PlantingWindowThe American Planting Almanac

When to plant mustard greens in Denver, Colorado

April 2 – April 16

Direct-sow 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. Direct-sow mustard greens 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

Direct-sow outdoorsApr 2 – Apr 16
Fall plantingAug 17 – Sep 14
  • indoors under lights
  • in the garden
  • frost-free season
  • 1-in-10 years still freeze to May 13
Mustard greens in Denver · freeze context from Denver Water Dept, CO, 1991–2020 normals.

Every method, with its sources

Direct-sow outdoors

Apr 2 – Apr 16

Purdue lists mustard under both hardy (4–6 weeks before the last frost) and semi-hardy (2–4 weeks before); the later semi-hardy start is used. Anchored to the median last spring freeze (Apr 30). Purdue University Extension

Fall planting

Aug 17 – Sep 14

Purdue Table 2: plant fall mustard greens at least 4–8 weeks before the first fall frost. Anchored to the median first fall freeze (Oct 12). Purdue University Extension

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 (1)

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