We did but it's still twigging me out since I haven't been able to root out the movie/show/book.
Hrm, you think maybe you're merging memories of different movies/books together? From your description it sounds a bit like Day of the Triffids, as mentioned previously, but, aside from the plant aspect, it also sounds a bit like Invaders from Mars, and It Came From Outer Space, and... but I can't see to find anything that has the people being turned into plants...mindless zombies, food sources, pod peoples, but not plants! Hrm, you've stumped us, Doomsie! Here, have a toaster.
You said this was a book? Novel, story book, short story, comic book? Maybe it was an episode of The Twilight Zone or The Outer Limits?