Jan...this what I read a few years ago.....
Watermelons: It is extremely difficult to tell if a watermelon is ripe by just looking; it must be examined. Watermelons will not continue to ripen after harvest. Hold the harvested fruits at 50 degrees to 60 degrees F. Here are indications you can look for:
1. Thump it. If the watermelon sounds hollow (if you hear a dull thump/thud), the melon is usually ripe. This is difficult for less-gifted ears. The unripe melon will have a tighter, metallic ringing or hollow sound. This technique is not perfect however, because the dull sound you hear doesn't indicate if the melon is overripe.
2. Use the criteria of approximate size for variety,
3. Ceasing of growth.
4. Look at the color on the top. The watermelon is ripe when there is little contrast between the stripes. Another indication is when the surface color of the fruit turns dull.
5. Look for the spot where the melon rested on the ground; a yellow-white, yellow or a cream-yellow color spot suggests ripeness and a white or pale green spot indicates immaturity. A green watermelon will have a white bottom; a ripe melon will have a cream- or yellow-colored bottom. Those fruit that show a change of color from green or olive-grey to yellowish brown should be considered ready to harvest. Also look for a breakup of green bands at the blossom end of the fruit. For best quality, walk the patch daily.
6. The rind at the soil spot should toughen and resist denting with a fingernail when the melon is ripe. Scratch the surface of the rind with your thumbnail. If the outer layer slips back with little resistance, showing a green-white color under the rind, the watermelon is ripe. Feel for development of ribbed indentations that can be felt with finger tips. It should be firm but not a rock. If soft or soft spots it�s too far gone. Sponginess is bad.
7. Press on it. If the watermelon sounds like it gives a little, it's ripe. (This method can also ruin the quality of the fruit.)
8. Check the tendril. If the tendril is green, you should wait to pick the melon. Harvest when the curled tendril near the stem, the "pigtail" or tendril closest to the melon on the vine begins to shrivel and dry up. If it dries while the leaves and rest of the vine looks good, the melon should be ripe.
9. Harvest when a small crack appears in the stem just above the melon indicating the melon is ripe. If it's half-dead, it could mean that the watermelon is nearly ripe or ripe. If the tendril is fully dead, it could mean that anthracnose or some other fungus killed the melon, or that it's ripe or overripe. The drying of the stem tendril nearest the attachment point and green color tone are also indicators of ripeness. Sign vary with cultivars.
10. Count the number of days from anthesis (flowering) or the number of days from planting. This works pretty well if you know the variety of watermelon and how many days it's supposed to take for that variety to ripen under normal temperature and fertilizer regimes.
11. The slipping of the stem from the melon with slight finger pressure is an excellent indicator of melon ripeness in the field.
Hold a melon up to your ear, if you can feel it squeeze and hear a slight mush instead of a crack it is ripe.
7 Check the size. It's not necessarily true that when a watermelon is big enough, it's ready; but under good conditions, it should be normal size. If it's not, you're probably too anxious.
8 Crack a few. You've got a whole field of watermelons, and you can practice a little, right?
9 Is the vine dead or dying? Well, the watermelon is not going to get any riper, so you might as well pick it
10. Rely on your nose, and look for a melon with the strongest fragrance, for this will most likely be the best tasting. Breath deeply and follow your nose to the sweet ripe melon. Sniff the aromatic one out. Next, look for a melon that is heavy for its size, because if you have two melons of equal size, the heavier one is almost assuredly the riper and better tasting melon. Smell is something you learn with experience.
11. Still confulsed? Guess. All indicators will not always work. Take your best shot and go with it.��
