I read a novella many years ago that I think was written in the '50s or '60s. The story centers around a couple of teenagers who live in a small desert town. They notice that adults in their town are getting very lethargic, and one by one they turn into plants (cacti I think?). Turns out that there was an alien invasion of sorts, and anyone who did any harm to the environment got turned into a plant. The kids were safe because they didn't drive. I think it had a kind of cheesy title, but that is all that I remember.
It may be The Plant People , by Dale Carson. First published in 1977.
I don't see a simple synopsis, but according to the Retroist link above it does have people turning into cacti.