The vines will be killed by the first frost. The fruit can not stand much frost either. But you can leave the fruit on the vine until the first frost. The fruit won't ripen much during cool weather. I'd get them off the vine and inside before it drops below 30F.

The Fruitnut

Thanks!

Keepitlow:

The best watermelons ripen in August, in the dead heat of summer. Even in September, as the days become shorter and the nights cool down, watermelons stop growing and accumulating sugars. That is why, except for the deep South, watermelon growers do everything possible to get the plants off to an early start.

After mid-September, you can leave watermelons in the field until the cows come home and they won't get any better. They are toast and material for the compost pile. Sure, you can eat them if you don't mind flavorless white flesh and thick rinds, but I don't think you will like them much.

Don Yellman, Great Falls, VA

I agree that as the days get shorter and cool, they really slow down. My melons this year had several setbacks and the earlier ones all got trampled by sheep then eaten by deer while small, so they didn't set fruit that stuck until early/mid August. Picked one last week a couple days after the vines partially frosted, then picked the rest Monday after 6" of snow from Sunday night melted. The one picked last week was ripe and very good, but it doesn't have the super sweetness earlier ones do. They don't seem to ever ripen any further once picked, so I always leave them on the vine until frost takes the leaves or we have a hard freeze coming to get whatever sugar and ripeness they can.

If the vines look okay after a frost, will the watermelon fruit survive

Did not say where you are but a frost will kill the vines. If vines are not dead you did not have a frost. Might have had it on other parts of the garden but not where the watermelon was located. The watermelons are not going to ripen anymore due to lack of sunlight and heat. I had cantaloupes that just would not ripen until a frost took them out. That was when I learned fall melon crops are a bad idea.