Your local weather forecast will tell you when there is a chance of frost. 32 degrees is the freezing point. You shouldn't have to worry about covering anything until the weather forecasts say it will be that cold. Even then it will depend on how long the temperature stays that cold.
Here in southeast SD it got down to 32 degrees about 6AM this morning, but didn't stay that cold long enough to be considered a hard freeze. Today it only got up to about 60 degrees. When did you plant them? It seems to me like they should already be harvested. Here in South Dakota, the home grown melons of any type were being sold in roadside stands and stores for more than two weeks.
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