When to plant watermelons in Rapid City: 6 June – 3 July

In Rapid City, watermelon: sow for seedlings from 18 April to 5 June; plant into a greenhouse from 16 May to 3 July; plant out into open ground from 6 June to 3 July.

Watermelon in Rapid City: soil, season and weather

In Rapid City the soil, not frost, sets the date: the last frost is over around 7 May, but the top 7 cm of soil reaches 18 °C — the minimum on the watermelon card — only around 6 June, 30 days later.

Last spring frost	7 May
Top 7 cm of soil at 18 °C	6 June, 30 days after the last frost
Into an unheated greenhouse	16 May – 3 July; May outside: 7.3 °C at night, 19 °C by day; card 18–35 °C
First autumn frost	12 October, 128 days after the start
Days in the bed to the first harvest (card)	59
First harvest, if planted in this window	4 August – 31 August
Days left for picking	69
Daylight	14:30 on 9 May, 15:21 on 6 June
Watering outdoors when fruiting (card)	every 5 days; the driest month, September, has 5 rainy days

The same dates in Rapid City hold for Cucumber, Bell Pepper, Eggplant, Zucchini and Pumpkin.

Watermelon in Rapid City: care after planting

Planted on 6 June: in the first weeks water every 5 days.

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

6 June — 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 Rapid City 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. Months that stay below 18 °C even by day on average: October (15.9 °C). Nights average below 18 °C in every month of the season, from 4.5 °C to 16.8 °C.

Month	Day, °C	Night, °C	Rain, mm	Rainy days	Daylight on the 15th
June	26.5	13.8	69	10	15:28
July	29.7	16.8	49	9	15:11
August	28.5	15.9	39	7	14:03
September	24.9	12.3	31	5	12:34
October	15.9	4.5	39	6	11:04

ERA5 2016–2025, Open-Meteo

Watermelon in nearby cities

Around Rapid City, Pierre is 12 days earlier and Cheyenne 2 days later.

City	Distance	Date	Difference
Pierre	233 km	25 May – 15 July	12 days earlier
Cheyenne	351 km	8 June – 4 July	2 days later
Bismarck	360 km	31 May – 6 July	6 days earlier
Fort Collins	420 km	7 June – 12 July	1 day later
Billings	457 km	2 June – 5 July	4 days earlier
Denver	503 km	28 May – 6 July	9 days earlier

Where to buy watermelon plants and seeds near Rapid City

Buy plants ready to set out from 6 June to 3 July (from 16 May to 3 July for an unheated greenhouse); seed for raising your own is sown from 18 April to 5 June. Within 15 km of Rapid City OpenStreetMap lists 1 market (collected 29 October 2026).

Blackhills Farmers Market — market
East Omaha Street 245, 57701 Rapid City · Nov–Apr Sat 09:00–12:00; May–Oct Sat 08:00–13:00; Jul–Oct Wed 08:00–13:00 · 2 km · blackhillsfarmersmarket.org

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