I am installing a three foot, above ground, redwood planter. It runs the lenght of the backyard (about 60 feet) and is about five feet wide. I am installing 4x4 redwood posts buried in concrete and screwing horrizonal planks of redwood to them to form the planter. The question is: when I fill in the planter with dirt and install a sprinkler system, how do I keep the inside of the planter from rotting because of damp dirt? Any help will be appreciated, thanks.