When to plant zinnias, sunflowers and dahlias where you live
Zinnias, sunflowers and dahlias all wait for the same thing in spring: the end of frost. Enter your ZIP code and the calendar works out that date from the weather station nearest you, then counts the indoor sowing window and the first flowers from it.
- Frost danger past (1 year in 10)
- Average last frost
- Hardiness zone
Zinnias
Sunflowers
Dahlias
Spring dates by USDA hardiness zone
Median dates across the ZIP codes in each half zone. Your own ZIP code above is the better answer, because frost dates vary a lot within a zone.
| Zone | Frost danger past | Zinnias indoors | Zinnias, sunflowers outdoors | Dahlia tubers |
|---|---|---|---|---|
| 3b | May 31 | Apr 19 – May 3 | May 31 | From May 31, soil 60 °F |
| 4a | May 27 | Apr 15 – Apr 29 | May 27 | From May 27, soil 60 °F |
| 4b | May 22 | Apr 10 – Apr 24 | May 22 | From May 22, soil 60 °F |
| 5a | May 17 | Apr 5 – Apr 19 | May 17 | From May 17, soil 60 °F |
| 5b | May 16 | Apr 4 – Apr 18 | May 16 | From May 16, soil 60 °F |
| 6a | May 14 | Apr 2 – Apr 16 | May 14 | From May 14, soil 60 °F |
| 6b | May 9 | Mar 28 – Apr 11 | May 9 | From May 9, soil 60 °F |
| 7a | May 3 | Mar 22 – Apr 5 | May 3 | From May 3, soil 60 °F |
| 7b | Apr 22 | Mar 11 – Mar 25 | Apr 22 | From Apr 22, soil 60 °F |
| 8a | Apr 14 | Mar 3 – Mar 17 | Apr 14 | From Apr 14, soil 60 °F |
| 8b | Apr 6 | Feb 23 – Mar 9 | Apr 6 | From Apr 6, soil 60 °F |
| 9a | Mar 25 | Feb 11 – Feb 25 | Mar 25 | From Mar 25, soil 60 °F |
| 9b | Mar 5 | Jan 22 – Feb 5 | Mar 5 | From Mar 5, soil 60 °F |
There are no rows for zone 10 and up: frost is rare that far south, and the closest station that records it can sit a long way from your garden.
Hardiness zones come from the PRISM Climate Group (Oregon State University) ZIP code data behind the 2023 USDA Plant Hardiness Zone Map. We derived this table from that data; it is not the official USDA map.
Zinnias
Wisconsin Extension advises sowing zinnias in the garden after the danger of frost has passed in spring, or starting them indoors 4 to 6 weeks earlier. Clemson Extension adds that zinnias sown straight into the garden flower 8 to 12 weeks later and like soil and air around 70 °F, so a cold spell after the frost date slows them down.
Sunflowers
Oregon State Extension advises sowing sunflowers after the last frost, once the soil has reached at least 50 °F. They can be started indoors, but they grow fast and turn rootbound in pots if started too early, so the calendar gives no indoor date.
Dahlias
Oregon State Extension plants dahlia tubers outside when the soil stays around 60 °F or warmer for several days in a row, 4 to 6 inches deep with the eye facing up. That usually comes a little after the frost-free date, so treat the date here as the earliest day, not the target.
Which frost date the calendar uses
The sources say to wait until the danger of frost has passed. The calendar reads that as the safe last frost date: the day after which, in NOAA's 1991–2020 normals, a 32 °F night came in only one year out of ten. The average date is earlier, and in half of all years there is still a frost after it.
Frost dates are for the weather station nearest your ZIP code. Zones are matched from PRISM Climate Group data behind the 2023 USDA Plant Hardiness Zone Map; this is a derived table, not the official USDA map.
Planting tulips, daffodils and peonies or digging dahlias in fall? Use the fall bulb calendar.
Where the dates and rules come from
Frost dates are NOAA's 1991–2020 climate normals for the weather station nearest your ZIP code. The calendar counts from the safe last frost date, the one after which a frost came in only one year out of ten. The rules for each flower come from the extension services listed below.