SketchUp file: ivy.skp
See how the leaf component's axes are set up.
Random distribution with Wrap (UV) mode
Use “Pointing Direction” all the way to “Normal”.
Check “Swap object’s Y and Z axes”
Use random X rotation between -30 and 0 (or other values) to control how much the leaves will lean to the ground. You can also put some random Y and Z rotation for more diversity.
More discussion and examples on the forums.