Well, don't give up all hope. I live in northern Minnesota and was able to grow tropical bitter melon this summer by staring the plants inside the house a few months before our average last frost date. That extends the growing season for us. I'm not saying that the bitter melon grew as well as they would in the tropics, but we did manage to get quite a bit of nice fruit before it got cold this fall.
Also, I think it's best to plant any tropical plants maybe 2 weeks after your last frost date, just to make sure you don't chill the plant.
Next year we plan on starting some more of those tropical plants in the house, transplant them outside 2 weeks after our average last frost date, and just hope for the best.
Picture of some of the tropical bitter melon grown in my northern Minnesota (Zone 3b) garden...
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