Boost Temperatures. 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.oosting the soil's fertility levels before planting watermelon seeds ensures quick germination and seedling establishment. For the best results with watermelons, use 3 pounds of 5-10-10 fertilizer for every 100 square feet of planting space.