The bug
All types of saplings can be placed on dirt, grass, and farmland. But not all of them can grow to trees on farmland.
Affected trees
Last updated for 1.11.2
2 × 2 spruce and jungle
dark oak
acacia
Code analysis
Based on 1.11.2 decompiled using MCP 9.35 rc1
The problem is that the blocks a tree can grow on are hardcoded and not retrieved from the sapling.
Comments 7
Issac, [yes|MC-4482].
Confirmed for 15w46a & 1.8.8
An update to the description would be that jungle saplings can now grow on farmland. It's fixed, but it affects dark oak saplings instead.
The fact that it only checks for the northwerstern-most sapling is now MC-112975
In 1.14.4, all 1x1 sapling except acacia can grow on farmland. I guess it has some connections with the shape of acacia.
In 1.8.3, single Jungle saplings will now grow if planted on farmland. Acacia still will not, nor will 2x2 trees if the northwest sapling is on farmland.