AboutMike Mascio Expertise I have been an avid gardener since 1985 and an AllExpert volunteer since
1998. I specialize in soil preparation, seed starting and plant
propagation, flowers, vegetables, and general landscaping. I am a strong
advocate of the square foot method of gardening and the use of organic
controls for pests and diseases.
Question I have a concrete patio that measures 25ft x 40ft. I would like to Build four raised beds on my concrete pad for vegetables. how much top soil will I need for the beds to do well. Is 12 inches sufficient or do I need more. I plan to grow cabbage, broccoli, tomatoes, Peppers, and collards
Answer I usually recommend 8" or more when building a raised bed on soil. For concrete, you definitely want to go down to a minimum of 12". The issue is drainage. The deeper the soil, the better the drainage. In case this is your first raised bed, I am copying my instructions for construction and soil. If you are already versed in building one, please disregard.
I personally prefer raised beds. The soil stays friable since you never walk in the beds; drainage is much better than traditional beds; you save space since raised bed are perfect for intensive gardening methods such as square foot gardening; and soil conditioning is continuous since your always building the beds up with organic matter such as compost, manure, and humus. They are also very neat and more attractive in the landscape.
I always recommend framing a raised bed. You can use treated lumber or a non-rotting natural wood such as cedar. Either of these woods will last up to 20 years. I personally use treated 1”X3”s screwed together in 4’X5’ beds and filled with well amended soil.
If you choose to use treated lumber, make sure to ask for wood treated with ACQ. This wood is treated with borax preservatives. Borax is non-toxic to humans and will not adversely affect your plants, even in the first year. Recent studies have shown that it is best to stay away from wood treated with arsenic (CCA Wolmanized) or woods treated with creosote. The chemicals can leach into your soil and have ill effects on both you and your plants.
The down side to using unframed raised beds is that the sides constantly need to be raked in after a hard rain. If left untended, the plants on the sides can eventually wash away. The decision is certainly yours. But my recommendation is to take the time and build the structures. Over the long term, you’ll be happy you did.
