When to Plant Onion Seeds: Complete Seasonal Guide

Plant onion seeds 8-10 weeks before your last spring frost date for indoor starts, or directly in garden soil when it reaches 50°F (10°C) in early spring. Northern gardeners should plant in March-April, southern regions can plant in fall for winter harvest.

Knowing exactly when to plant onion seeds makes the difference between a bountiful harvest and disappointing results. This comprehensive guide delivers precise planting schedules based on your specific climate zone, backed by agricultural research from university extension programs. Whether you're growing sweet Vidalias, pungent red onions, or storage varieties, timing your planting correctly ensures robust bulbs with maximum flavor development.

Understanding Your Climate Zone for Onion Planting

Onions are sensitive to day length and temperature, making your geographic location the primary factor in determining onion seeds when to plant schedule. The USDA Plant Hardiness Zone Map provides the foundation for timing your planting, but you'll also need to know your average last frost date.

USDA Zone	Last Frost Date	Indoor Seed Start	Outdoor Planting	Fall Planting Option
3-4	May 15-30	March 1-15	May 1-15	No
5-6	April 15-May 15	February 15-March 1	April 1-15	Late Sept
7-8	March 15-April 15	January 15-February 1	March 1-15	Sept-Oct
9-10	Feb 15-March 15	Dec 15-January 1	Feb 1-15	Oct-Nov
11-12	Rare frost	Year-round	Year-round	Oct-Dec

Source: USDA Plant Hardiness Zone Map and University of California Agriculture and Natural Resources

Optimal Soil Conditions for Onion Seed Germination

Onion seeds require specific soil conditions to germinate successfully. The ideal onion seed planting temperature ranges between 50-75°F (10-24°C), with 68°F (20°C) being optimal. Planting when soil temperatures consistently reach 50°F ensures proper germination without risking seed rot from cold, wet soil.

Before planting, prepare your garden bed with these steps:

• Test soil pH (onions prefer 6.0-6.8)
• Amend with 2-3 inches of compost
• Add balanced organic fertilizer (10-10-10)
• Ensure soil is loose to 12-inch depth

Step-by-Step Onion Seed Planting Guide

Follow this proven method for growing onions from seeds with maximum success:

1. Indoor starting (for short-season climates): Sow seeds 8-10 weeks before last frost in seed trays with quality potting mix
2. Seed depth: Plant seeds 1/4 inch deep - onions need light to germinate properly
3. Watering: Keep soil consistently moist but not soggy (drying out causes poor germination)
4. Thinning: When seedlings reach 2 inches, thin to 1 inch apart
5. Hardening off: Gradually expose seedlings to outdoor conditions over 7-10 days
6. Transplanting: Move to garden when seedlings are pencil-thick, spacing 4-6 inches apart
7. Direct sowing: For longer seasons, plant seeds directly when soil reaches 50°F

Onion Growth Timeline: What to Expect

Understanding the onion seed germination timeline helps manage expectations and identify potential problems:

• Days 1-7: Seeds germinate when soil temperature is adequate
• Days 7-21: Seedlings establish root systems and develop first true leaves
• Weeks 4-8: Bulb formation begins as daylight increases
• Weeks 8-12: Bulbs rapidly expand with proper watering and nutrients
• Weeks 12-16: Bulb maturation - tops begin to fall over signaling harvest time

Most onion varieties take 100-170 days from seed to harvest, depending on variety and growing conditions. Short-day varieties mature faster (100-120 days) while long-day storage onions take 150-170 days.

Regional Planting Strategies for Success

Your geographic location determines the best onion planting schedule by zone. Understanding these regional differences prevents common mistakes:

Cold Climate Regions (Zones 3-6)

Start seeds indoors 10-12 weeks before last frost. Use long-day varieties that require 14-16 hours of daylight to form bulbs. Plant outdoors 2-4 weeks before last frost date when soil is workable. In northern areas, consider using black plastic mulch to warm soil.

