Mickey Murch’s organic fields are lush with life that he didn’t plant: weeds. Growing where carrots are about to emerge in the rich soil of the Pine Gulch Creek delta in Bolinas, the weeds can’t be killed by herbicide. So he’s devised a strategy. First, he preps the beds to encourage weed seeds to sprout. Then, right before the carrots germinate, he conducts “flame weeding,” walking slowly along each 200-foot-long row with a hot propane…