Birch trees planted next to blocks with fences on them stay as saplings forever and won't mature.
Seems like if it isn't going to mature in that spot it should prevent you planting it in the first place.
MC-32815
Birch trees planted next to blocks with fences on them don't mature
Resolved
Works As Intended
0
Birch trees require a 5x6 to 5x8 growth area and begins checking 1m above the sapling's base. A fence is considered 1.5m to prevent escapees, even though it only appears to occupy a 1m space.
While not a validation source, the wiki can help understand why things operate as they do.
http://minecraft.gamepedia.com/Birch_tree#Birch_trees