I thought that plants respire so slowly that their surface temperatures must be pretty much the same as ambient. However, on windy nights, I have experienced three different PIR security lights in three different positions that are triggered by nearby leaves when the wind blows them about. None of the three PIRs are pointing at the house and there are no houses in the direction they point. What could possibly be the explanation, I wonder? Could the leaves be reflecting IR that originates in the house? Difficult to believe in one case because it is solar operated and 10m away from the house, pointing along a deserted lane. The problem goes away when I trim the growth back to a spacing of more than a metre. Possible answer: I found one paper (this link) that suggests values of heat evolution of respiring plants of the order of a few mW per gram of plant material. A small branch of the size I am cutting off, would perhaps have a leaf mass of 200g, which would imply less than a Watt overall. For just a couple of leaves (mass, say 10g), the heat evolved would be only a few tens of mW. Would that raise the surface temperature appreciably? Any opinions?