To be honest, I have only been using redwood for several months, maybe about 6 months or so at the most.
I have been mildly concerned about whatever it is that protects redwood from decay and its possible effect on orchids.
So far, no noticeable bad effects.
But, I haven't been using it all that long.

I also had that concern for cedar when I began to use it years ago.

When I was in my early 20s, I lived on the West side of the Olympic Peninsula in a very high rainfall area, 240"/yr.
My experiences working in timber harvesting, wood products manufacturing and just playing outdoors proved to me that old growth cedar was VERY resistant to rotting.

When I made my mounts from cedar, I ripped the strips from a bunch old growth cedar boards that I had saved for many years.
The orchids never noticed as far as I could see.

It is possible that redwood is not as benign as cedar.
So far it is not proving to be so.
http://i131.photobucket.com/albums/p...psa9f0c8ce.jpg
http://i131.photobucket.com/albums/p...ps98adb1b2.jpg
http://i131.photobucket.com/albums/p...ps6c810718.jpg