When to Plant Zinnias in Colorado
The cut-and-come-again champion. The more you cut zinnias, the more they bloom. A cottage garden essential.
The Short Answer
Colorado Frost Dates
Your planting dates depend on which part of Colorado you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Front Range (Denver) | 5a, 5b, 6a | May 1 - May 15 | Sep 25 - Oct 10 |
| Western Slope | 4b, 5a, 5b, 6a | May 10 - Jun 1 | Sep 15 - Oct 5 |
| Mountain Regions | 3a, 3b, 4a, 4b | Jun 1 - Jun 20 | Aug 25 - Sep 15 |
Zinnias Planting Schedule for Colorado
Front Range (Denver) (Zones 5a, 5b, 6a)
Average last frost: May 1 - May 15 · Average first frost: Sep 25 - Oct 10
Western Slope (Zones 4b, 5a, 5b, 6a)
Average last frost: May 10 - Jun 1 · Average first frost: Sep 15 - Oct 5
Mountain Regions (Zones 3a, 3b, 4a, 4b)
Average last frost: Jun 1 - Jun 20 · Average first frost: Aug 25 - Sep 15
Growing Zinnias in Colorado
Zinnias in Colorado's Climate
Zinnias are one of the best annual cut flowers for cold climates — they grow fast enough to bloom for months even in short seasons. Direct sow after your last frost when soil reaches 60°F. The more you cut, the more they bloom, producing armloads of color from July through first hard frost.
Soil Considerations for Colorado
Alkaline clay soils common along Front Range. Rocky, thin soils in mountains. Amend heavily with compost and sulfur to lower pH. Make sure soil has warmed to at least 60°F before planting zinnias outside.
Colorado Climate & Growing Season
High altitude means intense sun but cool nights. Low humidity. Hail risk in late spring. Short but intense growing season at elevation. Zinnias cannot tolerate any frost, so wait until all frost danger has passed before transplanting outside. Watch local forecasts carefully in spring.
Growing season length varies across Colorado: Front Range (Denver) (5a, 5b, 6a) has a last frost around May 1 - May 15, while Mountain Regions (3a, 3b, 4a, 4b) sees frost end around Jun 1 - Jun 20. This difference matters for zinnias — transplant timing shifts by several weeks across the state.
Growing Tips
Water at the base to prevent powdery mildew. Cut flowers just above a leaf node to encourage more branching and blooms.
Companion Planting
Plant zinnias alongside these companions for better growth:
The Bottom Line
Last reviewed: March 29, 2026