Flashing EPDM roofing up a short arched vertical portion?
I would run your EDPM sheet right up the back side of that arch and onto the top surface using the same adhesive you use to hold it down to the flat surface. I'd pull the membrane about 1/2 way across the horizontal top of the arch. Then I'd use the same flashing you're going to use for the sides to flash over the vertical edge of the arch.
Note: I'm not a roofer, this is just me &quot;thinking logically&quot;, so wait for some confirmation before doing this.
Also, I'd suggest that while it might look nice, the wooden planter isn't doing your roof leaks any good, and it will most definitely not be good for the rubber membrane you're about to lay down.