It’s been through quite a few iterations 
I started off with ‘random point in bounding box’. If you make a thin collision box and put it on one side, this works pretty well.
In the end, because of the ivy mesh, and random Z rotations, having them in a grid shape actually looked better Although I’d recommend start of with random bounding box, especially if Z rotation doesn’t move the mesh much.
I’ll put the BPs in a ‘snippet’. Gimme 5…