How to Grow Onions from Sets, Seeds, and Starts

Onions are the backbone of nearly every savory dish, and they are surprisingly easy to grow — once you pick a method. You can start them three ways: from sets (small bulbs), from seed, or from transplants (young plants). Each has trade-offs in cost, effort, and bulb size, and the right choice depends on your climate and patience. This guide covers all three routes, step by step, so you can grow a full season of onions no matter where you start.

What You Will Need

• Onion sets, seed, or transplants: pick one route — or two, to spread your harvest window.
• Loose, fertile soil: onions hate compacted ground; a raised bed or well-tilled bed with compost is ideal.
• Balanced fertilizer: a steady, moderate feed through the growing season.
• Mulch: straw or shredded leaves to hold moisture and block weeds.
• Patience with day length: choose short-day, long-day, or intermediate varieties to match your latitude.

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Sets vs Seed vs Transplants: The Trade-Offs

• Sets (small bulbs): the easiest and fastest route. Plant a bulb, get an onion. Downsides: fewer varieties, and sets can bolt (send up a flower stalk) if exposed to a cold snap after planting.
• Seed: the cheapest and most varied route, and the only way to grow most storage and heirloom varieties. Requires starting indoors 8 to 10 weeks before planting and more care early on.
• Transplants: young plants that skip the trickiest early stage. They cost more than seed but less than the time you would spend nursing seedlings, and they mature a bit earlier than direct-sown sets.

Step-by-Step: Growing Onions from Sets

1. Choose the right day length

Onions bulb in response to day length. In the North, grow long-day varieties; in the South, short-day; in between, intermediate (day-neutral) types. Getting this wrong is the most common cause of onions that grow huge tops and tiny bulbs.

2. Plant sets in early spring

Sets are tough — they tolerate light frost. Plant them 1 inch deep, about 4 to 6 inches apart, in full sun and loose soil. Pointed end up.

3. Water and feed steadily

Keep soil evenly moist. Side-dress with a balanced feed every few weeks until the bulbs start to swell, then stop nitrogen — too much late nitrogen makes soft, poor-storing onions.

4. Stop watering as tops begin to fall over

When tops flop naturally, stop watering and let the soil dry. A week or two later, lift the onions and cure them in a dry, airy, shaded spot for a couple of weeks before storing.

Step-by-Step: Growing Onions from Seed

1. Start indoors 8 to 10 weeks before your last frost

Sow seed a quarter-inch deep in a seed-starting mix, several seeds per cell. Onion seedlings look like grass and grow slowly — keep them under bright light and trim tops to about 3 inches to encourage thick stems.

2. Harden off and transplant

Once outdoor soil is workable, harden off seedlings over a week, then plant them 4 to 6 inches apart, just deep enough that the white base is covered. Healthy transplants are pencil-thick.

3. Treat them like sets from here

Same watering, feeding, and curing routine as set-grown onions. Seed-grown onions often store better and give you access to far more varieties.

Step-by-Step: Growing Onions from Transplants

1. Order or buy transplants at the right time

Transplants arrive in early spring by mail or at garden centers. Plant them as soon as the soil is workable — they handle frost well.

2. Plant at the same spacing

Set them 4 to 6 inches apart, covering the root base. Trim any floppy tops by a third to reduce transplant shock.

3. Water in well

Transplants need consistent moisture for the first two weeks while roots establish. After that, follow the standard watering and feeding routine.

Common Mistakes to Avoid

• Wrong day length. Northerners planting short-day onions get leaves, not bulbs.
• Planting too deep or too close. Deep planting yields long necks and small bulbs.
• Overfeeding late. Late nitrogen makes soft bulbs that rot in storage.
• Skipping the cure. Un-cured onions shrivel and spoil quickly.

Onions thrive in the same kind of loose, living soil that makes the rest of a new garden work, and they are an excellent first crop for a raised bed where the soil stays soft and workable.

FAQ

Are onion sets easier than growing from seed?

Yes — sets are the most forgiving method and the best place to start. Seed gives you more varieties and better storage onions but needs indoor space and 8 to 10 weeks of lead time. Many gardeners use sets for cooking onions and seed for special storage varieties.

Why are my onions flowering instead of bulbing?

Bolting usually comes from sets exposed to cold (below about 45°F / 7°C) for a stretch after planting, or from wide temperature swings. Plant sets at the right time for your area, and remove any flower stalk as soon as you see it so the bulb keeps growing.

When should I harvest onions?

Harvest when about half to two-thirds of the tops have fallen over naturally. Dig, don’t pull, to avoid bruising; then cure in a dry, shaded, airy spot for 2 to 3 weeks before storing braided or in mesh.

How much space do onions need?

About 4 to 6 inches between plants in rows or a block. A 4 x 4 foot bed holds 40 to 60 onions, which is a year’s supply for most households.

Bottom Line

Start with sets if you are new to onions — plant, water, feed lightly, and stop watering as tops fall. Move to seed when you want more varieties or better storage quality, and use transplants to save time if you miss the seed-starting window. Match day length to your region, keep the soil loose and evenly moist, and cure every bulb before storing. Do those four things and you will never buy another onion.

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