i normally try to avoid the clone tool only because of how precise you have to be to avoid wandering into areas you don't want to touch. this was a bit challenging with the vegetation sticking up but i was able to quick select the background pretty accurately (so as to prevent wandering into the foreground) then do the clone within the selection. it was easier than i thought..
If I understand you correctly adjusting the "Hardness" setting of your Clone tool might be helpful. If the Hardness setting is way low the feathering will allow your source-selection to "bleed" into the destination. Conversely, too much hardness will cause clearly defined selection lines that probably wouldn't look good on the vegetation. Maybe try a Hardness setting of, say, 10% - 15% for the vegetation and 50% for top of the wall if there's any bleed over from Cloning the vegetation. I'm pulling those exact numbers out of the air, but you get the point: the vegetation is "busy" enough you can get away with a softer setting, the wall will probably require something much harder to keep the lines looking clean and crisp. Like most things in Photoshop, it's a balancing act and while there are surely numerous ways to attack problems like this, using the Clone Tool seemed like the logical way to go to me.
