For many people it is the case that you may have never thought about dampness from the ground rising through building foundations into the walls, since this is taken as homogenous in all homes and so builders automatically install damp resistant materials. However, this ignorance is apparent when individuals design and build their own garden wall, for instance, when they have poor knowledge of all the materials needed. But why is damp such a problem?
This video highlights the main reasons why all modern buildings are rising-damp resistant. Damp is never good for any wall as it can cause mould, crumbling of the wall and can even lead to a loss of insulation as it all gets soaked, leaving your house in a mess. This is the same for your garden wall, which is why the same attention needs to be given to damp proofing it as you would give to damp proofing your house.
The industry solution is to place damp proof course in the wall between the foundation and the wall bricks, made invisible by covering it in cement. This material is thin and has grooves in order to bond to the cement better. Unfortunately, some people on the internet claim that you should not put this in your walls, for it causes the wall to just slip over the course, but this is only the case if cement has not been applied correctly or sufficiently.
Secondly, builders lay damp proof membrane on top of the building foundation layer of concrete, to stop water rising into the floor of the house, although you will not be using this material in your garden wall but you could use it if you were building a patio for instance.
Lastly, recently damp proof building blocks have been developed to be placed instead of foundation bricks and remove the need for damp proof course, although overall this solution is more expensive. You could however use these in your garden wall and will surely provide a sturdy base.
Overall, what is sure is that you cannot build any structure which meets the ground without forgetting to impede the rise of damp for if not, the whole structure will be left vulnerable.