DibbleWhen to plant cosmos in Zone 4
In Zone 4, the last spring frost usually passes around May 10 and the first fall frost arrives around September 21, leaving about 134 frost-free days. The windows below are computed from those frost dates, with the same math the Dibble app uses.
Cool climate with a brief warm window. Indoor starts and season extension help.
Planting windows for cosmos in Zone 4
| Task | Window |
|---|---|
| Direct sow | May 17 to May 31 |
| Estimated harvest | Jul 26 to Oct 4 |
The harvest estimate counts 70 days to maturity, then a picking window of about 70 days. These windows use typical frost dates for Zone 4; Dibble pins them to your exact ZIP code.
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How to grow cosmos
Sow cosmos after last frost in lean soil, too much fertility means leaves, not flowers. Deadhead for continuous bloom.
Companion plants
Good neighbors for Cosmos:
Cosmos in nearby zones
Keep exploring
Cosmos: the full growing guide · The Zone 4 planting calendar · Every crop and zone
Plant at the right time this season
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Dates are estimates based on typical frost dates for the zone. Your yard has its own microclimate, so use them as a starting point and adjust as you learn your space.