Here is a different approach to improving a single color… make it more colors. I do this with RawTherapee by stretching the L^*a^*b^* Hue-by-Hue curve around the greens at a good slope.
So instead of just lighter and darker greens, you also have the yellower and bluer greens. Green is sampled much more densly than red and blue by the camera sensor, so there is plenty of extra green luminous dynamic range to utilize.
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Absolutely true. I used to use CPL religiously, but recently I don’t even have one for my current favorite lens.
Beautiful production. In darktable I think this guess this would be done with the color zones module.
I don’t know scientific explanation why CPL filters can give better starting point. Probably something to do with controlling light reflections from the green leafs.
Here is quick demo I took this morning. It was dull and pretty soft light, so maybe not best example. Tried to edit it with darktable for muted green tones, but I need more practice with that 
Left is taken with Circular Polarizer Filter and right is without it. Edited only CPL version and then copied history to other one.
A polarizer more or less suppresses specular reflections of non-metallic surfaces. So with shiny leaves, you can remove part of the white reflections. As visible in the images, the shadow side of the leaves hardly changes. So no idea why a CPL would mute the greens…
Same lens but different focal length and CPL use. 1st is with CPL and the 2nd is without.
(but now that I am thinking about this, I actually doubt myself because the 12mm and 24mm on that lens needs a different filter, and the 24mm one is much easier to install) I actually don’t remember what I did.
