I made the mistake of cutting off 3 watermelons off their vines when they seemed big enough. It was a waste. Had I waited another 2-3 weeks, they would have been perfect!

You should wait for the dried tendrils/browning of the nearby leaf. Also, the yellowing occurs when you put the watermelon on the ground. Since one of my vines is pretty short, the watermelon sits on the granite and it doesn't have any yellowing, however I know its maturing because this was the very first watermelon I saw about 1 month ago.

Thanks so much Omni. That's exactly what I am afraid of...picking it too early. Do you thump on the watermelon at all to test for ripeness? Is there any correlation to a slowing or stop in the growth of the fruit?

The bottom should still be a lighter color if it's laying on something, but IMO that is the worst test for ripeness. The bottom is lighter almost the whole growth cycle of the watermelon. The dried tendril is better. One thing I (and it seems many others) tend to overlook is the maturity date right on the seed package. If the maturity date is 85 days, I can almost be sure that the melon will be close to ripe 85 days from setting out, or add 2 weeks if started outside from seed. In southern zones an experienced grower told me they ripen 5 weeks from fruit set, but in the north, in my experience, it's more like 7 weeks. There are several threads on watermelon ripeness. Experience helps a lot. I grew in containers this year, and with the hot summer we had the melons were actually a week ahead of schedule.

Also, I would look for BOTH, dried tendrils and yellowing. Thumping isn't really an accurate way of doing this, as you will have a different way of thumping.

The mistake I made was that I picked the watermelon when it had dried tendrils but was green all around (no yellowing). As I cut the watermelon open, I saw that only the center was red, and there was some green whereas the rest was all white.

Yellowing gives you a sense of age. I'm still experimenting as this is my first time growing sugar babies. I just brought in a melon today that had some yellowing and dried tendrils and it was great!I have two more watermelons outside where the tendrils are drying but no yellowing atm. They've been on the vine for quite a while so I'm really curious to see how they come out. Again, this will only make me more knowledgeable for the next growing season (hopefully I won't forget these lessons, lol)

Thank you all! That is some great information and insights you provided. Now, if only I had paid attention to the fruit set date :>) Guess I'll be waiting until I see some signs of lightening in color somewhere and those dried tendrils. Just worrying because it is getting so large. Plenty more to come after this one so I guess the world wouldn't end if it weren't quite ripe when picked, but I am growing this one for a 2 and 4 yr old that have been following its progress quite closely so I don't want to disappoint with an under ripe melon.