Starting Onions From Seed: A Complete Guide

Starting onions from seed is one of the most rewarding (and budget-friendly) ways to stock your kitchen with homegrown flavor. While many gardeners buy onion sets, starting onions from seed gives you more variety, stronger plants, and better long-term storage.

If you love cooking, preserving, or filling your pantry with homegrown ingredients, learning how to grow onions from seed is a game changer.

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Why Start Onions from Seed Instead of Sets?

Before we dig in, here’s why seeds are worth it:

• ✅ More varieties available
• ✅ Better storage life
• ✅ Stronger root systems
• ✅ Less risk of bolting
• ✅ More economical for large gardens

Onion sets are convenient, but seed-grown onions typically produce larger, longer-storing bulbs.

Choose the Right Onion Variety

The most important factor when growing onions from seed is day length. Onions form bulbs based on how many hours of daylight they receive.

• Long-Day Onions
  • Best for northern regions (like Pennsylvania and most of the northern U.S.)
  • Bulb when days reach 14–16 hours
  • Examples: Walla Walla, Yellow Sweet Spanish
• Intermediate-Day Onions
  • Good for middle regions
  • Bulb at 12–14 hours
• Short-Day Onions
  • Best for southern states
  • Bulb at 10–12 hours

Pro tip: If you’re gardening in a northern climate, choose long-day varieties for best results.

When to Start Onions Seed

Onions need a long growing season.

• Start seeds 8–12 weeks before your last frost date
• In colder climates, that’s typically January–March
• If direct sowing outdoors, plant as soon as soil can be worked

Starting indoors gives you a major head start and larger bulbs at harvest.

Starting Onions From Seed Indoors

Supplies You’ll Need:

• Seed starting trays or containers
• High-quality seed starting mix
• Grow lights (highly recommended)
• Spray bottle or gentle watering can

Instructions:

• Fill containers with moist seed-starting mix.
• Sprinkle seeds evenly across the surface.
• Cover lightly with ¼ inch of soil.
• Mist gently to avoid disturbing seeds.
• Cover with a humidity dome or clear plastic wrap until seeds germinate and pop through the soil.
• Place under grow lights immediately.

Onions need bright light to prevent leggy growth. Keep lights just 1–2 inches above seedlings. Check out how to make an inexpensive grow station HERE.

Germination Time:

• 7–10 days
• Ideal temperature: 65–75°F

Caring for Onion Seedlings

Once seedlings emerge:

• Keep soil moist but not soggy
• Maintain strong light
• Fertilize lightly every 2–3 weeks with diluted fish emulsion or balanced liquid fertilizer

Trim the Tops

When seedlings reach 4–5 inches tall, trim them back to about 3 inches

This:

• Prevents flopping
• Encourages stronger root growth
• Makes transplanting easier

Don’t throw away the trimmings — they’re edible and delicious in salads or soups!

Hardening Off

About 1–2 weeks before transplanting outdoors:

• Place seedlings outside in a sheltered spot for a few hours.
• Gradually increase sun exposure daily.
• Protect from strong wind and frost.

This reduces transplant shock.

Transplanting Onions Outdoors

Plant onions outside 2–4 weeks before your last frost date. They tolerate light frost well.

How to Transplant:

• Separate seedlings gently.
• Plant 1 inch deep.
• Space 4–6 inches apart for full-size bulbs.
• Space rows 12–18 inches apart.

Plant in full sun and well-drained soil.

Feeding and Watering

Onions are heavy feeders.

• Side dress with nitrogen every 2–3 weeks until bulbs begin forming.
• Water consistently — about 1 inch per week.
• Stop fertilizing once bulbs start swelling.

Consistent moisture = larger bulbs.

Harvesting and Curing

Onions are ready when tops fall over and begin to dry.

• Stop watering.
• Gently lift bulbs.
• Cure in a warm, dry, airy space for 2–3 weeks.
• Trim roots and tops once fully dry.

Properly cured onions can store for months.

Common Mistakes to Avoid

❌ Starting too late
❌ Using weak lighting
❌ Planting the wrong day-length variety
❌ Overwatering
❌ Skipping fertilizer

FAQ: Starting Onions from Seed

Can I direct sow onions instead of starting indoors?
Yes, but bulbs may be smaller in northern climates.

How many onion seeds per hole?
You can plant thickly and separate later, or sow 3–4 per cell and divide at transplant.

Do onions need full sun?
Yes — at least 6–8 hours daily.

Starting onions from seed takes a little planning, but the payoff is huge — bigger harvests, better storage, and more variety. If you love cooking with homegrown ingredients or preserving your own food, onions from seed are absolutely worth the effort.

Ready to grow your best onion crop yet?