Watermelon Citrullus lanatus, my F1 hybrid (Sugar Baby x White Seeded Besvirino Assyrian Watermelon).
Today we ate the first watermelon, 3kg.
It was sweet, but seeds were not all mature, so a few more days had perhaps made it even better. I also think the rind is very thick.
For me, watermelon ripeness is bit of a mystery. It does not smell aromatic, so there must be something else to look for.
Tendril
On the clever internet, I have read that you should harvest watermelon when the tendril closest to the stalk wither. This seems to be the case.
Yellow spot
Elsewhere on the clever internet, I have read that the bright spot where the watermelon is resting on the ground, should change colour to yellow. This has happened, but exactly how yellow should the yellow be??
One should also be able to tap and listen. I did too, and this watermelon had a much deeper quiet sound than the obvious immature watermelons crisp bright tones. But for us who do not have absolute pitch, it’s a bit difficult.
Fresh watermelon seeds
The last method I haven’t seen on the internet. The seeds ought to be mature when the watermelon is ripe. It doesn’t seem to be the case here. Pity I couldn’t see the seeds before I harvested the watermelon.
It is easy to collect and clean the seeds. Spit them into a bowl when eating the watermelon. Afterwards, rinse them in clean water, so they’re ready to dry in a thin layer on a plate.
Will the seeds germinate next year? I believe in the darkest of the seeds will, but the lighter are probably not mature enough. I have to wait and see.
In the toad's garden 
August 10, 2011 at 17:01
A clear sound when tapping/knocking is the safest sign. The one pros use to confirm other signs you listed and what educated consumers do at store.
August 11, 2011 at 11:36
My husband – who does not grow melons, but indulge in them, always collect the ones with dry stalk/place where the stalk should sit and hollow sound. So maybe you should wait for the stalk to start drying out? I do that with pumpkins.
August 27, 2011 at 23:53
Thanks, I guess I just have to knock on a lot of watermelons to learn the trick.
September 30, 2011 at 18:25
Tapping on its rind and listening for a hollow sound is a good indication that a watermelon is ripe. Another great way which I think is even better is by holding on to either side of it and squeezing firmly. If that produces a crunchy / crispy sound, the watermelon is good to go.
Of course, we always want a perfectly ripened watermelon. But if you’ve ever encountered an overripe one, you’d realize that in the absence of perfection, they taste far better when they are under-ripe than over.
October 3, 2011 at 20:19
The squeezing trick seems good to me! I will remember that next year.
I my cold summers, over-ripening isn’t really an issue for watermelons – even in the greenhouse.