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 mins, Goodwin 19 minutes
In an interview, producer Bernard Glasser said that in order to satisfy a British government requirement that productions shot in British 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.
The triffids inspired the E.T. plants of E.T.: The Extra-Terrestrial
When the film was finished it was too short, so the entire sequence where the triffids attack the lighthouse was added. This sequence was directed by an uncredited Freddie Francis