You can easily stack blocks like these (find them in lowes/home depot) to make nice flower planters. It’s very easy to assemble, and requires only very basic DIY skills.
The critical First Row
First, you need to lay the first row of blocks in a way that’s “near level”. By near level I mean it doesn’t have to be perfect, but when you do put the next row of blocks on top it should not wobble too much. The rule is, the taller your wall is, the more diligent you should be in leveling your rows.
Depending on your terrain, you might have to dig with a straight edge shovel on some points to straighten the ground under the first row of blocks. If you’re building the planter on a hilly patch like I am, you can either bury the blocks into the hill or you can lay a partial first row and just continue with the 2nd row once your 2nd row can rest at least partially on the soil (backfill holes with dirt)
In the best case scenario, you’d lay the blocks s.t. the 2nd row of blocks would start at the midpoint of a block below, making the blocks look like a brick pattern. That’s very easy to do if you’re doing a straight wall, but in this case the blocks have a lip on the bottom (which interlocks them with the block below) and which forces you to lay the next row of blocks slightly behind the face of the row below…and in a circular/oval setting this has the side effect that each new row of blocks will have a shorter run to go around and then all of a sudden your brick pattern is no longer there. You could circumvent this problem in 3 ways:
1) by ignoring the lip and laying the blocks on top of each other with their faces on the same vertical plane, but that may compromise stability (since a lot of the blocks will be inclined) and the cosmetics (blocks may look like they’re falling)
2) by laying the blocks with gaps between them to accommodate for the differences in lengths
3) by laying the blocks with no gaps and cutting the last block to fit.
I went with choice 2, out of sheer laziness… maybe I’ll adjust to choice 3 sometime in the future if I ever get bored 🙂
Once your rows of blocks are placed and you’ve finished adjusting and readjusting the blocks so that it looks good, you’ll have to fill it up with dirt.
To keep the blocks looking tidy and avoid dirt stains running down between the blocks, get some vinyl flashing (or any kind of plastic) and place it all along the inside of the blocks. This will keep the dirt from getting to the blocks when it rains.
If you’re doing a real retaining wall (i.e. once that has to hold the ground, as opposed to just a few plants in a planter) you’ll have to follow manufacturer/township rules such as making cement footings under the freeze level, leveling the blocks properly, getting permits, etc). In my case, my planter wall is just 2 feet tall, so I can just use my judgement.