Dahlia,
Is this just for a guest attending the wedding, or for the groom?
As a guest, I think a nice linen suit would be gorgeous, and if he is on your arm, you could wear a very chic colorful sundress. Just try not to show up the bride and groom.
As the groom, if it is an American Style wedding and there are no traditional garments to be worn, I have been to some gorgeous summer weddings where the groom has worn a lite grey suit, of course the bride in a white gown (but summery.) what really popped was the flowers. the groom and all the groomsmen had beautiful bouteneirs made of "dahlias" (kind of fitting for you.
)
Hope this helps.