If you're dreaming of large, raised planters to grow your favorite plants in, think beyond wood planters. Concrete blocks are a strong, sturdy option. Unlike wood, concrete blocks never rot and can be used to make circular and curved planters of any shape. The design of the blocks makes installation simple, and you won't need screws, bolts or power tools.
One option is to build concrete block planters as free-standing structures on flat ground, like a raised bed garden. You can also build them into a slope, either as individual planting shelves or as a long, continuous structure that doubles as a retaining wall. Annual flowers and vegetables can be grown in planters only 8 inches tall, but shrubs and trees are better off in planters at least 18 inches tall. However, 18 inches is a actually a nice size for a vegetable garden, as well, because the plants can be accessed without bending down -- plus, this is a perfect height to use the top of the wall as an informal garden bench.
The style of the block is largely a matter of your own taste -- select a color and texture that complements your yard. You can choose from blocks with a sleek, modern aesthetic with sharp corners and smooth surfaces, or go with one that has been tumbled to look old and worn, like natural stone blocks from another century. When considering the options for size, the general rule is for the block to be in proportion with the size of the planter.
The first question when planning a concrete block planter is whether to use blocks made to be cemented together with mortar or to use a style intended for mortarless, or "dry stack," installation. The latter is generally easier and many manufactured wall blocks are designed specifically for this purpose. The key difference is that mortarless walls have to taper inwards from bottom to top, while mortared walls can be built perfectly vertical. The taper counteracts the weight of the soil pushing against the blocks, ensuring that the planter does not fall apart over time. Check earthquake regulations before you start.
Dig a trench several inches wider and deeper than the size of the block, keeping in mind that the planter will look better and remain sturdier if the blocks are level. On sloped ground, this means the base layer of blocks will have to be laid in sections, stepping up or down as needed in increments equal to the height of the block. Spread 2 or 3 inches of crushed rock or coarse sand in the trench as a firm base and to make it easier to install the bottom row level. This may be time-consuming, but it makes it much easier to stack the rest of the blocks. Fill in behind the wall of the planter as it goes up with a high-quality topsoil and make sure to place landscape fabric between the blocks and the soil -- otherwise the soil will leak through the tiny gaps between mortarless blocks and stain the outside face of the planter.