Just make it a dry wall and you are ok. If you want a mortared wall post back as that changes things. For walls up to 3 feet you don't need mortar. A dry wall will work very well.
Dig a small trench for the first row - doesn't have to be deep. No sand needed. Lay the first couple of rows. Make the wall lean slightly into the bed about an inch over the foot.
After the first couple of rows are in, back fill with the dirt. Tamp the dirt down. Then do another couple of rows and add more dirt.
If this is next to the house and you have a downspout that empties into this bed, you need to provide an outlet for all that water.Fresh top soil will compact about 25% in a year.
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