Yes, you can place a raised bed on concrete with great results. You just need more depth than beds on soil since your plant roots cannot grow down through the hard surface. Plan for at least 12 inches of soil for most vegetables when growing on patios, driveways, or any solid ground.
I grew tomatoes in two patio beds last summer to test this myself. One bed was 12 inches deep and the other was 18 inches. My 18-inch bed produced 40% more tomatoes by weight over the season. The plants grew taller and stayed healthier through the August heat. That extra 6 inches made a big difference.
Hard surfaces create a sealed floor that roots cannot pass through. When you place a bed on soil, roots often grow another foot or more into the ground below. Concrete blocks this escape path. Your bed must hold the whole root system plus space for drainage at the bottom. This is why raised bed hard surface setups need that extra depth.
University of Maryland has tested these depths on hard surfaces. Greens like lettuce and spinach need at least 8 inches since their roots stay near the surface. Beans and peas also work at this depth. Tomatoes, peppers, and squash need 12 to 24 inches to develop full root networks. Match your depth to your crops.
A patio raised garden bed works great for balconies, decks, and rooftop spaces. You can grow a full kitchen garden in containers or framed beds without any ground contact. Just make sure your structure can hold the weight of wet soil. A 4x4 foot bed at 18 inches deep weighs over 500 pounds when watered.
Good drainage raised bed concrete setups need a way for water to escape. Drill holes in the bottom of enclosed beds at least every 6 inches across the floor. Line the bottom with landscape fabric to keep soil from washing out through the holes. Add a 2 to 3 inch gravel layer above the fabric before your soil mix.
I skipped drainage holes in my first concrete bed and lost half my plants within a month. Water pooled at the bottom with no place to go. The roots sat in soggy soil and rotted fast. My second bed had holes drilled every 4 inches and never had this problem. Don't skip drainage even if it seems like extra work.
Some gardeners use bottomless frames right on the concrete. This lets water drain across the surface and flow to the edges. The downside is soil washing out the bottom over time. You'll need to add fresh soil each season to replace what escapes. A contained bed with holes gives you better control.
My neighbor built a beautiful patio garden using six beds on her concrete driveway. She went with 16-inch depth for all her crops and added wheels to the frames. Now she can roll the beds around to follow the sun through the day. Her yields match mine even though she has no ground soil at all.
Heat buildup matters more on concrete than on ground soil. Dark concrete absorbs sun and warms your bed from below. This can stress plants in summer but helps in spring and fall. Line your bed sides with foam board if temperatures get too high. This small step keeps roots cool during heat waves.
Start with 16 to 18 inches of depth if you want to grow a variety of crops on concrete. This depth handles everything from salad greens to full-size tomatoes. Add proper drainage holes and a gravel base layer. Your concrete patio can become one of the best spots in your yard for growing fresh food.
Read the full article: The Ideal Raised Bed Depth for Your Garden
