When to plant cosmos in Boston, Massachusetts
April 4 – April 18
Transplant outdoors · typical Boston yearClimate basis: NOAA station Boston Logan INTL AP, MA (USW00014739), 3 mi (5 km) from downtown Boston (ZCTA 02108 centroid), elevation 12 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 4). One year in ten still freezes after April 17.
The Boston year
- indoors under lights
- in the garden
- frost-free season
- 1-in-10 years still freeze to Apr 17
Every method, with its sources
Transplant outdoors
Apr 4 – Apr 18NC 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 4). NC State Extension · NC State Extension
Direct-sow outdoors
Apr 4 – Apr 18The 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 4). 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.
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."