When to plant watermelon in Denver

In Denver, typical last spring frost is May 8 and typical first fall frost is October 13. Typical last spring frost: May 8. Watermelon in Denver is not realistic outdoors: 0 suitable days are fewer than the 70 days this plant usually needs.

Watermelon

How to grow Watermelon Denver climate Weekly temperatures and windows

Where are you growing?

Not realistic outdoors

Tight fit in a cold frame

Watermelon in Denver is doable with planning under fleece. 85 suitable days are enough for fast varieties (70 days) but not slow ones (100 days).

When to plantMay 28 – May 31

Harvest: August 6 – September 27
Season vs crop: 85 grow days vs 70–100 needed
Typical frost (last spring – first fall): May 2 – October 23

Watermelon in Denver is doable with planning under fleece. 85 suitable days are enough for fast varieties (70 days) but not slow ones (100 days).

Estimate. Protection varies with the cover, the weather and how you vent it.

Fleece adds about 1–4 °C on frosty nights, depending on its weight. Remove it on warm days.

Fleece also cuts light: about 5–10 % for light fleece, up to 50–70 % for heavy grades. Take it off by day once frosts are over.

Cover dates are modelled from weather-model data using published greenhouse studies. They are not measured in your greenhouse. A max–min thermometer inside will tell you more.

Typical last frost estimate range May 3 – April 22, depending on the cover and the night.

Planting conditions

Direct sow
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 row85

Days to harvest70–100 days from seed

May 28 – May 31

Direct sow

Jun 1 – Aug 5

Grow outdoors

Aug 6 – Sep 27

Harvest

Activity	Window	Notes
Direct sow	May 28 – May 31	Plant outdoors in this window.
Grow outdoors	Jun 1 – Aug 5	Plants grow outdoors until harvest.
Harvest	Aug 6 – Sep 27	Expected harvest from typical days to harvest.

Direct sow
Grow outdoors
Harvest

Blue fill is usually too cold for this plant. Dashed lines are typical last and first frost (35 seasons (1991–2025)). Adjust a week or two for your conditions.

Protected frost and sowing dates are modelled estimates from weather-model data and published greenhouse studies, not measured in your greenhouse. A max–min thermometer inside will tell you more.

Planting and growing across the year

Planting is the sow / plant-out window. Growing is daytime air versus this plant’s ideal (21 – 35 °C70 – 95 °F21 – 35 °C (70 – 95 °F)) and tolerable (16 – 38 °C61 – 100 °F16 – 38 °C (61 – 100 °F)) ranges, from modelled cover temperatures for the growing method you selected.

Ideal / plantable
Tolerable
Too cold
Too hot
Outside window

Planting uses sow / plant-out rules; growing shows daytime air against this plant’s ideal and tolerable ranges.

Key dates

Typical last spring frostMay 2

Low-risk last spring frostMay 14

Typical first fall frostOctober 23

Low-risk first fall frostSeptember 27

Frost-free days173

Outdoor sow dateMay 28

Direct sow windowMay 28 – May 31

Suitable growing windowMay 28 – August 20 (85 days)

Expected harvestAugust 6 – September 27

Sow again for a steady harvest

Sow quick crops like radish and lettuce every 1–2 weeks, and slower ones like beans and carrots every 2–3 weeks. Stop when there is not enough time left before your first autumn frost for the crop to mature.

One sowing is enough here; there isn't time for a second.

These dates are deliberately cautious. They come out earlier than common rules like '8 weeks before first frost', because we use the low-risk frost date, leave time for picking and add two weeks for shorter autumn days.

How these dates are judged

Ideal conditions: Daytime air from 21 – 35 °C70 – 95 °F21 – 35 °C (70 – 95 °F).
Suitable conditions: Daytime air from 16 – 38 °C61 – 100 °F16 – 38 °C (61 – 100 °F).
Last frost and first frost: Typical spring and fall frost dates for Denver, shown in the key dates.
Days to harvest: 70–100 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. Air ratings on the weekly chart colour only weeks inside that same window — see weekly temperatures and windows.

Other plants for Denver

Calculated by Plan & Plant. Method. Data 35 seasons (1991–2025).

Frost and suitability dates are typical figures from climate statistics.

Based on 35 years (1991–2025) of ERA5-Land data from Open-Meteo, for an area of about 10 km. Typical dates, not a guarantee.

Dates for Denver use ERA5-Land cell 39.70, -105.00. How we calculated this · Algorithm 2026-10-season-10.

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