Q. The planting beds in my yard are raised and the soil and mulch are washing away with every rain. How can I prevent this erosion?
A. It sounds like you could use a retaining wall around your planting beds. Another name for this type of wall is a garden wall. Planting live ground cover in an area or installing mulch will also help. The roots from the plants help hold the soil in place. However, I would recommend a retaining wall if the slope is steep enough that the rain causes soil erosion. Retaining walls can be wood, stone or even concrete, and can be decorative as well as functional.
In our area of generally flat land most of the retaining walls built are less than 3 feet high. These shorter walls do not require as much of a foundation as walls over 3 feet and can be installed easily. Landscape designers and architects add raised planting areas and retaining walls to many designs to add interest to the landscape. The garden wall can separate areas of the landscape creating private spots to retreat to in the garden or help frame a focal point of the landscape. Island planters for shrubs, flowers or gardens can be built with retaining wall. Raised gardens allow easy access, which can be helpful to those people who have limited mobility. Raised patios have grown in popularity in the past few years with the introduction of concrete retaining wall, giving you an option other than a deck for above ground outdoor living areas.
There are many types of retaining walls available, most of which are installed on top of a leveled and compacted gravel base. Generally the higher the wall, the deeper the base needs to be. A wall 2-3’ high will need an average of a 4” base under the wall. If I build a patio retaining wall I will be more critical about my base than if I build a small raised planter. Retaining walls are built out of brick, boulders, thin or thick natural rock, or concrete blocks. The weight of the wall units keeps the wall in place. Larger boulders or blocks should be used on taller walls to adequately withstand the pressure against it. To help it stay in place we can slope the wall back, bond pieces together, use gravel behind the wall, and install part the first row below the soil level.
Landscape timbers have been used for many years to hold back the soil. Most of the timbers sold today are pressure treated pine. The wood is impregnated with a solution of chemicals that makes it more rot and insect resistant. The wood turns green from this solution, but it can be painted or stained to your color of choice. Timber retaining walls are secured together with large spike nails or by drilling holes through the timber and pounding metal re-rod through the holes. The life of a timber retaining wall is 10 – 15 years depending on how much moisture is in the ground next to the timbers. Sand or gravel under your timbers will add years to the life of the timbers by allowing the water to drain away.
Boulders can be used to build large walls as well as border a garden bed. Michigan fieldstone is the most common type of stone used, but there are other types available too. Weathered limestone called Moss Rock is unique because of the irregular holes and crevices. Moss and small plants can grow in these spaces adding interest to the stone. Other types of boulders and slab walls are lava rock, quartz and various types of hard sandstone. Canadian flagstone and Ohio bluestone are a couple of thinner retaining wall stone that can be stacked to make a wall.
Man made split-faced colored concrete retaining wall is the newest trend in building a retaining wall. There are many applications for this wall ranging from a short garden boarder to commercial walls over 15 feet. The blocks are available in a variety of colors and the face of the finished wall can be contoured or flat. The newest type of concrete retaining wall to emerge is a tumbled wall block by Lafarge called Fieldstone. The blocks are literally tumbled in a cement mixer type of machine to achieve an old-world look. They are very impressive when you see a wall built from this block. You do not need a skilled mason to build this wall and there is virtually no maintenance needed. After the first row of block is set on a compacted level layer of compacted stone or sand the remaining rows are just stacked up. The cost of labor is significantly less to build a man-made block wall compared to a natural stone wall.