Adjust for heat, rainfall, and soil drainage. Consistent moisture is especially important while roots establish and bulbs expand.

Do not stop watering simply because bulbs start to swell. Reduce and then stop irrigation as the crop reaches maturity and the tops begin to dry and fall naturally.

Weeding and Mulch

Hand-weed gently or cultivate only the soil surface to avoid damaging shallow roots and bulbs. A light layer of clean straw or shredded leaves can help retain moisture and suppress weeds. Keep mulch away from bulb necks.

Fertilizing

Use soil-test recommendations and fertilizer label directions. Onions need nitrogen for early leafy growth, but stop routine nitrogen feeding when bulbs begin to form. Excess late nitrogen can delay maturity and produce thick necks and bulbs that store poorly.

Bolting and Other Problems

A flower stalk means the onion has bolted. Do not rely on cutting off the stalk to restore normal bulb growth. Set affected onions aside for early use rather than long-term storage. Learn about prevention in why onion sets bolt.

Watch for onion maggots, thrips, and signs of rot or leaf disease. Good drainage, airflow, and crop rotation help reduce problems. Avoid growing onions in the same bed year after year. If birds pull newly planted sets, use a securely supported row cover until roots establish.

When and How to Harvest Onions

Harvest green onions whenever they reach a useful size. For storage bulbs, wait until the tops begin to yellow, dry, and fall over naturally. Many onions mature in about 90-120 days, but planting time, variety, day length, and weather affect the schedule. Do not force the tops down.

When about half the tops have fallen, check the crop for maturity. Lift bulbs carefully with a garden fork on a dry day, leaving the tops attached. Avoid bruising or cutting them, and use damaged bulbs promptly.

How to Cure and Store Onions

Lay onions in a single layer in a dry, sheltered, well-ventilated place. Protect them from rain and intense sun.
Cure until the necks are tight and fully dry and the outer skins are papery. Allow roughly 2-4 weeks, depending on conditions. Dryness matters more than the calendar.
Once dry, trim roots and cut tops to about 1-2 inches, or leave suitable dry tops attached for braiding.
Store sound, fully cured bulbs in a cool, dry place with good airflow. About 32-40°F is ideal; do not let them freeze. Use mesh bags or ventilated crates rather than sealed plastic bags.
Check stored onions regularly. Use bulbs that are sprouting or softening first, and discard rotten bulbs.

Storage life depends on the variety, growing conditions, and curing. Well-cured storage varieties can keep for months; sweet, damaged, or thick-necked onions are better used sooner.

Quick Tips for Success

Keep moisture consistent and weeds under control for healthy bulb growth.
Give full-sized onions 4-6 inches of space. Crowding limits bulb size.
Stop watering at maturity, not at the start of bulb growth.
Rotate crops rather than planting onions in the same bed year after year.

Quick Answers About Onion Sets

How deep do I plant onion sets?

Plant sets shallowly, usually about 1/2-1 inch deep, with the pointed tip just at or barely above the soil surface. Avoid covering them with a thick layer of soil.

How far apart should I space onion sets for big bulbs?

Allow 4-6 inches between sets in rows 12-18 inches apart. For green onions, sets can be spaced 1-2 inches apart.

How many onions grow from one set?

One onion set generally grows into one full-sized onion. Harvest it early if you prefer a green onion.

When are onions ready to harvest?

Harvest storage onions when the tops yellow, dry, and begin to fall over naturally. Many varieties mature in about 90-120 days, but variety, planting time, and climate affect timing. Do not force the tops down.

What if an onion starts to bolt?

A flower stalk means the onion has bolted. Removing it does not reverse bolting. Use affected onions early rather than relying on them for long-term storage, and see our bolting prevention guide.

How long do I cure onions?

Cure until the necks are fully dry and the outer skins are papery, often about 2-4 weeks. Some onions dry sooner in favorable conditions. Check dryness rather than stopping after a fixed number of days.

Can I plant onion sets in fall?

Yes, in suitable mild-winter regions with varieties intended for overwintering. In colder climates, spring planting is generally the more practical choice.