The warmer the soil, the faster watermelon seeds germinate. For example, it takes approximately three days for a watermelon seed to germinate in 90 degree Fahrenheit conditions, and approximately 10 days when temperatures are at 70 degrees. Melon seeds will not germinate at a soil temperature below 65?F (18?C). Start melons indoors 4 to 3 weeks before planting out into the garden; sow seed in peat pots in seed starting mix. The indoor temperature should be between 80? to 90?F (27-32?C) until germination. Grow melon seedlings on at 75?F (24?C).