Houston area citrus expert Dr. Randall said he and others are seeing the same behavior in trees that are planted in the ground, and that he had never seen this problem before. He speculated that it may be because kumquats are not well adapted to 100+ degree temperatures like we have experienced this summer. The high for every single day of the month of August was over 100 degrees, with the highest temperatures reaching 108 degrees!
I couldn't find any literature on the high temperature tolerance of kumquats, but my observations seem to support Dr. Randall's theory. Even if the tree itself can handle extreme high temperatures, the fruit is getting cooked. This is really a shame, considering how much Boy #1 loves them. We've tried some of the fallen fruit, but they're dry and pithy inside, and don't taste good at all.
In addition to shedding its fruit, my kumquat trees have started blooming again. While I love the smell of the citrus blossoms, it is very strange to see them in September. Environmental stresses such as freezing temperatures or drought can trigger blooming, so I can only speculate that the flush of flowers I'm seeing now is due to the heat stress the tree experienced this summer. Even if they set fruit, they won't ripen before the first freezes of this coming winter, so I'll pick them off as they develop.