I really tried to save the blossoms on a heavily blooming Balaton pie cherry that's been in the ground for five years. It's about 8-10 feet tall and this was the first time I had a potentially large crop. I was really excited about the abundance of blooms and, like everyone else, very worried about the freeze. Here's what I tried, and how well (or not) it worked. What I did wasn't revolutionary, I got all the ideas from other people:

On the afternoon of the first predicted nighttime freeze, I decorated the tree with two strands of the small clear Christmas lights that are so popular these days. I put a tangled, wadded up third strand in the open center of the branches (been training for an open vase shape). I then covered the whole tree with a 2.5 ml sheet of black plastic. It wasn't quite as big as I needed, so I left it kinda loose and open at the bottom for fear of hurting branches by stuffing them in. That night the temps dropped to 23F or 24F. The next day I was sad to see almost every petal had a waxy, translucent look to it. I assumed it to be a total loss, but with even colder temps predicted that night I added more heat in the form of two light bulbs, one 60 and the other 100 watt. I also snugged up the plastic at the bottom and did a better job getting everything protected. It wasn't tight by any means, and I could still stick my head in there to check on things. That night was 18F, and I left everything in place for three more days and nights till the cold spell was over. When I finally removed the plastic I was thrilled to see a few live blooms. They were mainly in the center of the tree. Some buds were still unopened and have done so over the last day or so. Overall I'd say that I lost about 75% of the flowers. That probably happened on the first night without the additional heat and better-secured cover, even though the following night was colder. I'm happy to still have a few potential cherries, but bothered with myself for not doing it right in the first place. I think my success rate would have been very high if I'd done everything on the first night. All other plants around here, including the leaves of native spicebush and tulip poplar, are devastated. As far as leaving the plastic in place during the 50F sunny days, A few upper canopy leaves which were in direct contact with the plastic look wilted, but otherwise I don't think it caused any harm. Overall I'm pleased with how it turned out, and would recommend that others try it if they can. Be sure not to let the light bulbs come in direct contact with anything, and be prepared to lose a few leaves from the plastic if you leave it on during the day.