Question
Hello. thank you for being here. I have moved to central California area. I have three choices for wood used outdoors and in contact with the ground. Cedar, Redwood, and Pressure Treated. Of those three, which one would you use?

Answer
Your two big enemies of wood are termites and decay.  Installing fence posts, deck sleepers, or lawn edging? Go with pressure-treated (or PT) lumber.  

The redwood & cedar have tannins, a naturally rot-resistant chemical. Termites will still eat redwood & cedar, however. Older homes in California used to have redwood foundations directly on the ground, and architectural clear heartwood is crazy expensive!!  

The pressure-treated wood has more harsh chemicals.  the "green-looking" wood contains Chromated Copper Arsenate (CCA), which is arsenic-based.  Another "green" colored wood uses Alkaline Copper Quaternary (ACQ), but doesn't contain arsenic.  ACQ is still pretty nasty stuff.  The third  type is a "brown" colored wood, treated with Copper Boron Azole (CBA), a copper-based preservative with an organic fungicide.  

Don򴠢reath any dust created from cutting these lumber products!  PT lumber would be OK for use in retaining walls for ornamental plants.  But, none of these should not be in contact with plants or roots of plants you are going to eat -- fruit trees, berry bushes, or veggies.  In other words, these PT woods are NOT for use in a raised vegetable garden situation. This application would be the place to use the cedar or redwood.

If you are looking for a structural use that is not as utilitarian as a fence, like a swing-set or ornamental arbor?  Western Red Cedar has more structural toughness (less likely to shatter under loads), than redwood.  If you are going for top-dollar sexy? Then redwood is knockout gorgeous but can be expensive.  It will also require some maintenance over time to stay looking great.  I'd use a lower grade for landscape purposes, like construcion common/construction heart.

I Hope this helped ~m