When to plant sunflower in Denver, Colorado

April 30 – May 14

Transplant outdoors · typical Denver year

Climate basis: NOAA station Denver Water Dept, CO (USC00052223), 2 mi (3 km) from downtown Denver (ZCTA 80202 centroid), elevation 5228 ft, on the 1991–2020 climate normals. Transplant sunflower 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 13.

Last spring freezeApril 301-in-10 after May 13

First fall freezeOctober 121-in-10 before Sep 28

Frost-free season≈ 165 daysmedian at 32°F

Hard freeze, 28°Apr 18 – Oct 21kills even hardy greens

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

The Denver year

Start seeds indoorsApr 9 – Apr 16

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 13

Sunflower in Denver · freeze context from Denver Water Dept, CO, 1991–2020 normals.

Every method, with its sources

Start seeds indoors

Apr 9 – Apr 16

Illinois Extension's '2-3 weeks before planting outdoors' is measured from the after-frost outdoor sowing date, which places the indoor window at 21 to 14 days before last frost. Anchored to the median last spring freeze (Apr 30). University of Illinois Extension · NC State Extension · University of Georgia Cooperative Extension · North Carolina Cooperative Extension, Pender County Center

Transplant outdoors

Apr 30 – May 14

NC Cooperative Extension (Pender County) names sunflowers among the large-seeded plants 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

Direct-sow outdoors

Apr 30 – May 14

Illinois Extension and the NC State Extension Gardener Plant Toolbox both direct-seed sunflowers after the last spring frost, giving a two-week window from the last-frost date; UGA's 'several weeks after the last frost' is later but not a number, so it could not be encoded and only argues for a later start within or beyond this window. Anchored to the median last spring freeze (Apr 30). NC State Extension

Variety assumption: Assumes annual garden and cut-flower sunflowers (Helianthus annuus); the indoor window is deliberately short because sunflowers resent root disturbance.

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 (4)

NC State Extension: Helianthus annuus (Common Sunflower) | North Carolina Extension Gardener Plant Toolbox (accessed 2026-07-25)
"This annual is best planted by seed after the last frost in the spring, and harvested before the first frost in the fall."
University of Illinois Extension: How to grow sunflowers at home - A great addition to any garden | Good Growing (accessed 2026-07-25)
"Sunflowers are typically direct-seeded after the danger of frost has passed (early to mid-May). They can also be started indoors, typically 2-3 weeks before planting outdoors, if you want to get a head start."
University of Georgia Cooperative Extension: Growing Sunflowers in the Home Garden (Circular 1121) (accessed 2026-07-25)
"Sunflowers should be planted several weeks after the last frost."
North Carolina Cooperative Extension, Pender County Center: Growing Flowers and Vegetables From Seed (accessed 2026-07-25)
"Large seeded flowers and vegetables are the easiest to grow and include cucumbers, zucchini, squash, melons, cosmos, marigolds, nasturtium, zinnias, and sunflowers. To get a head start on the season, seeds of these varieties can be sown indoors in mid-March to transplant outdoors after the threat of frost."

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