- "I have a 10' x 10' concrete slab that was designed for a hot tub, but in these troubling economic times, we've decided to simplify and use the space for a vegetable garden. My question: Can I build a planter box directly over the concrete? And if so, how deep would it need to be to be successful? I'd rather not remove the slab; who knows, someday we may be back to ordering groceries from WebVan and toasting the new economy in the 'tub."
Raymond, your champagne dreams are as refreshing as a spring breeze. And who knows? Perhaps your windfall will come from cornering the market in microgreens or breeding the Next Big Thing in broccoli — in your new vegetable garden!
I absolutely think you can build a planter box on top of the pad, and here are a few things I would do to ensure success:
Here's a quick sketch of one possible layout on your slab, two boxes at 48"L x 36"W x 24"D plus one at 120"L x 30"W x 30"D:
To prevent rot of both the boxes and the plants, improve drainage by doing the following:
I would use untreated redwood for the boxes. Your trellis could be of either redwood or copper pipe; and you may also want to consider building "cages" framed with wood, copper, or plastic pipe to protect your investment from critters.
Good luck, and happy growing. Please send photos of whatever you create! And when you are rolling in cash again, I accept your invitation for a soak in that hot tub.