What Do I Put at the Bottom of a Raised Bed Before I Add Soil?
Trellises surround a raised bed garden
You need not place anything in the bottom of raised beds. In fact, mixing the soil that you add to the bed with the existing soil will improve rooting of the plants that grow in the bed.
An exception might be if you have severe gopher problems. In that casem you might want to place hardware cloth at the bottom of the bed to prevent them from tunneling into the bed.