When to Plant Cosmos in New Hampshire
Delicate, daisy-like blooms that dance in the breeze. Cosmos thrive in poor soil and practically grow themselves.
The Short Answer
New Hampshire Frost Dates
Your planting dates depend on which part of New Hampshire you're in. Here are the frost date ranges by region:
| Region | Zones | Last Frost (Spring) | First Frost (Fall) |
|---|---|---|---|
| Northern New Hampshire | 3b, 4a, 4b | May 20 - Jun 5 | Sep 10 - Sep 25 |
| Southern New Hampshire | 5a, 5b, 6a | May 1 - May 20 | Sep 25 - Oct 10 |
Cosmos Planting Schedule for New Hampshire
Northern New Hampshire (Zones 3b, 4a, 4b)
Average last frost: May 20 - Jun 5 · Average first frost: Sep 10 - Sep 25
Southern New Hampshire (Zones 5a, 5b, 6a)
Average last frost: May 1 - May 20 · Average first frost: Sep 25 - Oct 10
Growing Cosmos in New Hampshire
Cosmos in New Hampshire's Climate
Cosmos bloom reliably even in short-season gardens — direct sow after last frost and enjoy graceful flowers from July through hard frost. They actually prefer poor soil and minimal water, making them one of the lowest-maintenance flowers in any climate. Self-seed for volunteer plants next year.
Soil Considerations for New Hampshire
Rocky, acidic New England soils. Glacial deposits. Granite bedrock. Raised beds essential in many areas. Make sure soil has warmed to at least 60°F before planting cosmos outside.
New Hampshire Climate & Growing Season
Short growing season especially in north. Cold winters. Cool summers. Hardy varieties important. 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 New Hampshire: Northern New Hampshire (3b, 4a, 4b) has a last frost around May 20 - Jun 5, while Southern New Hampshire (5a, 5b, 6a) sees frost end around May 1 - May 20. 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