How to Build a Simple Cinder Block Raised Garden Bed

A raised-bed garden made of cinder blocks has numerous benefits. The bed warms earlier than an in-ground garden in spring, extending the planting season. You can customize the bed's planting medium, and the mixture stays loose, allowing oxygen and water to reach plant roots more effectively. A cinder-block raised bed requires less maintenance than a bed made of other materials, and you can easily build the raised bed in one afternoon.

Using Cinder Blocks for a Raised Bed

A standard cinder block
credit: Kitsune06/iStock/Getty Images
Cinder blocks make a durable raised bed.

Cinder blocks have a higher thermal value than other kinds of blocks, making cinder blocks a suitable material for a raised bed. A cinder block typically measures 8 inches wide by 8 inches tall and 16 inches long. Although other sizes are available, this size is large enough to build a one-layer raised bed. Each block weighs , making the blocks simple for most people to lift and place. A block usually has two square holes; that characteristic decreases the block's weight but not its stability.

Cinder blocks are composed of a mixture of powdered portland cement, water and granulated coal or volcanic cinders. Test the soil inside the raised bed every one or two years because cinder blocks can leach lime, requiring amendment of the soil to adjust its pH level for your plants.

Making the Raised Bed

Building a simple raised bed using cinder blocks requires only a few tasks and materials. Then you can plant your garden. Eight cinder blocks are needed to make a 40-inch-square bed.

Step 1 Select a Location

The best site for a cinder-block, raised-bed garden receives a minimum of eight hours of direct sun exposure daily, though many vegetable and flowering plants perform best with even more sunlight.

Step 2 Prepare the Site

Skim weeds, grass and other plants in a 40-by-40-inch space where you want to put the raised bed. Use a shovel or spade for that task. Remove the roots of all large plants, including weeds.

Step 3 Build the Bed's Frame

Place one cinder block, with its holes facing upward, in one corner of the 40-inch-square space. Place another cinder block's short end against the first block's short end that isn't in the raised bed's corner. Place a third block perpendicular to the second block to create the bed's next corner. Place another block the same way, and then another block perpendicular. Continue the pattern back to the beginning block.

Step 4 Lay a Weed Barrier

Set a single layer of cardboard or a 6-sheet-thick layer of newspaper flat in the bottom of the bed. The material will act as a weed barrier and break down over time, allowing plant roots and water to pass through easily.

Step 5 Make the Planting Medium

Mix 3 cubic feet each of topsoil, compost and vermiculite on a tarp. Shovel the mixture into the bed and the cinder block holes.

Building a Bigger Raised Bed

If you want to make the raised bed bigger, then only add to its length. Keeping its width under 4 feet makes reaching each plant for maintenance simple. To calculate how many cinder blocks you'll need to extend the length, divide the total desired length for the bed in inches by 16. Because the blocks' placement is staggered at the corners, also add 8 inches for the final length. For example, you could build a bed with 16 blocks that measures 40 inches wide and 104 inches long. Use a simple soil calculator to figure the total soil mixture you'll need. Divide that number by three to figure the amount of soil, compost and vermiculite needed.