palisade vs picket fence

palisade

noun
  • A wall of wooden stakes, used as a defensive barrier. 

  • An even row of cells. e.g.: palisade mesophyll cells. 

  • A line of cliffs, especially one showing basaltic columns. 

  • A long, strong stake, one end of which is set firmly in the ground, and the other sharpened. 

verb
  • To equip with a palisade. 

picket fence

noun
  • A simple fence made from wooden pickets, connected by horizontal bars. 

  • A series of spaced perpendicular striations that occasionally occur in aurorae. 

How often have the words palisade and picket fence occurred in a corpus of books? (source: Google Ngram Viewer )