Answer: If your watermelons are soggy on the inside, they are over-mature. In other words, they should have been harvested before they got soft and soggy. There are so many varieties out there that it's difficult to know how many days to harvest your particular melon is, but size is not always an indication of ripeness or maturity.
Many home gardeners experience difficulty in determining when watermelons are ripe. Use a combination of the following indicators: (1) light green, curly tendrils on the stem near the point of attachment of the melon usually turn brown and dry; (2) the surface color of the fruit turns dull; (3) the skin becomes resistant to penetration by the thumbnail and is rough to the touch; and (4) the bottom of the melon (where it lies on the soil) turns from light green to a yellowish color. These indicators for choosing a ripe watermelon are much more reliable than "thumping" the melon with a knuckle. Many watermelons do not emit the proverbial "dull thud" when ripe. For these, the dull thud may indicate an over-ripe, mushy melon.
Watch each melon carefully and harvest when the curly tendrils turn brown and dry and you'll avoid mushy melons.
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