Disney world does this! Lowes sells the 4 to 6″ round PVC pipe with holes already drilled. Purchase an end cap, fill with rock, soil, and plant. I will often put these in the center of a very large pot to stabilize, and add amazing height and color to a container that has trailing plants (no end cap or rock needed if you are placing in a container) @ DIY Home Ideas
Put newspaper over the dirt 3-4 pages thick & then cover it with mulch. The newspaper will prevent any grass & weed seeds from germinating, but unlike fabric, it will decompose after about 18 months. By that time, any grass & weed seeds that had been present in the soil will be dead. It’s green, it’s cheaper than fabric, & when you decide to remove or redesign the bed later on, you will not have the headache you would with fabric.