We have 2 of our many fruit trees, a nectarine and an apricot, that set fruit every year but never produce. Invariably, we'll have a late freeze and lose it all. The freezes are just enough below 32 to nip the fruit. I was about to give up and offer the nice trees to anyone willing to dig them out and move them out of our little river valley. Yes, people who live just a few tens of feet higher in the valley get beautiful fruit. Here in the low spot, the drupes just don't get the early heat they need to keep their fruit.
If you haven't browsed Low Tech Magazine, I recommend it. There are some great ideas. My latest experiment is a knock-off of a European fruit wall. The article is under the 'obsolete technlogy' header. These were essentially high stone walls that function as heat-sinks to allow fruit to grow well out of their normal range. Back in the 1600's, when transportation was too slow to move many perishables, fruit walls allowed even citrus to be grown as far north as England.
So, our experiment, in the spirit of fruit walls, is to place thermal mass, in the form of dark lava rocks, under our trees. We're hoping that we've put enough rock under them to raise the night-time temperature by 4 degrees on a freezing night. That's all we need.
The rocks are stacked in a ring that starts about 3 feet from the trunk, and is about a foot wide and a foot high. Already, buds have popped open above the 'wall' but not yet open higher up the tree. I'll let you know if it gets us some fruit!