The couple will tie the knot at the 'Hunger Games: Mockingjay' actor's mother Diana Ross' estate in Greenwich, Connecticut, on the evening of August 31, with white lanterns lining the walk way to the alter and hanging from overhead tree branches.
A source also told E! News that Ashlee - who has a five-year-old son, Bronx, from her marriage to ex-husband Pete Wentz - is opting for mismatched dresses for her bridesmaids.
Evan, 25, proposed to Ashlee in January after six months of dating but he previously admitted he wanted to marry the 29-year-old star almost as soon as they met.
He said: ''I was giving it time only because I wanted to give it time, but I would have asked her a month after I was with her.''
And the 'Wilderness of James' actor couldn't be happier.
He gushed: ''We just want to be married. We're in love. This is the most incredible time I could ever think of in my life and I'm young, but the same time, I've lived a different life than most and I can't think of a happier moment.''
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