We wonder how many florists have fallen foul of this requirement when trying to put a Royal Wedding together? Whereas a bride will usually have control over which flowers go in the bouquet to be thrown at the end of the wedding ceremony, tradition makes this choice for royalty. You can garland and surround it with whatever you wish, but there better be myrtle right at the core!
There's nothing special about myrtle in particular, but every Royal bride since Victoria herself has observed the tradition, and the myrtle all comes from the same place; Victoria's own flower garden at the palace, which is now 170 years old. She planted the myrtle herself, and it still blooms without fail year after year. The plant symbolizes love and marriage, and so couldn't be any more appropriate.