How to Grow Onion
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🛠️ Supplies You'll Need
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🌿 Plant Overview
☀️ Growing Requirements
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Sun
Full Sun (6+ hours)
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Water
Moderate
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Spacing
4" apart
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Depth
0.5" deep
📏
Height
12-18"
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Germination
7-10 days
Ready to Grow Onion?
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Common Onion Varieties
Popular cultivars to look for at your local nursery or seed supplier.
Yellow Sweet Spanish
Large, mild onion with golden skin; the most widely grown long-day onion in North America.
Red Burgundy
Short-day red onion with mild, sweet flavor; excellent for fresh eating and salads.
Candy
High-yielding intermediate-day onion with sweet, mild flavor; stores well.
Walla Walla Sweet
Famous Pacific Northwest sweet onion with very low pungency and high sugar content.
Cipollini
Small, flat Italian onion with intense sweetness; prized for roasting and caramelizing.
Days-to-maturity figures are approximate and vary by climate and growing conditions.
Common Onion Problems
What to watch for — and how to fix it before it spreads.
Pests
Tiny insects rasp leaf surfaces leaving silver streaking; transmit iris yellow spot virus.
Apply spinosad or insecticidal soap; use reflective mulch; avoid planting near grass.
Fly larvae tunnel into bulbs causing plants to collapse; worst in cool, wet springs.
Use row covers; rotate crops far from previous onion beds; till soil to expose pupae to birds.
Soil-dwelling larvae bore small tunnels into developing bulbs.
Till deeply before planting; plant buckwheat as a trap crop; use beneficial nematodes.
Diseases
Gray mold at the neck of stored bulbs; spreads through the bulb in storage.
Cure bulbs thoroughly (2–3 weeks in warm, dry, airy space) before storage.
Water-soaked white lesions that turn purple; leaves collapse and dry in wet weather.
Apply copper fungicide; avoid overhead watering; plant in well-draining soil.
Pink-red discoloration at the base of the bulb; common in hot soils.
Rotate crops; improve drainage; plant resistant varieties; avoid injuring bulbs at planting.
Common Mistakes
Short-day onions won't form bulbs in northern regions; long-day varieties stay tiny in the South.
Choose long-day varieties (14+ hours) for north of 35°N, short-day for south of 35°N, intermediate for in between.
Deep-planted sets or transplants have energy diverted away from bulb development.
Plant sets or transplants so the top of the bulb/root mass is just at or slightly below soil surface.
🌾 Seed Saving Guide
🟡 ModerateWhen to Harvest Seed
Allow seed heads to form in year 2 (onions are biennial). Harvest when most seeds are black and visible.
Processing
Cut seed heads into bags, dry 1 week, shake to release seeds.
Seed Viability
Up to 1 year when stored cool, dark, and dry.
Pro Tip
Onion seed is very short-lived — use within 1 year. Cross-pollinates with other onion varieties via insects.
Seed Storage Supplies
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Raised Bed Planting Guides
A 4×8 bed fits 9 per sq ft of Onion. See plant counts, spacing grids, and companion pairings for every bed size →
Onion Planting Calendar by Zone
Onion Planting Calendar by State
Other Plants
Frequently Asked Questions
When should I plant Onion? +
Start Onion seeds indoors 8 weeks before your last frost date, then transplant outdoors 2 weeks after the last frost. Use the search on this page to find exact dates for your ZIP code or city.
How long does Onion take to grow? +
Onion typically matures in about 100 days from planting. Exact timing varies by variety, soil conditions, and local climate.
Should I start Onion indoors or direct sow? +
Onion should be started indoors 8 weeks before the last frost date. This gives seedlings time to establish before being transplanted outside after frost danger has passed.
How much sun does Onion need? +
Onion requires full sun (6+ hours) to thrive.
How far apart should I plant Onion? +
Space Onion plants 4 inches apart to allow adequate airflow and root development.
How much water does Onion need? +
Onion has moderate water needs. Consistent moisture is important especially during germination and early growth.
Can you save seeds from Onion? +
Yes — seed saving difficulty for Onion is rated moderate. Allow seed heads to form in year 2 (onions are biennial). Harvest when most seeds are black and visible. Cut seed heads into bags, dry 1 week, shake to release seeds.
How long do Onion seeds last in storage? +
Properly dried and stored Onion seeds remain viable for up to 1 year. Store in a cool, dark, dry location in an airtight container with a silica gel packet for best results.
Are Onion seeds open-pollinated? +
Yes — Onion is open-pollinated, meaning seeds saved from your harvest will grow plants true to the parent variety. This makes it an excellent candidate for building a self-sustaining garden.
About Onion
Essential kitchen staple. Choose short-day or long-day varieties based on your latitude.
Expect your Onion to be ready for harvest in approximately 100 days from planting.
Use the search above or browse by zone or state to find the perfect planting dates for your location.