When to plant cosmos in Hartford, Connecticut

April 17 – May 1

Transplant outdoors · typical Hartford year

Climate basis: NOAA station Hartford Brainard Fld, CT (USW00014752), 2 mi (4 km) from downtown Hartford (ZCTA 06103 centroid), elevation 19 ft, on the 1991–2020 climate normals. Transplant cosmos outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (April 17). One year in ten still freezes after May 1.

Last spring freezeApril 171-in-10 after May 1

First fall freezeOctober 261-in-10 before Oct 12

Frost-free season≈ 191 daysmedian at 32°F

Hard freeze, 28°Apr 3 – Nov 5kills even hardy greens

USDA zone6b-5 to 0°F winter low — not frost timing

The Hartford year

Transplant outdoorsApr 17 – May 1

Direct-sow outdoorsApr 17 – May 1

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to May 1

Cosmos in Hartford · freeze context from Hartford Brainard Fld, CT, 1991–2020 normals.

Every method, with its sources

Transplant outdoors

Apr 17 – May 1

NC Cooperative Extension (Pender County) names cosmos among the large-seeded flowers to transplant outdoors after the threat of frost, giving a two-week set-out window from the last-frost date. Anchored to the median last spring freeze (Apr 17). NC State Extension · NC State Extension

Direct-sow outdoors

Apr 17 – May 1

The NC State Extension Gardener Plant Toolbox (both C. bipinnatus and C. sulphureus entries) and Utah State University Extension agree that cosmos seed goes into the ground once all danger of frost has passed, giving a two-week window from the last-frost date; the accompanying 65F soil requirement is not frost-anchored and was not encoded. Anchored to the median last spring freeze (Apr 17). NC State Extension · NC State Extension · Utah State University Extension

Variety assumption: Covers both common annual garden species (C. bipinnatus and C. sulphureus); no indoor_start rule is encoded because no extension page was found that states a frost-relative indoor-sowing offset for cosmos - Pender County's indoor date is a fixed North Carolina calendar date (mid-March), which the frost-anchored engine cannot honestly express.

These windows describe typical years (1991–2020 climate normals), not this year's forecast. Freezes happen outside them — especially in frost pockets, valleys, and urban edges. Watch the forecast before committing tender plants.

This week, not a typical year:

Crop rule sources (3)

NC State Extension: Cosmos bipinnatus (Cut Leaf Cosmos, Garden Cosmos, Mexican Aster, Tall Cosmos) | North Carolina Extension Gardener Plant Toolbox (accessed 2026-07-25)
"Sow the seeds outdoors after the danger of frost is past and when the soil has warmed to at least 65 degrees."
NC State Extension: Cosmos sulphureus (Klondike Cosmos, Orange Cosmos, Sulphur Cosmos, Yellow Cosmos) | North Carolina Extension Gardener Plant Toolbox (accessed 2026-07-25)
"Sow the seeds outdoors after the danger of frost is past and when the soil has warmed to at least 65 degrees."
Utah State University Extension: Cosmos in the Garden (accessed 2026-07-25)
"Grow plants from seed sown directly into the soil once all danger of frost has passed."

Keep exploring

This crop, other cities

Related crops in Hartford

Local overview