When to plant watermelons in San Juan: 18 September – 10 March

In San Juan, watermelon: sow for seedlings from 31 July to 11 February; plant into a greenhouse from 28 August to 10 March; plant out into open ground from 18 September to 10 March. For plants and seeds, the places nearby include Vivero Yamanouchi (garden centre, 2 km) and Fedemi (farm supply, 2 km); addresses and opening hours are below.

Watermelon in San Juan: soil, season and weather

In San Juan the soil, not frost, sets the date: the last frost is over around 27 July, but the top 7 cm of soil reaches 18 °C — the minimum on the watermelon card — only around 18 September, 53 days later.

Last spring frost	27 July
Top 7 cm of soil at 18 °C	18 September, 53 days after the last frost
Into an unheated greenhouse	28 August – 10 March; August outside: 5.9 °C at night, 19.9 °C by day; card 18–35 °C
First autumn frost	19 June, 275 days after the start
Days in the bed to the first harvest (card)	59
First harvest, if planted in this window	16 November – 8 May
Days left for picking	216
Daylight	11:07 on 21 August, 11:58 on 18 September
Watering outdoors when fruiting (card)	every 5 days; the driest month, September, has 1 rainy day

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

Watermelon in San Juan: care after planting

Planted on 18 September: in the first weeks water every 5 days.

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

18 September — 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 San Juan during the watermelon season

No month of the season averages above the watermelon card's 18–35 °C by day; the warmest, January, averages 34.2 °C. Months that stay below 18 °C even by day on average: June (15.4 °C). Nights average below 18 °C in March–June and September–November (4–17.8 °C).

Month	Day, °C	Night, °C	Rain, mm	Rainy days	Daylight on the 15th
September	23.7	9.3	4	1	11:52
October	27.4	13.1	15	2	12:50
November	31.7	17.8	5	1	13:43
December	34	20.3	14	3	14:11
January	34.2	21.8	27	5	13:59
February	33.3	20.9	15	3	13:12
March	30	17.8	17	3	12:19
April	24.7	13.1	9	2	11:20
May	19.1	8.3	7	2	10:32
June	15.4	4	4	1	10:06

ERA5 2016–2025, Open-Meteo

Watermelon in nearby cities

Around San Juan, La Rioja is 19 days earlier and Valparaíso 41 days later.

City	Distance	Date	Difference
Mendoza	153 km	19 September – 7 March	1 day later
San Luis	284 km	25 September – 13 March	7 days later
La Rioja	286 km	30 August – 5 March	19 days earlier
Santiago	292 km	14 October – 8 March	26 days later
Valparaíso	335 km	from 29 October	41 days later
San Rafael	343 km	22 September – 24 February	4 days later

Where to buy watermelon plants and seeds near San Juan

Buy plants ready to set out from 18 September to 10 March (from 28 August to 10 March for an unheated greenhouse); seed for raising your own is sown from 31 July to 11 February. Within 15 km of San Juan OpenStreetMap lists 9 garden centres, 5 farm-supply shops, 3 plant nurseries and 6 markets (collected 27 October 2026).

Vivero Yamanouchi — garden centre
Tucumán 1111 (N), J5400EKW San Juan · Mon–Sat 09:00–13:00, 16:00–20:00 · 2 km
INSEMI — plant nursery
Sargento Cabral 3671 (O), 5407 Marquesado · 8 km
Fedemi — farm supply
25 de Mayo 1272 (E), J5400AGZ San Juan · 2 km · fedemi.com.ar
Almacén UTT El Palomar — market
Mon–Fri 09:00–14:00, 17:00–21:30; Sat 09:00–14:00 · 2 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.

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