Hello, Wukon.:-D
Welcome to UsingEnglish.com.
I agree with Nickle that we'll need context to know what "roses always work" means.
I don't think it's an idiom, either. I think it's just a sentence.
In your sentence, 'to work' means "to have the intended effect or result":
Merriam-Webster's Learner's Dictionary
So, I think it basically means "Roses are effective on every occasion(, for example, Mother's Day or your girlfriend's/wife's birthday etc)".
I hope I'm not talking nonsense.