Basically, Germans used hedgerows as a method of slowing down allies and concealing their positions. At the beginning of the invasion of France, many allied forces lost their tanks and armor because they were blocked off by the hedgerows, so tanks had no choice but to climb over them. When that happened, under-belly of the tank (usually the thinnest part of the armor) was exposed to the enemy troops, tank crews can't see what's blow them and germans were able to shoot their anti tank guns or weapons directly to the belly of tanks.
This is from a website describing hedgerow battles.
"This feature of the terrain caused the Allies great problems during the fight for Normandy in World War II. Allied units could not see beyond the next hedgerow. The Germans defended each embankment. The hedgerows made Normandy a natural fortress. During battles, tanks could not climb the embankments of the hedgerows without exposing their undersides to anti-tank fire. Tanks moving down the narrow, tree-covered lanes would get blasted by German tanks, assault guns, or anti-tank guns. Disabled tanks would block the lane for following tanks
A small German unit with a few machine guns supported by anti-tank weapons could prevent an entire battalion from advancing. In July 1944, tankers began welding iron beams to the front of Sherman tanks (like a set of teeth) to overcome the hedgerow problem. The tank could then drive directly at the hedgerow and plow through the mass of earth and vegetation without exposing itself. This was known as a "Rhino" conversion or Rhino tank."
As for gun placements, I am not really sure whether germans dug up the section of hedgerow to place their guns or not. Keep in mind that Hedgerow was rows of dirt/vegetation mounds and in between were open farming fields and narrow roads.