You didn't get any answers because the socialists and social democrats use a red carnation and not a red rose. It was first used on the foundation congress of the Second International in Paris in 1889. Red flags were banned so they have taken the flowers then abundant in public gardens.

source: an old issue of Vorwärts paper cited on the website of the German Social democrats.

Today this flowers is also associated with the international worker's day and the international woman's day since both were established by the Second International.