I haven’t used the flat wall before, but have populated many 45 to 60 degree slops of mountains or hills by tilting the terrain or object.  Unfortunately, getting the item back to its original position, in exactly the same place, does make for a challenge.  You will want to write down the Location Coordinates of X,Y,Z, before you start.

 

I find it easier to make a copy of the object, move it away, populate it and then turn the base material transparent.  Then I can select the original material and the secondary material and align them to the correct X,Y,Z, coordinates to match it all up.  This will also work for multiple objects that must all be aligned to the first. 

You could also create a flat cube or other shaped object make it transparent, populate it and then align it with the Wall, much easier all around.

 

Another method is to create the Ivy separate and then align it to the wall.  I take one of the basic dead trees and turn it into a very long thin branching vine and then I can populate that item with leaves and add it to a wall. 

 

I am on my other computer now, as the other is rendering so cannot give you an example. 

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