If you are making an accessible garden with an open base, as shown in Figure 1, you should consider how the raised garden bed will drain excess water.
Figure 1 - Open base raised garden bed
Without proper drainage, in your open base accessible garden, you will end up having to replace the base material often because the platform that holds the raised earth will rot out very quickly.
There are a couple of methods that can be used to drain the raised garden bed.
- Fiberglass Screen & Landscape Fabric
- Drainage Pipe & Gravel
Drill some 1 inch holes, spaced approximately 12 inches on center, in the bottom of the sheet of plywood that holds the earth for the flower bed, as shown in Figure 2.
Figure 2 - 1 inch holes drilled in base of accessible open base raised garden
Place a layer of fiberglass screen over the bottom of the base and on top of the fiberglass screen lay a layer of landscape fabric, as shown in Figure 3. On top of the landscape fabric add a layer of gravel.
Figure 3 - Drainage materials assembled
The gravel acts as the water path. The landscape fabric prevents the earth from washing out through the 1 inch holes in the bottom and the fiberglass screen does not allow the landscape fabric to eventually protrude through the 1 inch holes, due to the weight of the gravel and earth.
The problem with this drainage design is that the water will just drip down to the ground or surface below the raised garden, which could stain concrete and other natural stones. The drained water could also cause puddling which could create an accessibility obstacle for the gardener.
You can add PVC drainage pipe to the bottom of the raised garden bed.
Standard PVC drainage pipe is sold in 3 and 4 inch diameters, which is usually too big for a raised garden bed.
Use some 1 1/2 or 2 inch PVC pipe and drill 3/8 inch holes in it. The holes should be all around the diameter to allow excess water to enter the pipe.
Build a manifold using PVC fittings, so that the finished manifold will provide drainage for the entire base of the raised garden bed.
Install a PVC male fitting to the second PVC
T at one end of the manifold. The PVC male fitting will go through a hole that has been drilled in the base of the raised garden.
Place the manifold inside the raised garden bed and put a piece of wood under the end farthest from the male fitting so that the water will drain towards the male fitting.
Figure 4 shows the manifold and Figure 5 shows the manifold installed in the raised garden bed.
Figure 4 - Drainage manifold made of PVC pipe and fittings
Figure 5 - Drainage manifold mounted on base of open base raised accessible garden
Cover the drainage pipe with gravel and cover the gravel with landscape fabric.
You can allow the excess water to just exit the male PVC fitting or you can connect some PVC pipe to the male fitting and direct it away from the raised flower bed. You can even direct the PVC piping under walkways to avoid producing a pond which could easily become an accessibility barrier.