Answer: If the set the blocks directly over the grassy area it should drain well so the only thing you need to do is measure the area where you want the planter to be. Take your measurments to a staff member of Home Depot and they can help you figure out how many blocks or key stone blocks you will need. If the planter isn't very high you won't need mortar but can simply stack the blocks up. If it is over 24" high, you may want to cement them together so they don't topple over. You'll need to fill it with topsoil prior to planting and if you want to get really creative, you can install a drip system prior to planting. Run your ideas through a staff member at Home Depot and they'll help you choose all the items you need for your project.
Best wishes with your project!
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