When used to ask a question, (quand) is always followed by an inversion. If the subject is a noun (here: les roses) then you will use complex inversion. That is, you invert the verb (here meurent) with a dummy pronoun (here "elles" because "les roses" is feminine plural )

Simple inversion: il meure --> quand meure-t-il?

Complex inversion: les roses meurent --> quand les roses meurent-elles?

If you want to do things simply you can always use est-ce que and not bother with the inversion: quand est-ce que les roses meurent?

"Quand les roses meurent?" would be "When the roses die?" Adding the extra pronoun "-elles" makes the question about "les roses," in the general sense, hence "When do roses die?" Compare "Roses, when do they die?" vs. without a pronoun.

Thanks.

I put quand meurent les roses and it was accepted. I think the meurent-elles only occurs at the end of a question but not when you put the verb second.

Roses are feminine, la rose. And since that is where elles refers to, it should be feminine.

Well I know that talking about roses in general needs "les" which is why it's the primary translation here... Whereas saying "des roses" means "some number of roses" since roses are countable, just like "du pain" means "some amount of bread" since bread is uncountable. That explains "il aime le pain" - he likes bread in general. Someone else can probably give you a better explanation for why "des roses" is also acceptable here though.

You certainly can't say "rose", you have to say "roses". But apparently you have to also say "-elles" at the end (I'm not sure why). [EDIT: see Muzorewi1984's comment, which explains the complex inversion justifying the -elles.]