When to plant broccoli in Denver

Where are you growing?

Comfortable

Not realistic in greenhouse

Comfortable in a cold frame

Broccoli in Denver is a good fit outdoors. The suitable growing window of 84 days covers the 55–75 days this plant usually needs.

Start indoors: January 28 – March 24
Transplant outside: March 18 – May 12
Harvest: May 12 – July 26
Season vs crop: 84 grow days vs 55–75 needed
Typical frost (last spring – first fall): April 21 – September 8

Broccoli in Denver is a good fit outdoors. The suitable growing window of 84 days covers the 55–75 days this plant usually needs.

Planting conditions

Start indoors
Harden off
Transplant outside
Grow outdoors
Harvest

Local activity windows for this place and growing method. Blue fill is usually too cold for this plant. Dashed lines are typical last and first frost.

Suitable growing days in a row84

Days to harvest55–75 days from transplant

Jan 28 – Mar 24

Start indoors

Mar 11 – Mar 17

Harden off

March 18

Transplant outside

Mar 19 – May 11

Grow outdoors

May 12 – Jul 26

Harvest

Activity	Window	Notes
Start indoors	Jan 28 – Mar 24	Sow indoors before the outdoor window.
Harden off	Mar 11 – Mar 17	Acclimate seedlings outdoors before planting.
Transplant outside	March 18	Move plants outside on this day (start of the plant-out window).
Grow outdoors	Mar 19 – May 11	Plants grow outdoors until harvest.
Harvest	May 12 – Jul 26	Expected harvest from typical days to harvest.

Blue fill is usually too cold for this plant. Dashed lines are typical last and first frost. Adjust a week or two for your conditions.

Planting and growing across the year

Planting is the sow / plant-out window. Growing is daytime air versus this plant’s ideal (59 – 75 °F) and tolerable (39 – 81 °F) ranges.

Planting uses sow / plant-out rules (frost, soil germination, night, and last-sow): green when plantable, blue diagonal when nights or soil are too cold, amber cross-hatch when too hot, faint for other calendar limits. Growing shows daytime air against this plant’s ideal and tolerable ranges across the year.

Key dates

Typical last spring frostApril 21

Safer last spring frostApril 21

Typical first fall frostSeptember 8

Safer first fall frostSeptember 8

Frost-free days139

Earliest indoor seed startJanuary 28

Latest indoor seed startMarch 24

Transplant outdoorsMarch 18 – May 12

Direct sow windownot recommended here

Suitable growing windowMarch 18 – June 9 (84 days)

Expected harvestMay 12 – July 26

How these dates are judged

Ideal conditions: Daytime air from 59 – 75 °F.
Suitable conditions: Daytime air from 39 – 81 °F.
Last frost and first frost: Typical spring and fall frost dates for Denver, shown in the key dates.
Days to harvest: 55–75 days from transplant, from typical days to harvest.
Sowing window: Dated sow and harvest bars use frost dates, soil minima, night limits, and last-sow count-back from typical crop timing. The year strip and weekly chart use the same window: air ratings only colour weeks inside it.

Growing window for Broccoli: Mar 18 – Jun 9 (84 days)

Planting window for Broccoli: Mar 18 – May 12 (56 days)

Usually too cold: Jan 1 – Mar 15; Oct 19 – Dec 31

Outdoors: weekly temperatures and windows

Temperatures across the year for Denver. Shaded bands are this plant’s ideal and tolerable daytime air ranges. Growing is daytime air suitability; Planting is sow / plant-out conditions (same colour language). Window brackets mark each stretch.

Ideal range
Tolerable range
Possible min to max
Average min to max
Suitable
Tolerable
Too cold
Too hot

Selected week · tap a week or use the arrows

Week 12 · Mar 16 – Mar 22

Average 22 to 47 °F · Possible 7 to 67 °F

Growing: too cold (inside window)

Planting: suitable

A week is in range when its average low and high sit inside the tolerance band 39 to 81 °F; it is suitable when that average range also overlaps the ideal 59 to 75 °F band and possible readings stay within 39 °F to 81 °F. Crossing a possible limit, or missing the ideal band, makes it tolerable; a possible low below 39 °F makes it tolerable. When a sowing window is calculated, weeks outside that window are uncoloured even if air is in range.

Based on average frost dates.

Show all 53 weeks

Cover method weekly charts

Outdoor weekly chart is above. Open a cover chart only if you grow under fleece, a cold frame, or an unheated greenhouse.

Other plants for Denver

Calculated by Plan & Plant · Method · Data 2019–2026

Frost and suitability dates are typical figures from climate statistics. Microclimates vary; they are not a guarantee.

How we calculated this

Data: ERA5-Land via Open-Meteo, cell 39.70, -105.00, years 2019–2026 (not a 30-year climate normal yet).
Typical and safer dates: Typical last frost is the date after which half of winters in this sample had no more frost; safer is later in the sample so fewer winters still frosted.
Crop rule: Local windows for Broccoli from frost dates, temperature ratings, and crop timing rules.
Confidence: Provisional — short or thin sample for this place.
Full methodology (algorithm 2026-10-season-6).

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