The answer to 'when to retaining wall' is that a retaining wall should be installed when you need to hold back soil, prevent erosion, or create a level area on a sloping landscape. Retaining walls are typically built when there is a significant change in grade on your property, such as a steep slope or hillside. They help stabilize the soil and prevent it from shifting or sliding down the slope.
The best time to install a retaining wall is typically in the spring or fall when the weather is mild and there is less chance of heavy rain or freezing temperatures. This allows the wall to be constructed properly and gives the soil time to settle and compact around it. Retaining walls can be built with a variety of materials, including concrete, stone, brick, or treated wood, depending on your budget and the specific needs of your landscape.