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Yes, dahlias bloom in August

Yes, dahlias bloom in August and it's a perfect flower for special events.

You can contact a cut your own farm like this one in Portland, OR.

http://www.krugersfarmmarket.com/flowers.html

or check out other gardens that sell flowers in bulk for special occations.

http://www.lemeragardens.com/ is one example.

You can also ask your friends and family to plant dahlia tubers in the spring and then cut the blooms for your wedding in August.

Good luck!