Raised beds on concrete

With more than one in three American households now participating in some form of food gardening, the promise of fresh, local produce is closer than ever before. But not everyone taking part in this gardening revolution has an acre or two of arable soil tucked away in their backyard. In fact, many people driving up this statistic don’t even have a backyard—and they’re growing something anyway. So how do they do it?

With outdoor space at a premium, urban and small-space gardeners have embraced portable and often temporary garden designs to make the most of the space they have available. Whether confined to a balcony, patio, or other paved area, these gardens are sprouting up everywhere using two mainstays of the urban farming movement: raised beds and patio planters or containers.

Raised Garden Beds

While raised beds are most commonly installed on top of bare soil to improve garden health and accessibility, more people are discovering that with a few adjustments, they can also work on top of paved surfaces. Not only do raised beds provide benefits like ample garden space at a reasonable price, they also offer versatility for those with a larger area to fill.

Are raised garden beds right for you?

Raised garden beds
Raised beds are right for you if:
  • You have a larger space and you want to grow a plethora of healthy, nutritious foods.
  • You have materials available to construct sides for your beds or you’re able to purchase ready-made raised garden bed kits.
  • You have permission to install a semi-permanent garden area.
  • You have access to rich soil to fill your beds and give your vegetables the best possible start.

Two important things you need to keep in mind when building raised beds on hard surfaces are drainage and staining.

Dealing with drainage

To ensure your raised bed won’t get waterlogged once installed, perform a simple test. Pour some water on your concrete or paved surface and see where it drains. Usually this will be toward the property perimeter or away from structures, but it’s important to mark this now to prevent future problems.

Once that’s completed, install your bed on the ground and set a level on top of one of the long sides. Have a few small blocks or wedges ready, then lift the end needed to bring the bed to perfect level, and slip a wedge under to block it in place. Be sure the bed is level both lengthwise and widthwise. Now you can slide a few small blocks beneath the boards where needed for permanent installation.

Preventing stains

Once you’ve installed your raised garden bed, monitor the surrounding area for leakage. There should be little to no runoff coming from beneath during regular gardening, but runoff can happen if the bed is left under a sprinkler or during heavy rains.

Any water that does run off from under the bed will be brown and will eventually stain the concrete. To avoid this:

  • Don’t overwater.
  • Cover the beds in winter.
  • Ensure the soil has adequate organic matter and nutrients to maximize absorption.

If runoff does occur, spray the soiled area with the hose to dilute and disperse it.

What about raised bed liners?

If you want to prevent leakage onto surrounding areas, you can include a raised bed liner when constructing your beds. However, ensure your soil has enough organic matter to absorb excess moisture or it might end up waterlogged.

Alternatively, you can use drainage material at the bottom of your bed. The Royal Horticultural Society recommends laying at least three inches of coarse gravel or stones covered with a geotextile membrane beneath a raised bed built over concrete or pavement. The membrane prevents the drainage material from clogging and getting mixed up with your soil.

What about soil depth?

When building raised garden beds on top of hard surfaces, ensure a depth of at least 18 inches. Salad and herb gardens can be grown in 12″ beds, but the deeper beds are better suited for a broad range of vegetable crops. Since plant roots will not have the luxury of reaching soil beneath ground level, you’ll need this depth to ensure healthy plant roots and to reduce watering needs. For more information, see our article about soil depth requirements for raised beds.

Things to keep in mind when installing raised beds on concrete or pavement:

raised beds on pavement
  • Concrete is alkaline. Over time, this may affect the soil in your raised beds. Test periodically with pH meter and adjust as required.
  • Raised beds built on dark pavement will usually have higher moisture needs than those built on a lawn or natural substrate. Keep this in mind when considering watering frequency.
  • Hard surfaces will prevent beneficial creatures like earthworms from entering raised garden beds from below. Thankfully there are…