Flaming weeds in the orchard

Being organic almond farmers means that we can't use herbicides to kill weeds around our trees and sprinklers. We've tried several methods to kill the weeds, but no method is perfect. Our current effort involves using a propane torch to flame the weeds. The flames don't actually burn the weeds, but instantly boil the water out of the plant, which causes it to die in about 24 hours.