What happens if you use green wood?

It will rot and collapse, then you will have to replace fairly quickly.

Not at all. what are you considering "green wood"? wolmanized treated boards are some what green in color from the chemicals, they are recorded to sustain substantial life in wet conditions, as well as dry apps. green wood is also referred to as fresh cut, with little to no drying or cure time. if you use green wood(wet) as it dries it possibly results in warping, shrinking depending on the application in which it is used.