A few years ago I was part of a production of Misalliance, which takes place in a conservatory, and we used trees borrowed from our campus's greenhouse. The big issue with the live trees was their care. After each performace they had to be heavily watered (as the hot stage lights were drying them up) and dragged out into the lobby so they could get sunlight.
We had our share of spilled dirt, spilled mud, broken pots, drooping and dropping leaves, but really, the effect or live plants on stage ended up making it worth it.
Good luck finding someplace to get your trees for free/cheap, if you do decide to go that route! Perhaps a trade for advertising space in the program might offer an incentive to local businesses?