Frost damage

As the reliable weather forecast had indicated severe frost, I installed a 'frost guard' heater in the tunnel.  I was over-optimistic as to its powers to prevent frost damage in the large space of the tunnel, so considerable damage was caused to the potato , tomato and sweet pepper plants; the temperature having fallen to 4.7 degrees below freezing.  It is put into perspective for me when I hear of the extreme losses to vineyards and orchards in France.