Preheat the oven to 425 degrees F. Whisk the flour, baking powder, baking soda, wheat germ, salt, lemon zest and thyme in a medium bowl. Using a pastry blender or your fingers, work the butter into the flour mixture until it is in pea-size pieces. Gently stir in the buttermilk with a wooden spoon until just moistened. The dough will be loose.
Turn the dough out onto a lightly floured surface and knead 3 to 5 times, just until it comes together. (Don't overknead or the biscuits will be tough.) Roll out to 1/2 inch thick, then cut out biscuits using a floured 2-inch-round cutter. Arrange on an ungreased baking sheet. Reroll the scraps and cut out more biscuits. Brush the tops with water and sprinkle lightly with salt and thyme.
Bake the biscuits until golden brown, 12 to 15 minutes. Serve warm.