Welcom to the forum, Wukon.
Are you sure that "Roses always work" is an idiom? Because I did not find something about this phrase. There are several idioms about rose, you can check them there The Free Dictionary - Rose. I do not think that is an idiom, because I have never heard it.
I think the teachers would be glad to see a full context where you have seen it.
I am not a native speaker.
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