On a few of my vintage bespoke suits the tailor made a boutonnière flower stem-loop - a small braided cord on the back of the lapel a few inches below the buttonhole to keep the flower stem in place.
As I occasionally wear a boutonnière flower also to work (e.g. if we have a bouquet of flowers in a vase at home) I make sure that all my jackets with a lapel buttonhole have a boutonnière loop, too; so I sew three pieces of thread into the back of the lapel, plait them into a cord and attach the end of the cord. Here on my 1960s bespoke suit:
Bertram Wilberforce Wooster strolling down Dover Street, London, on the way to his club.As always wearing a fresh boutonnière flower