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When to plant onion in Denver

How to grow OnionDenver climate
Where are you growing?

Tight fit

Not realistic in greenhouse

Tight fit in a cold frame

Onion in Denver is doable with planning outdoors. 115 suitable days are enough for fast varieties (90 days) but not slow ones (120 days).

When to plantMarch 10 – May 4

Harvest
June 8 – September 1
Season vs crop
115 grow days vs 90–120 needed
Typical frost (last spring – first fall)
April 21 – September 8

Onion in Denver is doable with planning outdoors. 115 suitable days are enough for fast varieties (90 days) but not slow ones (120 days).

Planting conditions

Onion planting activities across the year: Direct sowJFMAMJJASOND~Apr 21~Sep 8
  • Direct sow
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 row115
Days to harvest90–120 days from seed
Mar 10 – May 4
Direct sow
ActivityWindowNotes
Direct sowMar 10 – May 4Plant outdoors in this window.
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 – 86 °F) ranges.

Onion planting and growing weeks in DenverJANFEBMARAPRMAYJUNJULAUGSEPOCTNOVDECPlantingGrowing
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
Outdoor sow dateMarch 10
Direct sow windowMarch 10 – May 4
Suitable growing windowMarch 10 – July 2 (115 days)

How these dates are judged

Ideal conditions
Daytime air from 59 – 75 °F.
Suitable conditions
Daytime air from 39 – 86 °F.
Last frost and first frost
Typical spring and fall frost dates for Denver, shown in the key dates.
Days to harvest
90–120 days from seed, 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 Onion: Mar 10 – Jul 2 (115 days)

Planting window for Onion: Mar 10 – May 4 (56 days)

Usually too cold: Jan 1 – Mar 8; 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.

JanFebMarAprMayJunJulAugSepOctNovDec−40 °C−40 °F−30 °C−22 °F−20 °C−4 °F−10 °C14 °F0 °C32 °F10 °C50 °F20 °C68 °F30 °C86 °F40 °C104 °FTolerable 4–30 °CTolerable 39–86 °FIdeal 15–24 °CIdeal 59–75 °FWindowGrowingMar 10 – Jul 2PlantingMar 10 – May 4Last frost ~Apr 21First frost ~Sep 8
  • 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 11 · Mar 9 – Mar 15

Average 24 to 50 °F · Possible −10 to 65 °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 86 °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 86 °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 Onion 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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