The filmmakers wanted to donate something to the monks who acted in the film since they refused payment. According to Rossellini's daughter, he expected them to ask that the donation be something charitable, like setting up a soup kitchen. Instead, the monks surprised everyone by asking for fireworks. Rossellini saw to it that the town had an enormous, elaborate fireworks display that was the talk of the region for years.
The roles of St. Francis and his disciples were played by monks from the Nocere Inferiore monastery.
Roberto Rossellini and 'Ingrid Bergman' were involved in an highly-publicized, strongly-condemned love affair during the making of this film and Bergman was pregnant by Rosselini while being married to another man. These circumstances in Rosselini's personal life made the shoot more difficult.
Roberto Rossellini was one of a group of pioneering filmmakers of the neo-realist era. Rather than shooting on stage, they shot on locations and quite often used non-professional actors. Most of the performers in the film, with the exception of Aldo Fabrizi, are real Franciscan monks.
"The Flowers of St. Francis" was edited for its 1952 US release, with Rossellini's chapter-like inter-titles and a couple of scenes that had been removed from the original Italian version. They have been reintroduced in the Criterion and in the Masters of Cinema edition.