When to Plant Cosmos in Oregon
Delicate, daisy-like blooms that dance in the breeze. Cosmos thrive in poor soil and practically grow themselves.
The Short Answer
Oregon Frost Dates
Your planting dates depend on which part of Oregon you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Willamette Valley | 8a, 8b | Mar 25 - Apr 15 | Oct 15 - Nov 10 |
| Portland Metro | 8b, 9a | Mar 15 - Apr 5 | Oct 25 - Nov 15 |
| Central/Eastern Oregon | 5a, 5b, 6a, 6b | May 10 - Jun 1 | Sep 10 - Oct 1 |
| Southern Oregon Coast | 8b, 9a, 9b | Mar 5 - Mar 25 | Nov 5 - Nov 25 |
Cosmos Planting Schedule for Oregon
Willamette Valley (Zones 8a, 8b)
Average last frost: Mar 25 - Apr 15 · Average first frost: Oct 15 - Nov 10
Portland Metro (Zones 8b, 9a)
Average last frost: Mar 15 - Apr 5 · Average first frost: Oct 25 - Nov 15
Central/Eastern Oregon (Zones 5a, 5b, 6a, 6b)
Average last frost: May 10 - Jun 1 · Average first frost: Sep 10 - Oct 1
Southern Oregon Coast (Zones 8b, 9a, 9b)
Average last frost: Mar 5 - Mar 25 · Average first frost: Nov 5 - Nov 25
Growing Cosmos in Oregon
Cosmos in Oregon's Climate
Cosmos love your heat and sun. Plant from March and enjoy blooms for 6+ months. They handle drought well — one of the most water-efficient flowers for warm-climate gardens. Let some flowers go to seed for naturalized volunteer displays in subsequent years.
Soil Considerations for Oregon
Rich volcanic soils in Willamette Valley. Dry, alkaline soils east of Cascades. Acidic forest soils on coast. Generally excellent for gardening in valleys. Make sure soil has warmed to at least 60°F before planting cosmos outside.
Oregon Climate & Growing Season
Dramatic east-west climate divide. Wet, mild winters west of Cascades. Dry, cold winters east. Year-round gardening possible in mild western valleys. Cosmos 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 Oregon: Willamette Valley (8a, 8b) has a last frost around Mar 25 - Apr 15, while Southern Oregon Coast (8b, 9a, 9b) sees frost end around Mar 5 - Mar 25. This difference matters for cosmos — transplant timing shifts by several weeks across the state.
Growing Tips
Don't over-fertilize — rich soil produces more leaves and fewer flowers. Let some flowers go to seed for next year's crop.
Companion Planting
Plant cosmos alongside these companions for better growth:
The Bottom Line
Last reviewed: March 29, 2026