When to plant brussels sprouts in Denver, Colorado

April 2 – April 16

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 brussels sprouts 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 indoorsFeb 19 – Mar 5

Transplant outdoorsApr 2 – Apr 16

indoors under lights
in the garden
frost-free season
1-in-10 years still freeze to May 13

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

Every method, with its sources

Start seeds indoors

Feb 19 – Mar 5

Sow 4–6 weeks before the set-out window opens (UNH 4–6 weeks; Missouri G6570 6–8 weeks). Anchored to the median last spring freeze (Apr 30). University of New Hampshire Extension · University of Missouri Extension

Transplant outdoors

Apr 2 – Apr 16

UNH: set out 2–4 weeks before the last frost; Purdue's hardy class allows 4–6 weeks before. The later UNH range is used. Anchored to the median last spring freeze (Apr 30). University of New Hampshire Extension · Purdue University Extension

Variety assumption: A long-season crop (about 90–110 days from transplant); sprouts sweeten after light fall frosts.

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 (3)

University of New Hampshire Extension: Growing Vegetables: When to Plant Your Vegetable Garden [fact sheet] (accessed 2026-07-24)
Per-crop 'time from seed to transplant' and 'safe to set out in garden' relative to last frost.
University of Missouri Extension: Starting Plants Indoors from Seeds (G6570) (accessed 2026-07-24)
Weeks needed to grow vegetable transplants indoors (Table 2).
Purdue University Extension: Indiana Vegetable Planting Calendar (HO-186-W) (accessed 2026-07-24)
Hardiness classes: hardy = plant 4-6 wk before last spring frost; semi-hardy = 2-4 wk before; tender = after average last frost; very tender = at least 2 wk after. Table 2: fall planting at least 4-8 wk before first fall frost.

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