Q.building a flower bed
after digging out the sod, do yo treat the soil with anthing before building tbe dirt mound? put plastic down first?
Mixing in compost or other amendments is very advantageous, depending on the soil texture, pH, and nutrient levels you start with:
Using plastic at the bottom of a raised bed is an option, but not required. It can be advantageous if there are roots underneath that are likely to grow into the raised bed, or if you're building above treated lumber or another source of contamination. But it will make the bed shallower, and the plants' roots won't have as much space to explore. You'll need to make sure your bed is a minimum of 6 inches (but preferably a lot more) above the plastic.