When to plant zinnia 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 zinnia 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
Transplant outdoors
Apr 30 – May 14NC Cooperative Extension (Pender County) names zinnias among the large-seeded flowers sown indoors to transplant outdoors after the threat of frost, giving a 14-day set-out window opening at the last spring freeze; UW-Madison also mentions setting young transplants out, but attaches no frost anchor to that clause, so it does not move the window. Anchored to the median last spring freeze (Apr 30). North Carolina Cooperative Extension, Pender County Center
Direct-sow outdoors
Apr 30 – May 14NC State's Extension Gardener Plant Toolbox says to sow zinnia seed directly in the ground after the last frost date and UW-Madison Extension independently places direct sowing after the danger of frost has passed in spring, so the two agree on the anchor and the window opens at the median last spring freeze and runs the standard 14-day span. Anchored to the median last spring freeze (Apr 30). NC State Extension · University of Wisconsin-Madison Division of Extension
Start seeds indoors
Not calculatedNo indoor_start window is encoded: the UMN zinnia page used in the earlier draft is not cited because it never uses the word frost and fixes its outdoor date by soil temperature (70 degrees F), and UW-Madison's companion clause about starting seed indoors 4 to 6 weeks earlier is measured from the direct-sow date rather than given as its own frost-anchored range, so no unambiguous offset from the median last freeze follows.
Variety assumption: UW-Madison additionally conditions outdoor seeding on the soil warming and gives a 65 to 70 degrees F germination temperature, so this frost-anchored window can open before the soil is actually warm enough in a cool spring.
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: Zinnia elegans (Common Zinnia, Zinnia) | North Carolina Extension Gardener Plant Toolbox (accessed 2026-07-25)"Sow seed directly in the ground after the last frost date."
- University of Wisconsin-Madison Division of Extension: Zinnias - Wisconsin Horticulture (accessed 2026-07-25)"Seed can be direct sown in the garden or landscape after the danger of frost has passed in spring, and it can also be started indoors 4 to 6 weeks earlier."
- 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."