You would be better off building new flowerbeds a few inches away from the brick. So that you don't get pests like termites.
Never build flowerbeds or lay dirt against the exterior of a house.
Dirt is wet and water will get through the brick and into your sheathing, framing, insulation, and eventually inside the house and creates all kinds of problems. I had to rework a lot of brick and sheathing due to the previous owners doing this.
Don't use rocks.
Build them out of pressure treated lumber. They can be against your house, as long as the PT is there as a barrier. You don't want to build three walls and use your house as the third.
If they already have four walls, you can build an add-on box to put on top of your existing.