Start growing onions from seed by planting them 8-10 weeks before your last expected frost date. Sow seeds ¼ inch deep in well-draining soil with consistent moisture, maintain 60-75°F temperatures, and provide 12-16 hours of daily light. Transplant seedlings outdoors when they reach 4-6 inches tall and soil temperatures exceed 50°F. Expect harvest 100-170 days after planting depending on variety.
Discover exactly how to grow onions from seed with this comprehensive guide designed for both beginner and experienced gardeners. Growing onions from seed rather than sets gives you access to more varieties, reduces disease risk, and produces stronger, longer-storing bulbs. This complete resource covers every step from seed selection to harvest, including professional tips you won't find in generic gardening articles.
Why Grow Onions from Seed Instead of Sets?
While many gardeners opt for onion sets for convenience, growing from seed offers significant advantages. Seed-grown onions develop stronger root systems, have better disease resistance, and provide access to hundreds of varieties unavailable as sets. According to the University of Minnesota Extension, "Onion sets often bolt prematurely and produce smaller bulbs compared to seed-grown plants."
| Method | Advantages | Disadvantages |
|---|---|---|
| From Seed | Wider variety selection, stronger plants, better storage, lower disease risk | Longer growing season, requires indoor starting |
| From Sets | Quicker harvest, easier for beginners | Limited varieties, higher bolting risk, shorter storage life |
Essential Materials for Starting Onion Seeds
Before you begin growing onions from seed, gather these essential supplies:
- High-quality onion seeds (choose varieties suited to your daylight zone)
- Seed starting trays with drainage holes
- Well-draining seed starting mix (not garden soil)
- Heat mat (optional but recommended)
- Grow lights or sunny south-facing window
- Watering can with fine rose attachment
- Fertilizer with balanced NPK ratio
Step-by-Step Guide to Growing Onions from Seed
1. Timing Your Planting Correctly
Timing is critical when learning how to grow onions from seed. Start seeds indoors 8-10 weeks before your last expected frost date. The USDA Agricultural Research Service confirms that "onion seed germination is optimal between 60-75°F," so maintaining consistent temperatures during germination significantly improves success rates.
2. Sowing Onion Seeds Properly
Fill seed trays with moistened seed starting mix. Plant seeds ¼ inch deep, spacing them ½ inch apart. Cover lightly with soil and water gently. Maintain consistent moisture—onion seeds need steady hydration to germinate properly. Unlike many vegetables, onion seeds germinate best in complete darkness, so cover trays with opaque material until sprouts appear.
3. Providing Adequate Light and Temperature
Once seedlings emerge (typically 7-14 days), provide 12-16 hours of daily light. Position grow lights 2-4 inches above plants, raising as they grow. Maintain temperatures between 60-75°F—cooler temperatures encourage stronger growth but slow development. The Cornell University Cooperative Extension notes that "onion seedlings tolerate light frosts but grow best with consistent temperatures."
4. Thinning and Transplanting Seedlings
When seedlings reach 2-3 inches tall, thin to 1 inch apart by snipping weaker plants at soil level. Transplant to individual containers when they reach pencil thickness. This "how to grow onions from seed" technique prevents root disturbance that can cause bolting later.
5. Hardening Off Before Outdoor Planting
Begin hardening off seedlings 2-3 weeks before transplanting. Start with 1-2 hours of outdoor shade daily, gradually increasing exposure to full sun. This critical step prevents transplant shock and ensures your seed-grown onions thrive in the garden.
6. Planting Onion Seedlings in the Garden
Transplant seedlings when they reach 4-6 inches tall and soil temperatures exceed 50°F. Space plants 4-6 inches apart in rows 12-18 inches apart. Plant at the same depth they grew in containers—onions develop better bulbs when planted shallowly. Water thoroughly after planting.
Onion Growth Timeline: What to Expect
Understanding the complete growth cycle helps manage expectations when growing onions from seed:
- Days 1-14: Germination period requiring consistent moisture
- Days 15-60: Seedling development indoors with regular thinning
- Days 60-90: Early growth stage after transplanting outdoors
- Days 90-120: Bulb formation begins as daylight increases
- Days 120-170: Bulb maturation and harvest preparation
Regional Considerations for Growing Onions from Seed
Your geographic location significantly impacts success when growing onions from seed. Onions are categorized by daylight requirements:
- Short-day onions (10-12 hours): Best for southern regions (below 35°N latitude)
- Intermediate-day onions (12-14 hours): Suitable for mid-latitude regions
- Long-day onions (14+ hours): Ideal for northern regions (above 35°N latitude)
Planting the wrong type for your region results in poor bulb formation. The Texas A&M AgriLife Extension confirms that "selecting varieties matched to your latitude is the single most important factor in successful onion production."
Common Problems and Solutions
Even experienced gardeners encounter challenges when growing onions from seed. Here's how to address frequent issues:
- Damping off: Prevent with sterile seed starting mix and proper air circulation
- Bolting: Avoid by maintaining consistent temperatures and choosing appropriate varieties
- Purple leaves: Indicates phosphorus deficiency—apply balanced fertilizer
- Thrips damage: Control with insecticidal soap or neem oil applications
- Poor bulb formation: Often caused by incorrect day-length varieties or overcrowding
Harvesting and Curing Your Homegrown Onions
Proper harvesting ensures maximum storage life for your seed-grown onions. When tops naturally fall over and begin yellowing (typically 100-170 days after planting), carefully lift bulbs from soil. Cure onions in a warm, dry, well-ventilated area for 2-3 weeks until necks are completely dry and papery. Store cured onions in mesh bags in a cool, dark place with good air circulation.
Frequently Asked Questions
How deep should I plant onion seeds?
Plant onion seeds ¼ inch deep in seed starting mix. This shallow planting depth allows the tiny seedlings to reach the surface easily while maintaining adequate moisture contact with the seed.
Can I grow onions from seed in containers?
Yes, you can successfully grow onions from seed in containers. Use pots at least 8 inches deep with drainage holes, and space plants 3-4 inches apart. Container-grown onions require more frequent watering and feeding but work well for patios and small spaces.
Why are my onion seedlings falling over?
Onion seedlings naturally become top-heavy as they grow. This is normal and doesn't indicate a problem. If seedlings are excessively weak or yellowing, it may indicate insufficient light—move them closer to grow lights or to a sunnier window location.
How long do onion seeds remain viable?
Onion seeds typically remain viable for 1-2 years when stored properly in a cool, dry place. For best germination rates, use seeds within one year of purchase. Older seeds may have reduced germination percentages, requiring you to plant more seeds to achieve desired plant counts.
Do onions grown from seed require different care than those from sets?
Seed-grown onions require more consistent moisture during early growth stages compared to sets. They also benefit from slightly higher nitrogen levels during leaf development. However, once established, care requirements are similar for both methods.
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