Retaining walls, such as those that hold back soil in a tiered landscaping system, can often be unsightly and difficult to hide. A concrete block wall will do a great job of retaining soil, but it is a struggle to get plants to grow in the gap behind the wall. When planting behind a retaining wall, the trick is to add mulch and plenty of it! The gap behind your wall will quickly dry out, leaving the roots of your plants exposed to drought conditions. Digging a small trench behind the wall and filling it with a water-retaining mulch will give your plants the best possible chance of thriving.