Enchantment by Camille Peters

When Maren runs away from the threat of a forced marriage, the last place she expects to end up is the Malvagarian Palace, home to the enchanted gardens, a cursed prince, and a magical rose that traps her there. Crown Prince Briar isn’t pleased to be stuck with a troublesome guest, especially one as mischievous and curious as Maren. She, on the other hand, is determined to escape, but instead finds herself inconveniently falling in love with him. Despite her lack of beauty, feelings steadily blossom between her and the prince.
Their budding romance is soon threatened when sinister magic begins to eclipse the enchanted gardens, a darkness which quickly spreads not only to the kingdom, but to the king himself. In order to stop it, Briar and Maren will both be forced to make the utmost sacrifice, only to realize that the gardens’ requirements may prove too high a price.