When to plant nasturtium in Denver, Colorado
April 30 – May 14
Transplant outdoors · typical Denver yearClimate 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 nasturtium 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.
The Denver year
- indoors under lights
- in the garden
- frost-free season
- 1-in-10 years still freeze to May 13
Every method, with its sources
Start seeds indoors
Mar 19 – Apr 2UW-Madison states seed can be started indoors in peat pots 4 to 6 weeks before the last expected spring frost, giving 42 to 28 days before last frost. Anchored to the median last spring freeze (Apr 30). University of Wisconsin-Madison Division of Extension · University of Illinois Extension · Iowa State University Extension and Outreach · North Carolina Cooperative Extension, Pender County Center
Transplant outdoors
Apr 30 – May 14NC Cooperative Extension (Pender County) names nasturtium 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). University of Wisconsin-Madison Division of Extension · University of Illinois Extension · Iowa State University Extension and Outreach · North Carolina Cooperative Extension, Pender County Center
Direct-sow outdoors
Apr 30 – May 14Sources disagree - UW-Madison and Illinois Extension sow nasturtium only once all danger of frost has passed, while Iowa State treats it as a cool-season annual that can go in 3 to 4 weeks before the last frost - and the later, more frost-conservative after-frost window wins, opening at the last-frost date and running two weeks. Anchored to the median last spring freeze (Apr 30). Iowa State University Extension and Outreach
Variety assumption: Covers both climbing (T. majus) and bush (T. minus) garden nasturtiums; Illinois Extension notes nasturtium does not transplant well, so the transplant window assumes peat or paper pots planted intact, as UW-Madison specifies.
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)
- Iowa State University Extension and Outreach: Direct Seeding Vegetables and Annuals - Preparation and Care | Yard and Garden (accessed 2026-07-25)"Cool-season annuals, like viola and nasturtium, can be sown 3 to 4 weeks before the last frost."
- University of Wisconsin-Madison Division of Extension: Nasturtium, Tropaeolum species - Wisconsin Horticulture (accessed 2026-07-25)"Seed can be sown directly in the garden in late spring to early summer once all danger of frost has passed or started indoors in peat pots 4 to 6 weeks before the date of the last expected spring frost."
- University of Illinois Extension: Nasturtium | Herbs (accessed 2026-07-25)"Because nasturtium does not transplant well, it is suggested to directly sow seeds into the garden after danger of frost is past."
- 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."