I think that's a great idea and with the real prom decoration you can save so much money. I would try to talk to some local flower shops if they could make some black roses or maybe they got some decayed flowers you could have for free.

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Deep into the darkness peering, long I stood there, wondering, fearing, Doubting, dreaming dreams no mortal ever dared to dream before.
Edgar Allan Poe, The Raven