According to the website of the Swedish Social Democratic Party, a rose has been used on and off by the party since the early 20th Century. The red colour is said to be a sign for the stuggle againt material poverty and the flower for the struggle against the spritual poverty. Elias Grandqvist, 7 August 2001
The poppy symbolizes (nowadays) soldiers dead in combat. It seems that until the early part of the 20th century the red flower (no matter the type) was a socialist symbol. Since most articles tend to agree it was only after WW2 that the (socialist) rose imposed itself, could it be that it was a conscious effort not to be confused with the "poppy-bearers" that made the socialists chose the red rose? Marc Pasquin, 28 November 2004
les membres du Gouvernement provisoire et toutes les autorités porteront la rosette rouge, laquelle sera placée aussi à la hampe du drapeau.
meaning: "The members of the provisonal government and all the autorities shall wear the red rosette which shall be place at the hoist of the flag." Marc Pasquin, 26 November 2004
The fact that "rosette" in French ethymologicaly means "little rose" might be a reason why it would have stayed as a militant worker symbol afterward. However, reading through the article, it appears that the exact type of flower being worn during worker demonstration in the second half of the century was less important than its colour. My final hypothesis then would be that the rose in particular being adopted might have been because (in addition to a confluence of the other items mentioned) that as a flower it is a well known one, widespread and it tend to invoke the colour red right away. Marc Pasquin, 26 November 2004
It is quite possible that nothing was meant or came out of the fact that instead of a circular piece of cloth they used rose-shaped one but I just I mentioned as this seem to be the earliest association rose + red = proletariat. ("Cockard" in French is "cocarde" I checked a French-English dictionary and "rosette" was the translation for the same word in French.) Marc Pasquin, 28 November 2004