In fact, you are able to use "greening" here. A google search for "greening" returns a great number of websites and articles about "greening" in exactly the sense you describe. There is nothing strange about it.
This google search also led me to another wonderful English compound word: "greenwashing" [0]. Undoubtedly, if this were instead a German compound ("Grünwaschen" oder so), we would fawn over the inventiveness of German and lament the inability of English to produce such compounds.
In fact, you are able to use "greening" here. A google search for "greening" returns a great number of websites and articles about "greening" in exactly the sense you describe. There is nothing strange about it.
This google search also led me to another wonderful English compound word: "greenwashing" [0]. Undoubtedly, if this were instead a German compound ("Grünwaschen" oder so), we would fawn over the inventiveness of German and lament the inability of English to produce such compounds.
[0] https://en.wikipedia.org/wiki/Greenwashing