Published back during the unbridled excesses of the late 60’s or early 70's (definitely before 1972) there was a book I read in which some kind of climatic change caused sentient plants to overrun the Earth. It was quite a shocking find for a High School library of the time.
I believe it began in South America (there is an intense impression of green), and the only way mankind was able to reach an accommodation (and avoid extinction) was to join forms of sexual reproduction (putting it delicately). I’m sorry, but that is the most I remember; I think possibly I only skimmed it.
If someone has a suggestion where to look I would appreciate it: I do not want to unconsciously plagiarize original work.
It is not “Greener than you Think” ,“Day of the Tryfids”, “Midworld”, or “Vaster than Empires”.