Gardeners grow watermelon vines (Citrullus lanatus) when there's no threat of frost and the soil and weather are warm. The variety Crimson Sweet produces rather rounded, but slightly oval melons with a striped green skin and seeded, sweet red flesh. Faster melon development occurs when watermelon vines bask in all-day sunshine and the soil is evenly moist but fast-draining. Planting Crimson Sweet watermelons in a gritty to sandy soil enriched with organic matter is ideal.
Crimson Sweet watermelon plants need approximately 85 days of warm growing conditions to be productive in the vegetable garden. It takes that many days for sown seeds to sprout, develop leafy vines, bloom and then produce fully ripe melons ready for harvest. If the weather is cloudy or rainy and cool, more than 85 days may be needed. Temperatures in the range of 80 to 90 degrees Fahrenheit in the daytime and nighttime lows in the 65- to 70-degree range promote timely plant development.
The garden axiom of thumping a watermelon to determine ripeness has been passed down from generation to generation. Unfortunately for Crimson Sweet watermelon growers, thumping is an unreliable indicator of when to harvest, and provides inconsistent insight. Presumably, a dull and low-sounding thud means the melon is ripe, but a higher-pitched, metallic sound means to leave the melon on the vine. Once they're picked from the vine, watermelons do not further sweeten or improve their flesh texture.
Monitor the Stem
A more reliable way to determine when to harvest a ripe Crimson Sweet watermelon is to look at the color and form of the stem. When the stem connecting the melon to the vine is green and plump, the plant is still moving water and sugars into the fruit flesh and developing the seeds. Once the stem begins to naturally shrivel and turn beige to brown, the melon's seeds are mature and the flesh surrounding them is fully colored and sweet. The curly tendrils on the vine closest to the melon also shrivel and turn brown.
Melon Skin Indicators
Once a Crimson Sweet watermelon is picked and taken to market, you don't have the luxury of knowing when the melon was harvested. Examine the melon's skin to discover additional indicators of ripeness. The rind is firm and doesn't give when pressed with your thumb fingertip. The light and dark green-striped skin looks dull, not glossy. Turn the Crimson Sweet watermelon over. The bald spot on the melon bottom becomes cream or pale yellow in color when it's ripe. If the bald spot is still pure white or light green, choose another melon.