Kieron Moore and Janette Scott were only added to the cast when it was discovered upon completion of filming that there was only 57 minutes of good usable footage available. The whole lighthouse sequence, directed by veteran Cinematographer Freddie Francis, was only added to help extend the movie's running time - even though these scenes contain the movie's surprise- twist denouement. Presumably this was a last-minute script change. Freddie Francis, when asked about his uncredited contribution to the film, implied strongly that the whole production had been chaos.
Although delighted to be reunited with Howard Keel (whom she'd met as a child), Carole Ann Ford found the film difficult to work on, citing last-minute script revisions and changes of director as particular problems. She also suffered when an action sequence left her back accidentally raked by a stuntman's nails, drawing blood.
The book ends with the Triffids still a threat and Masen leaving for the Isle of Wight, hoping to eradicate them from the face of the Earth someday.
In an interview, producer Bernard Glasser said that in order to satisfy a British government requirement that productions shot in Britain have a certain percentage of British executives, George Pitcher, a retired film executive, was hired as basically a "frontman"; although credited as producer, he had no actual authority over the production.
Although credited with "additional music", composer Johnny Douglas actually supplied more music than the main composer, Ron Goodwin. According to the official music cue sheets, Douglas wrote 26 minutes, Goodwin 19.