Generally it is not enough to add a question mark at the end of a sentence to make it a question.
The sentence proposed here is a question. So you need to use one of the possible interrogative constructions that French has:
FORMAL : quand les roses meurent-elles ? STANDARD : quand meurent les roses ? ORAL : les roses meurent quand ?
As the sentences, and the rules for constructing those sentences get more complex, it behooves us less and less to translate literally. When I read "Quand les roses meurent-elles ?" I thought "When the roses die?" as my "literal translation" but, knowing that French and English do not always use the same rules for sentence formation, smoothed out my mental translation to "When do the roses die?" Duo doesn't necessarily do this last step very well.
French uses articles a bit differently than English. Duo does not elaborate on this, which is a pity. But for example "J'aime les roses." Means that "I like roses [in general]." Not "I like the roses [that are present here]." So sometimes you need to omit the article in English. Unless I'm in error.
Die down is one possible way to refer to plants dying. It is a term used when referring to living plants. If you had a vase of of roses you wouldn't say that they were dying down, only that they were dying.
Using dying down in this example means you have reason to believe they are growing roses which are now at the stage of dying down back into the ground and not roses in a vase or whatever which are dying and therefore simply shriveling away.
There is nothing in this sentence that leads you to believe they are roses in the ground which is what dying down is limited to. All we know is that some roses of some type are dying. Therefore it is appropriate to refer to them as no more than simply dying.
Depends what you mean by dead. They are still carrying on some life functions. Initially, they still suck water up through their vascular system and distribute it to the parts that are important to us. By attaching capsules to the bottom of each stem, the flower will even continue to deliver nutrients.
When process stops as it inevitably does and evidence that it is no longer happening becomes visible we refer to them as dying.
As you can see we are now discussing the question raised by Duo, when are roses dying?
For learners of English, you must use "when do the roses die?" NOT "when are the roses dying". A native English speaker would never say the second one.
You absolutely must use the present simple here. The roses have a certain life span based on nature and science. It's a general truth that roses only last so long. That means we need to use the present simple. Please compare http://www.perfect-english-grammar.com/present-simple-use.html to http://www.perfect-english-grammar.com/present-continuous-use.html
'The roses, when do they die?' seems like a perfectly fine translation of this sentence - it is merely a different way to express as a sentence the same proposition. Why then was this translation marked incorrect? There are so many little things like this constantly plaguing my duolingo experience that I am considering leaving.
Not sure why -elles is required, but it does feel right - "feel" is something I think am getting from Duolingo, if only by repetition of segments 3 or 4 times in quick succession, without giving myself time to Think about it. BTW - really enjoying this course, but is anyone else bothered by the lack of emphasis on the end of words? The emphasis on the end so often gives a clear understanding of tense and meaning (or am I out of date on how the French speak French these days?)