Q&A: Planter Boxes on Concrete

Raymond in Sonoma County, who believes the return of circa-1998 wealth is more likely than romantic evenings in his hot tub, writes:

"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:

To keep all the plants within reach, make your planters no more than 30" wide (i.e. front to back) if you'll be accessing them from only one side, or 60" wide if you'll be able to access them from opposite sides.

Leave yourself at least 24" between planters for comfortable navigation.

Orient your planters along the east-west axis if possible (obviously this depends on the orientation of your slab).

Make the slab a heat sink by leaving the southern edge exposed; it will radiate warmth throughout the night, which should improve yields.

Although most of your veggies need only 6"-12" of root space (tomatoes and root crops will appreciate more), you'll garden more comfortably if the planters are 20"-30" tall. If you attach a trellis (to the northernmost planter edge), make sure its top doesn't exceed your reach.

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:

Raise the boxes off of the pad by attaching cleats (e.g. ripped 2x4s) to their bases. Depending on how long you expect your economic recovery to take, you could use redwood (probably a 5-year lifespan) or a composite lumber (forever lifespan).

Make the bottom of the boxes permeable, i.e. leave gaps between boards. Again, to increase lifespan I would consider a composite lumber for the bottoms, and I would line the inside of the box with weed fabric — spun polypropylene — to keep soil from draining away as well. If you want to get really fancy, try building the bottoms at a 1%-2% slope toward one corner, so all the water drains to that point.

Fill the bottom of the box with a layer of drain rock, topped with weed fabric, topped with potting soil. Try to leave a planting depth of at least 8"; if you're building the raised boxes pictured, that would be about 6"-12" of rock.

Because you're creating such good drainage, your potting soil may dry out; try coconut coir as a sustainable alternative to peat moss.

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.