When to Plant Onion in Box Elder, Montana (59521)

Location

Box Elder, Montana

ZIP Code

59521

USDA Zone

Last Frost

Friday, May 15, 2026

📅 Planting Calendar

Start Seeds Indoors Friday, March 20, 2026

Transplant Outdoors Friday, May 29, 2026

Pro Tip: Start Onion seeds indoors 8 weeks before your last frost date, then transplant outdoors around Friday, May 29, 2026.

🌿 Plant Details

Scientific Name Allium cepa

Days to Maturity 100 days

Mature Height 12-18"

Spread 4"

Type Bulb

Fruit Size Medium (2-4 inch bulb)

☀️ Growing Requirements

☀️

Sun

Full Sun (6+ hours)

💧

Water

Moderate

↔️

Spacing

4" apart

🌱

Planting Depth

0.5" deep

Seeds typically germinate in 7-10 days

Ready to Plant Onion in Box Elder?

Get your seeds now so they're ready by Friday, March 20, 2026.

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🤝 Good Companions

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⚠️ Avoid Planting Near

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Other Plants for Box Elder

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Growing Onion in Zone 4a

Onion (Allium cepa) is a popular garden vegetable. In Box Elder, Montana, which is in USDA Hardiness Zone 4a, you should start seeds indoors around Friday, March 20, 2026.

Your average last frost date is Friday, May 15, 2026. Plan to transplant your Onion seedlings outdoors around Friday, May 29, 2026, about 2 weeks after your last frost date.