So if you take Rare Breed "A" to make seeds with a flower that's a regular breed, with the stem from Rare Breed "B", it's possible that the seed will give you "A" with the stem of "B". That's the only way to mix rares! So if I can grow a Bite Weed with the stem of a Prickly Pear (a single huge bloom, pointing straight up), it'll resemble a Piranha Plant from Super Mario.