Watermelon
Citrullus, Sugar Baby, Crimson Sweet, Charleston Grey
Needs the most heat of any common vegetable — soil must be 70°F+. Start 4 weeks early in short-season climates. Use black plastic mulch to warm soil. Check ripeness: dull thud when tapped, yellow ground spot, dried tendril nearest fruit. Reduce water 1–2 weeks before harvest for sweeter fruit.
- Germination
- 3–10 days
- Soil temperature
- 21–35 °C
- Harvest after sowing
- 70–90 days
- Spacing
- 90–185 cm
When to plant watermelon
Sow outdoors 1–3 weeks after your last spring frost.
Dashed line: typical last spring frost in zone 7 (Apr 5). Zone dates are averages; most of the UK is zone 8–9, central Europe 6–7 and the Baltics and Scandinavia 4–6. In the app, Tomatograph uses your garden's own frost dates.
How to grow watermelon
- 1
Warm the soil
Watermelon needs more heat than any common vegetable. Wait for soil of at least 21 °C, 1–3 weeks after the last frost; black mulch film warms it faster.
- 2
Sow
Sow 2.5 cm deep in compost-rich, sandy hills. In cooler climates start in pots 2–4 weeks earlier and transplant carefully.
- 3
Water
Water deeply and evenly while vines grow. Once fruit is sizing up, avoid big swings in moisture, which split melons, and cut back watering 1–2 weeks before harvest for sweeter fruit.
- 4
Harvest
Ripe melons, 70–90 days from sowing, have a dull rind, a cream-yellow patch underneath, a dried tendril next to the stem and a deep hollow sound when tapped.
Growing conditions
Light
Full sun
Water
High water
Temperature
18–38 °C
Frost tolerance
Frost-sensitive — protect below 0 °C
Days to maturity
75–100 days
Plant spacing
90 cm
How Tomatograph watches your watermelon
Tomatograph checks your local forecast every morning and only speaks up when one of these applies to your garden.
- Forecast of 2 °C or lowerFrost alert the evening before: cover plants with fleece or bring pots inside.
- 3 days at 35 °C or aboveHeat-wave alert while plants are flowering or fruiting: shade and water early.
- Humidity above 80% for 48 hoursCheck for fusarium wilt, which thrives in those conditions.
- Under 5 mm of rain forecast over 3 daysWatering reminder: water deeply at the base, early in the morning.
- Within a week of maturityHarvest-window reminder based on when you planted.
Watermelon questions
- When should I plant watermelon?
- Sow outdoors 1–3 weeks after your last spring frost.
- How long do watermelon seeds take to germinate?
- Usually 3–10 days at a soil temperature of 21–35 °C.
- How long until I can harvest watermelon?
- About 70–90 days after sowing, depending on variety and weather.
- How far apart should I plant watermelon?
- Allow 90–185 cm between plants.
- What grows well with watermelon?
- Good neighbours include corn, nasturtium, radish.
Growing guide written by Tomatograph. Planting figures from openplantdb (CC0 1.0). Zone frost dates are averages from openplantdb (CC0 1.0).