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When to plant onion in Colorado

Colorado spans USDA zones 5b–7a, so the right time to plant onion shifts by weeks across the state. The window to transplant seedlings outdoors runs from about March 17 in Grand Junction to May 24 in Edwards — below are local dates for 70 cities, each computed from its own frost dates.

In Denverthe onion transplant out window (April 6–April 20) has passed for this year.
CityZoneLast frostFirst frostTransplant out
Denver6aMay 4October 8April 6 – April 20
Colorado Springs6aMay 3October 8April 5 – April 19
Aurora6aMay 7October 8April 9 – April 23
Fort Collins5bMay 2October 7April 4 – April 18
Lakewood6aMay 3October 7April 5 – April 19
Thornton6aMay 2October 11April 4 – April 18
Arvada6aMay 5October 5April 7 – April 21
Westminster6aMay 2October 11April 4 – April 18

Onion in Colorado: FAQ

When can I plant onion in Colorado?

Across Colorado, the time to transplant onion spans roughly March 17 in Grand Junction to May 24 in Edwards, each following that city's local frost dates.

Does the onion planting date vary across Colorado?

Yes. Colorado publishes 70 cities with their own frost dates, so the right onion planting window shifts by weeks between the warmest and coldest parts of the state — use your city's page for the exact dates.

How long does onion take to grow in Colorado?

Onion takes about 90–110 days to reach harvest once planted — check that this fits inside your city's frost-free season on its place page.

Planting reminders

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An email when it’s time to start seeds, transplant, and sow — timed to your frost dates. Double opt-in, one-click unsubscribe, no spam.

Onion planting guide →All Colorado cities →Colorado planting calendar →
When to Plant Onion in Colorado — Frost-Based Dates by City — BlissGarden