Raised garden beds are an ideal way to teach children about gardening. Easily accessible, easy to weed and small in size, they are less intimating to children than large garden beds, and they can be installed almost anywhere -- even right over the top of concrete, as long as you provide good drainage.
One advantage of raised garden beds is that they help conserve water, according to The Ohio State University Extension. They also may prevent water-related diseases if the plants are not watered with overhead sprinklers. On the other hand, you don't want to conserve too much water -- overly wet soil can lead to fungal diseases such as root rot. For this reason, raised flower beds should be built in such a way that they drain well -- especially if built over concrete.
Raised garden beds can be framed with rocks or edging stones, but probably the most common material is lumber. Never use treated creosote or pentachlorophenol-treated lumber, as the chemicals can leach out and damage or contaminate the plants. This is especially important if you are growing edible plants such as vegetables or herbs. Ideally, raised garden beds should be narrow, facing north-south for low-growing plants and east-west for taller plants. Raised beds should not be wider than 4 feet, according to Purdue University, and should ideally be about 2 to 3 feet high. In best-case scenarios, they should also have about a foot of soil beneath them, but if not -- as in the case of concrete -- you can do some things to improve drainage.
Line the raised bed with landscape fabric. The microscopic holes in the fabric will allow water to flow out onto the concrete while at the same time keeping the soil in the bed. To allow water to flow out, leave space between the bottom of the raised bed and the ground -- the fabric should not be completely flush against the concrete. The wood used to build the bed might be thick enough to raise it an inch or so off the surface, or try placing the corners of the box on bricks. It's a myth that you need to line the bottom of the bed with pea gravel, according to PennLive. Instead, the quality of the soil will determine how well the water will drain.
Choose a soil mixture that contains peat moss for drainage and organic matter for nutrients. It should be composed of between 5 and 7 percent organic matter, according to Purdue University, and have a pH level of around 6.5. A good way to create soil for your raised garden beds is to mix about a third of the existing gardening soil, which usually contains a lot of important minerals, with equal parts compost and peat or sand for drainage.