Question: I have a small lot with a basketball pad from which I'd like to trim about 5 feet of concrete from two sides. Can I get around the manual labor aspect of this chore by installing concrete edging block raised beds 18 to 24 inches over the existing concrete, or does the concrete need to be removed?
The problem you face is to have water that enters the raised bed drain freely. In situations like these, water added to the raised bed will not drain freely unless you provide drainage paths that connect to exit points from the raised bed.
Otherwise, irrigation water will form a slow-draining water table, which will cause plants to die due to waterlogged soils. Water will drain, but it will drain slowly and will weep through the sides of the raised bed, usually at the lowest points.
You could put a planter box on top of a concrete pad, provided the soil on top of the pad will drain water freely. There are a couple of different ways you could do this. One way is to punch large holes through the concrete pad to the soil beneath the pad.
I would spray the pad with water until you see puddling. The puddles will indicate where you need to punch the holes. You will need several of these holes.
Remove soil beneath the holes to a depth of a couple of feet and pack these holes with coarse gravel. You may need three or four of these holes.
When you fill the raised beds with soil, put a 6-inch layer of the same coarse gravel on top of the pad to provide continuous draining. Fill the remainder of the raised bed with a blended topsoil that drains easily.
The second way is to install drain tiles on top of the pad, which allow water to exit through the bottom of the walls. The drain tiles must be sloped so that water can exit freely from the raised bed.
Bob Morris is an associate professor with the University of Nevada Cooperative Extension.
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