How can I get the plums on our northern MN plum trees to ripen? They stay green and hard and never ripen. Thx?

Question:

Answers:
Try wrapping the branches in blue tarps. This will hold in the heat and moisture, which will promote ripening.

I've seen this done with banana trees in Israel. It is the strangest thing to see all these blue tarps on the trees, but it works. I don't know about plums in MN though, but it is worth a try.