You're local Home Depot store manager can direct you to various types of brick and faux stone concrete that are the easiest way to make a low masonry wall around that bed. They have the supplies and experts on hand to tell you how to do it.
Once the low wall is done rake out the surface mulch and save it for re-mulching later. Then mix about 2 inches of compost into the soil to prepare it for planting and to improve the performance of your soon to be planted flowers!
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