Moderate Climate Regions (Zones 7-8)

Direct sow seeds in early spring as soon as soil can be worked (typically February-March). You can grow both intermediate-day and some long-day varieties. For fall harvest, plant seeds in late summer (August).

Warm Climate Regions (Zones 9-11)

Plant short-day varieties in fall (October-November) for spring harvest. In these regions, onions grow best as a winter crop. Avoid planting during summer heat when temperatures exceed 90°F (32°C).

Common Onion Planting Mistakes to Avoid

Even experienced gardeners make these onion seed planting errors that reduce yields:

• Planting too deep: Onion seeds need light for germination - never plant deeper than 1/4 inch
• Incorrect variety selection: Using long-day varieties in southern regions results in poor bulb formation
• Overcrowding: Onions need adequate spacing (4-6 inches) for proper bulb development
• Inconsistent watering: Fluctuating moisture causes splitting and disease
• Planting too early: Cold soil delays germination and increases rot risk

Troubleshooting Onion Growing Problems

Address these common issues with targeted solutions:

Poor Germination

If your onion seeds aren't sprouting, check these factors:

• Soil temperature below 50°F (10°C)
• Seeds planted too deep
• Soil dried out during germination
• Old seeds (onion seeds remain viable 1-2 years)

Thick Stems But Small Bulbs

This indicates incorrect day-length variety for your region. Short-day varieties form bulbs with 10-12 hours of daylight, while long-day varieties need 14-16 hours. Using the wrong type results in excessive top growth with minimal bulb development.

Premature Flowering (Bolting)

Bolting occurs when plants experience cold temperatures followed by warm weather. Prevent by:

• Using fresh seeds (older seeds more prone to bolting)
• Avoiding early planting in cold soil
• Choosing bolt-resistant varieties like 'Stuttgarter' or 'Walla Walla'

Maximizing Your Onion Harvest

For the best results with your onion seed planting schedule, implement these professional techniques:

• Succession planting: Sow seeds every 2-3 weeks for continuous harvest
• Soil warming: Use black plastic mulch to raise soil temperature 5-10°F
• Nitrogen management: Reduce nitrogen after bulb formation begins
• Water consistency: Maintain even moisture (1 inch per week)
• Weed control: Hand-weed carefully to avoid damaging shallow roots

Remember that onions are heavy feeders but sensitive to competition. Keep your onion beds meticulously weeded and apply a balanced fertilizer every 3-4 weeks until bulb formation begins.

When to Harvest Your Onions

Timing your harvest correctly ensures maximum storage life. Watch for these onion maturity indicators:

• Top leaves begin to yellow and fall over
• Neck of the onion becomes soft
• Bulb wrappers turn dry and papery

For storage onions, wait until at least half the tops have fallen naturally before harvesting. Cure onions in a warm, dry, well-ventilated area for 2-3 weeks before storing.

FAQ: Onion Seed Planting Questions Answered

Can I plant onion seeds directly in the garden?

Yes, you can direct sow onion seeds when soil temperatures consistently reach 50°F (10°C). This works best in regions with longer growing seasons (Zones 7+). Plant seeds 1/4 inch deep and keep soil consistently moist until germination.

How long do onion seeds take to germinate?

Onion seeds typically germinate in 7-14 days when soil temperatures are between 50-75°F (10-24°C). Optimal germination occurs at 68°F (20°C). Colder soil temperatures significantly delay germination and increase rot risk.

What's the difference between onion sets and seeds?

Onion sets are small, immature bulbs that grow faster but produce smaller onions with less storage life. Seeds produce larger, longer-storing onions but require more time and care. Sets are better for beginners or short seasons, while seeds offer more variety options and better storage potential.

Can I grow onions from kitchen scraps?

You can regrow green onion tops from kitchen scraps, but you won't get proper bulbs. For full-sized onions, you need to plant actual onion seeds or sets. Kitchen scraps lack the necessary energy reserves to form mature bulbs.