How to Grow Onion From Seed? Step-by-Step Guide for Beginners
Introduction: Why grow onions from seed?
Want to know how to grow onion from seed? Starting onions from seed is the easiest way to unlock dozens of varieties, save money, and control the exact planting time so you get larger bulbs and better storage onions.
Unlike planting sets, seeds let you choose sweet types, long-day or short-day varieties, and specialty colors. A single seed packet can produce dozens of seedlings, which is useful if you want to succession plant for a steady harvest. You also avoid pests and diseases that come with imported sets.
This guide walks you through every practical step, from choosing the right variety and sowing seeds indoors, to thinning, transplanting, feeding, hardening off, and curing for storage. I include timing tips, soil mixes that work, and quick troubleshooting for common problems so your first crop succeeds.
Grow from seed versus sets and transplants
Seeds, sets, and transplants each have clear tradeoffs, so pick based on time, taste, and patience.
Growing from seed gives the most variety, lower cost per plant, and the best chance at large storage onions. It does require more lead time and gear, start seeds eight to twelve weeks before last frost, use a grow light, and harden off seedlings. If you want heirlooms or long-term storage bulbs, seeds are worth it.
Onion sets are bulbs you plant for instant results, great for quick spring harvests, but they often produce smaller bulbs and may bolt.
Nursery transplants are a middle ground, easier than seeds, more reliable than sets, but pricier.
Choose seeds if you want control and bigger bulbs, choose sets for speed, choose transplants for convenience.
Choose the right variety and timing, understanding day length
If you want to know how to grow onion from seed, start by matching day length to your climate. Short-day onions form bulbs when daylight is about 10 to 12 hours, they are for southern regions. Try Texas 1015Y or Red Creole. Long-day onions need 14 to 16 hours, they are for northern summers. Try Walla Walla or Ailsa Craig. Intermediate-day varieties bridge the gap for mid-latitudes, try Candy or Copra.
Timing is simple, use your last frost date. For long-day varieties start seeds indoors 12 to 14 weeks before last frost. For intermediate-day start 8 to 10 weeks before. For short-day onions plan for fall planting in warm areas, start seeds 8 to 12 weeks before your typical transplant date in late summer or early autumn. Seedlings must be sturdy before moving outside.
What you need, supplies and soil preparation
If you search "how to grow onion from seed?" start with these essentials. Supplies: onion seeds, seed-starting trays with drainage, a sterile potting mix, grow light or sunny south window, heat mat for faster germination, labels, and a gentle liquid fertilizer.
For potting mix, use a light, well-draining blend, for example 50 percent compost or coconut coir plus 50 percent peat-free potting soil, with 10 percent perlite. Aim for pH 6.0 to 7.0.
Soil amendments to add to garden beds include well-rotted compost, a tablespoon of bone meal per square foot for root development, and lime only if soil pH is below 6.0.
Quick tips: sow 1/4 inch deep, keep soil evenly moist, thin seedlings to 1 to 2 inches for bunching onions, and harden off before transplanting.
Sowing seeds step-by-step, indoors and direct sowing options
If you wonder how to grow onion from seed, follow this clear process.
- Fill flats with a light, well-drained seed mix, moisten, then sow seeds 1/4 inch (6 mm) deep.
- Cover lightly, label, keep trays at 60 to 70°F, provide bright light or a grow lamp.
- Thin or transplant into individual cells when seedlings have two true leaves, keeping roots intact.
- Start hardening off seven to ten days before planting out. Transplant when seedlings are four to six inches tall and soil is workable, usually eight to ten weeks after sowing. Space transplants four to six inches apart for bunching onions, ten to twelve inches for full-size bulbs.
- Water gently after transplanting, mulch to retain moisture, and side-dress with balanced fertilizer four weeks later.
Direct sow when you need scallions quickly, when spring soil warms early, or in mild climates for overwintering. Direct sowing also saves time if you dislike transplanting.
Caring for seedlings, thinning, watering and feeding
When you figure out how to grow onion from seed, light is non negotiable. Give seedlings 12 to 14 hours under a grow light, set about 2 inches above the leaves, or put them in a sunny south window. Too little light makes leggy plants; move lights closer or add hours if stems stretch.
Keep the soil evenly moist, not soggy. Water when the top half inch feels dry, or bottom water trays for seedlings in cell packs. Overwatering causes damping off; if seedlings collapse, stop overhead watering and improve air circulation.
Thin when the first true leaves appear, usually at 2 to 3 inches tall. Snip unwanted seedlings at soil level, do not pull, then space to 1 inch for scallions or 3 to 4 inches for bulb onions. Thinning prevents competition for nutrients.
Feed lightly with a dilute fish emulsion or a balanced 10-10-10 every 10 to 14 days until bulbs form. Yellowing leaves usually mean nitrogen deficiency; leggy growth means low light; fungal rot means too wet or contaminated mix.
Transplanting outdoors, spacing and growing to maturity
If you want to know how to grow onion from seed, start hardening off seedlings 7 to 10 days before transplant. Put them outdoors in morning sun for two hours on day one, add two hours each day, bring them inside nights until they tolerate full day and cool temperatures.
Plant seedlings so the soil level matches the pot, never bury the neck. Space plants 4 to 6 inches apart in rows 12 to 18 inches apart, or use blocks in raised beds for easier watering. For larger storage bulbs choose wider spacing toward 6 inches.
Keep soil evenly moist, about 1 inch of water per week, and mulch 2 inches to suppress weeds. Cut back high nitrogen fertilizer once bulbs begin to swell to encourage bulbing. Harvest when tops yellow and flop over, then lift gently with a fork, cure in dry shade for 2 to 3 weeks, and store cured bulbs in a cool dry spot.
Harvesting, curing, storing and troubleshooting
Harvest when tops fall over and start to yellow, or when bulbs feel firm and the neck softens. For fresh eating, pull a few, brush off soil, rinse, and eat within a week. For storage crops, lift bulbs before heavy frost, shake off excess dirt, and leave in the sun a few hours to dry.
Curing, step by step
- Lay onions in a single layer on racks or old window screens in a warm, dry, well ventilated spot, 75 to 85°F if possible.
- Cure 10 to 14 days until necks are tight and skins papery.
- Trim roots and tops to about one inch, brush off loose skin.
- Store in mesh bags or crates at 32 to 40°F and about 60 to 70 percent humidity.
Quick fixes for pests and diseases
- Thrips, use insecticidal soap, row covers, and attract predatory insects.
- Onion maggot, rotate crops and use yellow sticky traps.
- Fungal leaf spots, improve air flow, avoid overhead watering, remove infected plants promptly.
Final insights and quick checklist to succeed
You now know the essentials of how to grow onion from seed? Here are the quick takeaways and a tight checklist you can follow, so you actually get onions this season.
Seed starting
- Sow thinly in trays 8 to 10 weeks before last frost, keep soil moist, give 12 to 14 hours of light or a sunny windowsill.
Transplant and spacing
- Harden off for a week, plant seedlings 4 to 6 inches apart for green onions, 6 to 8 inches for bulb types, set soil level same as in tray.
Growing and care
- Water about 1 inch per week, feed with nitrogen early, side-dress compost or balanced fertilizer at mid-season, pull weeds promptly.
Harvest and storage
- Harvest when tops flop, cure 1 to 2 weeks in dry shade, store in a cool, dry spot 32 to 40 degrees F for longer storage.
Realistic expectations
- First-year bulbs may be small, choose the right day-length variety for your latitude, and you will improve fast with each season.
