There are many different choices when building decorative landscaping planting and retaining walls in your garden.
Many people enjoy the look of natural stacked stone or stone with mortar. Other designs will require reinforced concrete that can have a decorative facing applied to its visible side.
For smaller projects you can use wood. Wood should go under the same design planing as any other wall. Most often you will build smaller planting areas with raised beds that may only be a foot or two high. In other instances you may use larger 10×12 treated wood for building retaining walls four feet or higher.
When you use wood you must always understand that it will breakdown over time.
Treated wood can come in many dimensions and can be pressurized lumber able to withstand direct ground contact or it may have a tar coating applied on site.
You should not use these chemicals or pressure treated lumber around vegetable gardens. The products do contain materials that can leach into the ground and cause harm but should not cause you problems if used in flower beds or retaining walls.
The same chemicals that are used in pressure treating lumber are available in liquid form and can be applied to cut ends but this products is best applied in a factory setting under pressure.
Some home owners enjoy the use of fresh cut rounds for building smaller decorative retaining walls. Although trees may live for a century once they are cut they do not hold up well to water and bacteria found in soils.
Applying a coating of petroleum or latex driveway sealer can help and it is an inexpensive and easy to find coating.
You should build the wall in place with enough room behind the wall to apply a good amount of sealer. You may also want to follow this up with a plastic liner.
Insects are also a problem so if you have your home treated for termites make sure they treat around the wood retaining walls that you expect to last for an extended period and are not decorative.
For better results you should provide weep holes at the bottom of the wall about a foot up from the ground to allow water to drain from behind the wall after a heavy rain. This will reduce the static pressure behind the wall as wet soil will tend to move walls.
Remember wood will rot due to water and bacteria and even concrete and mortar will break down over time into a sandy mix.
However with proper planing you can extend the life of your decorative wood landscaping 4 to 5 times that of untreated and unprepared wood.