Growing Onions from Seed: A Complete Guide

Learn how to grow onions from seed with expert tips on planting, care, and harvesting. Perfect for home gardeners.

Why Grow Onions from Seed?

• Greater variety than sets or transplants
• Cost-effective for large plantings
• Healthier plants less prone to disease
• Satisfaction of nurturing plants from the beginning

Choosing the Right Seeds

Before planting, select onion seeds based on:

• Type of onion: Short-day, long-day, or day-neutral varieties depending on your region
• Flavor profile: Sweet, pungent, or mild
• Disease resistance: Look for seeds labeled resistant to common onion diseases

When to Plant Onion Seeds

• Indoors: 8–10 weeks before the last frost
• Direct sow: In mild climates after the risk of frost has passed

Keep seedlings indoors in trays or small pots under grow lights for best results.

Preparing for Planting

• Use a high-quality seed-starting mix
• Fill seed trays or small pots ¾ full
• Moisten soil before sowing seeds
• Plant seeds ¼ inch deep and cover lightly with soil

Caring for Onion Seedlings

• Watering: Keep soil consistently moist but not waterlogged
• Light: Provide 12–14 hours of bright light daily
• Temperature: Ideal germination temperature is 65–75°F (18–24°C)
• Thinning: Once seedlings have 2–3 leaves, thin to 1–2 inches apart

Transplanting Outdoors

• Harden off seedlings by gradually exposing them to outdoor conditions for 7–10 days
• Space onions 4–6 inches apart in rows 12–18 inches apart
• Ensure full sun exposure and well-draining soil
• Mulch around plants to retain moisture and suppress weeds

Fertilization and Maintenance

• Use a balanced fertilizer rich in nitrogen
• Water regularly, especially during dry periods
• Monitor for pests such as onion maggots and aphids
• Remove yellowing leaves to encourage bulb growth

Harvesting Onions

• Onions are ready when tops begin to yellow and fall over
• Pull bulbs and allow them to cure in a dry, ventilated area for 2–3 weeks
• Store cured onions in a cool, dark place for long-term use

Tips for Success

• Rotate crops annually to prevent soil-borne diseases
• Avoid overwatering to prevent rot
• Start multiple batches for continuous harvest