I am directing a show  Snappy's Happy Half-Hour by Don Zolidis- we open in Feb.- At the end of the play, 2 characters virtually destroy the set (trees) in a staged fight.  The set is supposed to look poorly made- and  I have already collected lots of styrofoam and cardboard. Now the ??- How do I make the trees stand up by themselves 1 character picks up a tree and swings it like a bat! (fun show) 

If the tree were made of cardboard or plywood (something in a flate plane), the base can be cut with a center notch.  Similarly a second plywood/cardboard piece can be made to fit perpendicular to the first piece--this piece also has a notch cut.  The two notches fit together to make a base that fits in a cross configuration (when seen from above forms a cross).  Most of the time when swung, the second piece will fall off with a clatter saving you making it again, and also looking cheap.