Title.

Begonia is obviously an oversight since it's a double-tall flower just like the rest in the mod.
Helvola flowers are less obvious, but given that Lotus Flowers are required, it only makes sense that Helvola Flowers also be required.

Incidentally, the following vanilla #minecraft:flowers are also not required:
Torchflower
Wither Rose
Pink Petals
Spore Blossom
Pitcher Plant
Chorus Flower

Most of these make sense not to be included - the ancient flowers, Chorus Flowers, Wither Roses. All annoying to get. I think perhaps Pink Petals and Spore Blossoms should be required, however. Those two feel like an oversight, as they're fairly easy to get.