The filter cannot do much when the light and angle is not there. Of course the CPL can also affect the appearance of vegetation. But it depends on the angle of the sun and the specifics of the leaves, grass or whatever you see there. So while a banana leaf might not appear impressive other trees can look different when using the CPL.
CPL filters light in a specific way. Which means: there must be light from the sun and there must be a reflection that affects the polarization of light. Using the filter you can select which light (polarization layer) you want.
Secondly, set your White Balance to 'Daylight' to avoid any corrections by Auto WB.
Hi, correct me if i'm wrong but you can just increase the vibrance or saturation in photoshop for making the trees and grass more green. A CPL main purpose is for reducing reflection or glare from shiny surfaces. Also, it had a filter factor which slow down your shutter speed.