This would be a case of a classic mess, if it was a classic. Instead, it's just a mess, one of those movies where the story of its making is far more interesting than what ends up on the screen.
It's based on a popular book by John Wyndham (who also wrote the book on which Village of the Damned is based ... that movie is far better than this one). I have memories of reading this as a kid, and then seeing the movie on TV, back when I'd watch anything Creature Featurish. The plot features monster trees that eat people (the Venus flytrap is trotted out as scientific evidence such a thing is possible), and it turns out it wasn't easy making monster trees very scary. They move slowly, and, well, they're trees. The basic concept, of a world on the verge of apocalypse, can be intriguing, as we have seen from dozens of movies over the years. This particular execution of the concept, though, is anything but intriguing. It looks cheap (especially on the washed-out print I saw), and there's not much effort to crank out a low-budget classic ... nope, it's just cheap. While I don't really remember the book, the movie is apparently a dumber, more monster-oriented version of the original story, which doesn't help. Howard Keel is the lead ... Keel made his name in 1950s musicals like Show Boat, Calamity Jane, Kiss Me Kate, and Seven Brides for Seven Brothers. He doesn't get to sing here. In fairness, he isn't bad in Triffids, it's just hard to care. There's no one else of note in the cast, although my wife thought the female lead, French actress Nicole Maurey, looked familiar (a perusal of her filmography gave no indication of why this would be, unless my wife spent a lot of time in her childhood watching Secret of the Incas).
Whatever budget existed apparently ran out before they had finished the film, although "finished" may be an exaggeration ... when they were done, they had a movie that was less than an hour long. So back they went, with a different director (Francis), two new actors (Janette Scott and Keiron Moore, listed in the credits as appearing "By Special Arrangement", whatever that means), and a new subplot taking place in a lighthouse that, other than being a place where the triffids are attacking, is completely unconnected to the rest of the movie. The result was a movie that lasted 93 minutes, which was long enough to be released.
I admit I found unfortunate humor in a few scenes where people, blinded by the arrival of the triffids (don't ask), wander around tripping over things. Mostly I just kept wishing I was watching Quatermass and the Pit (aka Five Million Years to Earth) ... there is no connection beyond being English sci-fi from the 1960s, but I love the Quatermass film ... I often wish I was watching it.
There are stories about a possible restoration of the movie, which would make it look better but which would still be stupid. There was a BBC TV adaptation in the early 80s that is supposed to be better, and another TV version a few years ago, neither of which I've seen. One thing I can say with authority is that the 1963 film version isn't worth your time.