I am not sure you are planning your print the right way.
As I understand your project will be printing black and green on the cardboard:
So you should separate your files, a plate for the black and a plate for the green.
But if you want white over green, you need a white plate that will look simply the same as the black plate, using white opaque ink. There is a chance that you can use a flat green color, without the shape of the pot. Probably not. It depends on the absorption of the cardboard and how opaque the inks are.
You do not need a transparent PNG, no. You need a black and white plate.
Keep in mind that cardboard absorbs ink like water on a desert, so make some tests first.
There is a chance you need to "darken" your white plate.
