When to plant sunflower in Boise, Idaho
May 17 – May 31
Transplant outdoors · typical Boise yearClimate basis: NOAA station Boise 7 N, ID (USC00101017), 6 mi (9 km) from downtown Boise (ZCTA 83702 centroid), elevation 3885 ft, on the 1991–2020 climate normals. Transplant sunflower outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (May 17). One year in ten still freezes after June 10.
The Boise year
- indoors under lights
- in the garden
- frost-free season
- 1-in-10 years still freeze to Jun 10
Every method, with its sources
Start seeds indoors
Apr 26 – May 3Illinois 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 (May 17). University of Illinois Extension · NC State Extension · University of Georgia Cooperative Extension · North Carolina Cooperative Extension, Pender County Center
Transplant outdoors
May 17 – May 31NC 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 (May 17). NC State Extension
Direct-sow outdoors
May 17 – May 31Illinois 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 (May 17). 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.
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."