When to plant watermelons in Flagstaff: 12 May – 12 July

In Flagstaff, watermelon: sow for seedlings from 24 March to 14 June; plant into a greenhouse from 21 April to 12 July; plant out into open ground from 12 May to 12 July.

Watermelon in Flagstaff: soil, season and weather

In Flagstaff frost, not soil, sets the date: the top 7 cm of soil warms to 18 °C — the minimum on the watermelon card — around 30 April, 5 days before the last frost on 5 May, so planting waits only for the week after that frost.

Last spring frost	5 May
Top 7 cm of soil at 18 °C	30 April, 5 days before the last frost
Into an unheated greenhouse	21 April – 12 July; April outside: 3 °C at night, 17.4 °C by day; card 18–35 °C
First autumn frost	21 October, 162 days after the start
Days in the bed to the first harvest (card)	59
First harvest, if planted in this window	10 July – 9 September
Days left for picking	103
Daylight	13:01 on 14 April, 13:55 on 12 May
Watering outdoors when fruiting (card)	every 5 days; the driest month, June, has 3 rainy days

The same dates in Flagstaff hold for Tomato, Cucumber, Bell Pepper, Eggplant, Zucchini and Pumpkin.

Watermelon in Flagstaff: care after planting

Planted on 12 May: in the first weeks water every 5 days.

Feeding, by the card’s schedule from that day:

12 May — feed: balanced (NPK)

Pests and diseases of watermelon to watch for:

Dates count from the first planting day of the window above; a later planting moves them by as many days.

Weather in Flagstaff during the watermelon season

No month of the season averages above the watermelon card's 18–35 °C by day; the warmest, July, averages 29.7 °C. Nights average below 18 °C in every month of the season, from 6 °C to 16.5 °C.

Month	Day, °C	Night, °C	Rain, mm	Rainy days	Daylight on the 15th
May	21.4	7	23	6	14:00
June	28.4	13.7	15	3	14:31
July	29.7	16.5	53	10	14:19
August	27.8	15.3	51	10	13:30
September	24.7	12.1	27	5	12:26
October	18.4	6	24	4	11:20

ERA5 2016–2025, Open-Meteo

Watermelon in nearby cities

All the nearest cities on our list plant watermelon earlier than Flagstaff; the closest date is in Grand Junction, 6 days earlier.

City	Distance	Date	Difference
Phoenix	197 km	23 February – 19 September	78 days earlier
Tucson	335 km	7 March – 8 September	66 days earlier
Las Vegas	336 km	1 April – 2 September	41 days earlier
Albuquerque	452 km	18 April – 31 July	24 days earlier
Grand Junction	512 km	6 May – 18 July	6 days earlier
Santa Fe (United States)	518 km	3 May – 17 July	9 days earlier

Where to buy watermelon plants and seeds near Flagstaff

Buy plants ready to set out from 12 May to 12 July (from 21 April to 12 July for an unheated greenhouse); seed for raising your own is sown from 24 March to 14 June. Within 15 km of Flagstaff OpenStreetMap lists 1 plant nursery and 1 market (collected 28 October 2026).

Sprouts Farmers Market — market
South Riordan Ranch Street 1540, 86001 Flagstaff · Mon–Sun 07:00–22:00 · 4 km · sprouts.com
Viola's Flower Garden — plant nursery
8 km

Each window starts on the Greenly calendar date and ends on the last day that still leaves the crop’s days to harvest (from its card) before the average first autumn frost, plus six weeks of picking for crops harvested over weeks (tomatoes, peppers, aubergines, cucumbers, courgettes, pumpkins, watermelons, beans) — our rule; earlier in the window is safer.

Questions