How would an oil of plant growth work in regards to application.
The spell can effect a wide area, but the oil would be limited by the time it takes for application. I would think it would take at least a full-round action to apply an oil to 10 square feet, and I don't think one oil could be applied to much more than that.
What do you think?
I'd mix it with water and simply sprinkle that over the area.
You can only make an oil out of a spell that targets one or more creatures or objects. The plant growth spell targets an area, so you can't make an oil out of it.