When to plant watermelon in Ottawa

In Ottawa, typical last spring frost is April 28 and typical first fall frost is October 29. Typical last spring frost: April 28. Watermelon in Ottawa is doable with planning outdoors. 79 suitable days are enough for fast varieties (70 days) but not slow ones (100 days).

Watermelon

How to grow Watermelon Ottawa climate Weekly temperatures and windows

Where are you growing?

Tight fit

Not realistic in greenhouse

Comfortable in a cold frame

Watermelon in Ottawa is doable with planning outdoors. 79 suitable days are enough for fast varieties (70 days) but not slow ones (100 days).

When to plantMay 31 – June 20

Harvest: August 9 – October 17
Season vs crop: 79 grow days vs 70–100 needed
Typical frost (last spring – first fall): April 28 – October 29

Watermelon in Ottawa is doable with planning outdoors. 79 suitable days are enough for fast varieties (70 days) but not slow ones (100 days).

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 row79

Days to harvest70–100 days from seed

May 31 – Jun 20

Direct sow

Jun 21 – Aug 8

Grow outdoors

Aug 9 – Oct 17

Harvest

Activity	Window	Notes
Direct sow	May 31 – Jun 20	Plant outdoors in this window.
Grow outdoors	Jun 21 – Aug 8	Plants grow outdoors until harvest.
Harvest	Aug 9 – Oct 17	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.

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.

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 frostApril 28

Low-risk last spring frostMay 8

Typical first fall frostOctober 29

Low-risk first fall frostOctober 17

Frost-free days183

Outdoor sow dateMay 31

Direct sow windowMay 31 – June 20

Suitable growing windowMay 31 – August 17 (79 days)

Expected harvestAugust 9 – October 17

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 Ottawa, 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 Ottawa

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 Ottawa use ERA5-Land cell 45.41, -75.70. How we calculated this · Algorithm 2026-10-season-10.

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