Raised beds make growing vegetables easy. You don't have to get on your knees to care for the plants and it allows you to use high-quality soil, rather than the soil you have in your yard. Raised beds eliminate drainage problems and you can extend the growing season. Concrete block beds are simple to build but have both advantages and disadvantage compared with other materials.
Advantages of Concrete Bock
Concrete blocks promote better drainage than wood beds because water can drain through the spaces between blocks. They also hold heat, warming the soil so seeds sprout more quickly and the gardening season is longer. A concrete block bed is also easier to build and doesn't require power tools. Concrete blocks do not have any toxic preservatives so they're a good choice for organic gardening.
Disadvantages of Concrete Block
Concrete blocks are generally more expensive than wood, unless you have leftover blocks from another project or recycled blocks. The site must be level for the block walls to be stable and the blocks will become misaligned over time if you don't anchor them. In dry climates, the increased drainage can be a disadvantage because it's harder to keep the well-draining soil moist. Some people also consider concrete blocks less attractive than other building materials. Decorative blocks make attractive beds but are even more expensive.
Building a Concrete Block Bed
Lay out a rectangle of blocks, no more than 4 feet wide and as long as you like, on a level surface. Use string as a guide to make the sides straight and the corners square. Place a second layer of blocks on top of the first, offsetting the blocks so the joints do not overlap. Pound 36 inch long sections of 1/2 inch rebar into the openings in the blocks to anchor them. Pound the rebar in until it is just even with the top of the blocks.
Depth of Bed
Block walls less than 2 feet tall can be laid in place without mortar or footings. If you have to use mortar and footings, wall construction is considerably more complicated. A deep raised bed also requires a great deal of soil. Few vegetables need more than 16 to 18 inches of soil and some need only 3 inches. To be accessible to people with physical limitations, beds may need to be higher than 2 feet. Otherwise, one or two layers of 8-inch blocks should be enough.