In Mass Effect 3, how can I give Ashley the roses I bought in the hospital after she wakes up?
Buying the right item will show you giving the gift to your injured squadmate when you visit. For Ashely it seems you want to get the Tennyson, and if you have Kaiden, get the Whiskey. This thread indicates that you will give them during the conversation if you purchase them.
No you won't break your relationship if you get her the 'wrong' gift, or no gift at all. You will have time (through conversations with her later), to continue your romance. (Just be nice to her the rest of the way.)
As for the 'correct' gift to purchase for her? It is the Tennyson poetry books. Remember in ME1 all of the poetry she quoted? Buying her those books shows her that you 'remembered'. All of the other gifts listed are just a waste of credits and will never be used, or shown being given to her. And you will give her the gift on your 2nd meeting with her in the hospital. (Once she awakens from her concussion)