When to plant cosmos in Manchester, New Hampshire

April 30 – May 14

Transplant outdoors · typical Manchester year

Climate basis: NOAA station Massabesic Lake, NH (USC00275211), 4 mi (6 km) from downtown Manchester (ZCTA 03101 centroid), elevation 253 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 30). One year in ten still freezes after May 14.

Last spring freezeApril 301-in-10 after May 14

First fall freezeOctober 111-in-10 before Sep 30

Frost-free season≈ 164 daysmedian at 32°F

Hard freeze, 28°Apr 17 – Oct 24kills even hardy greens

USDA zone6a-10 to -5°F winter low — not frost timing

The Manchester year

Transplant outdoorsApr 30 – May 14

Direct-sow outdoorsApr 30 – May 14

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to May 14

Cosmos in Manchester · freeze context from Massabesic Lake, NH, 1991–2020 normals.

Every method, with its sources

Transplant outdoors

Apr 30 – May 14

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 30). NC State Extension · NC State Extension

Direct-sow outdoors

Apr 30 – May 14

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 30). 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 Manchester

Local overview