Let's suppose you have decided to have a character tell a tale in your story. And you decide to underscore one of your motifs. This can be adroitly handled because your character is, in fact, keenly aware of the motif.
A girl is thrilled to pieces to be going to her first white ball -- a dance with only girls, preparatory to her future dances in full society. While waiting, she tells a story. Cinderella, Catskin, Cap O'Rushes, or any of the other ones where the heroine goes to three balls. She will, of course, avoid the ones where the woman goes to church three times instead.
At that, she might tell one of the Sweetheart Roland ones where the heroine has to win back the hero, enchanted to forget her, by going to dance at his wedding with the false heroine. She has fewer choices then, since it's a less common variant in that tale.
A more sour girl might tell of the twelve dancing princesses, and how she hopes that the soldier, or the gardener's boy, or even the prince, rescues her from the necessity to dance.
In another story -- perhaps a tale of a family of rose breeders -- the oldest son wants to tell his younger brothers and sisters, and their cousins, a fairy tale. About roses.
There are no tales in which roses are as central a motif as dancing is. This can easily be shown by the way variants casually substitute flowers.
Beauty and the Beast may feature a rose, but in the original tale, the rose drops from the tale as soon as it has been the occasion of the offense. There are variants of it that have nothing more specific than a little red flower. As for the variants that far as far as East of the Sun, West of the Moon, they can go as far as a radish being the plant that causes offense -- that is, among the tales where a plant is the problem. There's no guarantee of that, either!
The Brothers Grimm may have named their sleeping beauty Little Briar-Rose, but the plants that grew over her are noted for thorns not flowers. Likewise Perrault's Sleeping Beauty is surrounded by brambles, which could be blackberries for all the evidence in the tale. (Not that you would know that from the illustrations, to be sure.)
Now, Rose-Red and Snow-White features two rose trees outside the two girls' mother's home, but that's a rather literary-derived tale, and the rose trees don't feature much in the tale. (And it didn't return to the folklore vein much; there are very few variants.)
On the other hand, its very lack of central importance can be put to use. Just as roses can be turned into other flowers in the course of variation, so too can other flowers be turned into the rose.
Perhaps the boy knows the Feather of Finist the Falcon. In the original tale, the heroine asked for a little red flower -- or else for a feather, which is actually more typical for this type of Prince-As-Bird tale. But if the boy likes it, he can make it a rose.
Or perhaps he wants the boy to be the hero and not the love interest rescued from his injuries by the heroine. The Twelve Dancing Princesses, besides having any number of princesses from one to twelve, can also have all sorts of heroes. Soldiers. Princes. Or gardener's boys, one of whose duties is gathering flowers and delivering bouquets to the princesses. If the tale does not specify roses, why, our teller of the tale can make them roses. If his family likes to talk up the language of the flowers, he can spell out what the bouquets contain so the younger children can deduce the message each one sends.
If the boy wants more adventure, there's always Iron Hans, also featuring a gardener's boy, though a prince turned gardener's boy. And if it tells of the princess and the hero of the tale
She saw the boy and called out to him, "Boy, bring me a bouquet of flowers."
He quickly put on his cap, picked some wildflowers, and tied them together.
well, there's still no reason why he could not pick roses instead. And even roses with a message in a flower language.
Wide reading can turn up the motif your story needs -- parsley? you can use Petrosinella -- but also tales that can be fitted to your needs, once you fathom their deep structure and know that something is filigree, not structural support.