When to plant watermelons in Helena: 3 July – 5 August

In Helena, watermelon: sow for seedlings from 15 May to 8 July; plant into a greenhouse from 12 June to 5 August; plant out into open ground from 3 July to 5 August. For plants and seeds, the places nearby include Keeping It Green (plant nursery, 1 km); addresses and opening hours are below.

Watermelon in Helena: soil, season and weather

In Helena the soil, not frost, sets the date: the last frost is over around 19 May, but the top 7 cm of soil reaches 18 °C — the minimum on the watermelon card — only around 3 July, 45 days later. 92 days separate planting out (3 July) from the first autumn frost (3 October); the card's 59 days to the first harvest leave 33 for picking, so a start 4 weeks late would still fit.

Last spring frost	19 May
Top 7 cm of soil at 18 °C	3 July, 45 days after the last frost
Into an unheated greenhouse	12 June – 5 August; June outside: 7.9 °C at night, 21.8 °C by day; card 18–35 °C
First autumn frost	3 October, 92 days after the start
Days in the bed to the first harvest (card)	59
First harvest, if planted in this window	31 August – 3 October
Days left for picking	33
Daylight	15:40 on 5 June, 15:45 on 3 July
Watering outdoors when fruiting (card)	every 5 days; the driest month, July, has 6 rainy days

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

Watermelon in Helena: care after planting

Planted on 3 July: in the first weeks water every 5 days.

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

3 July — 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 Helena during the watermelon season

No month of the season averages above the watermelon card's 18–35 °C by day; the warmest, July, averages 27.3 °C. Months that stay below 18 °C even by day on average: October (12 °C). Nights average below 18 °C in every month of the season, from 0.6 °C to 12.1 °C.

Month	Day, °C	Night, °C	Rain, mm	Rainy days	Daylight on the 15th
July	27.3	12.1	23	6	15:29
August	26.5	11.5	28	5	14:14
September	21.1	7.4	40	7	12:36
October	12	0.6	46	9	10:58

ERA5 2016–2025, Open-Meteo

Watermelon in nearby cities

All the nearest cities on our list plant watermelon earlier than Helena; the closest date is in Missoula, 3 days earlier.

City	Distance	Date	Difference
Missoula	155 km	30 June – 8 August	3 days earlier
Billings	287 km	2 June – 5 July	31 days earlier
Idaho Falls	345 km	4 June – 5 July	29 days earlier
Spokane	425 km	22 May – 6 July	42 days earlier
Boise	466 km	7 May – 18 July	57 days earlier
Calgary	517 km	18 June – 22 June	15 days earlier

Where to buy watermelon plants and seeds near Helena

Buy plants ready to set out from 3 July to 5 August (from 12 June to 5 August for an unheated greenhouse); seed for raising your own is sown from 15 May to 8 July. Within 15 km of Helena OpenStreetMap lists 2 farm-supply shops and 6 plant nurseries (collected 28 October 2026).

Keeping It Green — plant nursery
Columbia Avenue 2005, 59601 Helena · 1 km · keeping-it-green-helena.com
Gardenwerks: Nursery & Landscaping — plant nursery
2 km
Chadwick Nursery — plant nursery
4 km
Valley Farms — plant nursery
5 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