Can you put a raised bed on concrete, a patio or a deck?

The short answer

Yes. The University of Maryland Extension says raised beds on hard surfaces are fine if they are at least 8 inches deep for leafy greens, beans and cucumbers, and 12 to 24 inches deep for peppers, tomatoes and squash. With no soil below, every root is in the fill, so Washington State University Extension’s advice for that case is a bed at least 15 to 18 inches deep.

With no soil under the bed, three things change: the fill is all the root room there is, the water needs somewhere to go, and on a deck, balcony or roof the filled weight bears on a structure. The weights below are estimates to plan with; only someone who assesses the structure can say it will hold them.

What to fill it with

Maryland’s mix for hard surfaces is compost and a soilless growing mix in equal parts. Topsoil can make up to 20% of the mix, but only in beds at least 16 inches deep. The soil mix calculator has this recipe as a preset.

Let the water out

Water must leave the bed. The University of Minnesota Extension suggests lining the bottom with a material that lets water through, such as landscape fabric, rather than plastic. Do not add a gravel layer: an Ask Extension answer for planter beds on a patio says gravel causes a perched water table that keeps water in the soil. On a surface you want to protect, plan where the drainage water goes before filling.

On a balcony or deck, that water runs across the floor and can drip over the edge onto the space below. NC State Extension suggests self-watering containers for balcony gardening where draining water would be a problem. Ask the building’s owner or manager what is allowed before you set up a bed that drains.

What a filled bed weighs, and what that does not tell you

Weight is the question on a deck, balcony or roof, and a bed is heaviest just after it has been watered. Potting mix that has been watered and allowed to drain weighs about 40 to 84 lb per cubic foot. That estimate comes from the normal ranges NC State and USDA researchers give for most potting mixes: 0.19 to 0.70 g per cubic centimetre dry (12 to 44 lb per cubic foot), holding 45 to 65% of its volume as water, which adds 28 to 41 lb. Screened topsoil weighs 2,200 lb per cubic yard as one supplier, Sands and Soils, delivers it: about 81.5 lb per cubic foot before you water it.

Those figures are the fill alone. Add the bed itself, the plants, water standing in it after heavy rain and the people beside it. A maker’s own figure for a soil-filled bed is higher still: Vego says its Jumbo elevated bed, which holds 22.6 cubic feet, weighs up to 2,100 to 2,200 lb filled with soil and water, depending on the soil and how wet it is. The University of Maryland Extension notes that a 20-inch container of moist potting mix and plants can weigh 100 pounds.

None of this is a load approval. We cannot tell you what your deck, balcony or roof can carry, and neither can a planter’s own rating: Vego rates its 2 × 4 ft Classic elevated bed to support 700 lb, which says what the bed holds, not what the surface under it can take, and Vego itself asks for a structural engineer before a deck, balcony or rooftop. Have the surface assessed before you buy, and bring the bed’s footprint, where its legs or wheels stand and the weight estimate; the filled-weight checklist lists what to bring. The weight calculator turns your bed’s volume into pounds at any weight per cubic yard.

Elevated planters

A planter on legs keeps the soil off the surface and lets air under it, but it does not spread the weight: the whole filled bed comes down through its legs or wheels, each on a small patch of deck or tile. Its water drains through the floor onto the surface below. Gardener’s Supply says excess water drains through the slats of its elevated cedar planters, and Vego’s beds let it out at the sides. UGA Extension describes planters 10 inches deep, raised on legs or blocks, as a way to save soil. What to plant in a raised bed with legs estimates the watered weight of each planter we list.

References

• Growing Vegetables in Raised Beds, University of Maryland Extension.
• Raised Beds: Will They Benefit Your Vegetable Garden? (FS075E), Washington State University Extension.
• Raised bed gardens, University of Minnesota Extension.
• Planter beds on top of a patio (question 926805), Ask Extension (Cooperative Extension).
• 18. Plants Grown in Containers (Extension Gardener Handbook), NC State Extension.
• Healthy Substrates Need Physicals Too! (HortTechnology 15(4), 2005), USDA Agricultural Research Service and NC State University.
• Topsoil, Sands and Soils.
• Vego Classic Elevated Garden Bed, 2 × 4 ft, Vego Garden.
• Vego Jumbo Elevated Garden Bed, Vego Garden.
• Elevated Cedar Raised Bed, 2 × 4 ft, Gardener’s Supply.
• Growing Vegetables in Containers and Salad Tables, University of Maryland Extension.
• Raised Garden Bed Dimensions (Circular 1027-4), University of Georgia Cooperative Extension.