Symbolism, Imagery, Allegory
The Royal Blossoms
The twelve Eathesburian princesses are a lot like—you guessed it—flowers. They're beautiful (like flowers), and they are also considered a sight worth seeing (like flowers)—oh, and they all are named after flowers (with a few plants thrown into the mix) too. That last part pretty much seals the deal that flowers are symbols for the princesses.
When Lord Teddie visits them to solve the riddle, he reads to the girls from a book called The Eathesburian Holiday Guidebook. When he says, "It says if you're lucky, you might even see the rare flowers of Eathesbury!" (20.80), Hollyhock replies with a giggle, "That's us […] We're the flowers of Eathesbury!" (20.82). So you don't have to take our word for it—let Hollyhock herself convince you that flowers represent the twelve sisters.
Just getting a glimpse of how much the girls thrive in the gardens when they're finally allowed to go outside is also telling. Azalea has fun running "through the gardens, her black skirts billowing in the breeze of honeysuckle and lilac. She had forgotten how fresh and alive the gardens felt, with bright flowers bursting all over it like fireworks" (15.36). The royal gardens are spectacular, and so are the royal princesses, and just like flowers bloom there, so too do the girls. The princesses and the gardens go hand in hand.
Sunny Delight
But what do flowers need? Sunshine, among other things—which the girls don't get a lot of in mourning.
When mourning begins, the girls all have to wear black, and after Azalea finishes explaining the rules of mourning (which takes a while, since there's a lot of them), "the girls all looked like miserable, drooping black blossoms" (4.21). Uh-oh. By this point mourning hasn't been in effect for very long, but the girls already look like drooping flowers… not a good sign for how they'll hold up going forward.
Another things worth mentioning is that flowers aren't supposed to be black. At least, not most flowers, most of the time. This description thus hammers home the point that mourning is not the natural state for young, exuberant girls. Heck, we'll go so far as to say that flowers turn black when they die—so mourning is almost a deathlike state for the girls.
We get another glimpse of the girls being flower-like in their sadness. Right after Keeper shoots the king, "The girls knelt around the King, their skirts spread out like forlorn blossoms, swallowing, and whispering one word" (28.131). Again the girls are compared to flowers, though now they're sad flowers—the prospect of losing their father and his love makes them wilt like flowers cut from their roots, which is fitting since the King is their only remaining parent (or root). By the end of the book though, with mourning lifted and love in the air, we're thinking they'll all perk back up.