In the 1.8.9 versions of Forge, it seems that single spruce saplings cannot be grown with bonemeal.

 

I tested it in vanilla Minecraft 1.8.9 and the spruce tree grows as expected, but in Forge, tested version 11.15.0.1699 with no other mods installed, I can use over 20 stacks of bonemeal and the tree does not grow, even though there is plenty of room around it and above it.

 

The 2x2 spruce trees will grow easily, but a single sapling won't. Oak, birch, acacia and both single and 2x2 jungle trees grow as expected, as well as 2x2 dark oak trees. A single dark oak sapling will, of course, not grow.

 

So, in other words, a single spruce sapling behaves just like a single dark oak sapling, i.e. not growing at all.

 

Of course, I have made sure that 5x5x8 area around it is clear, as it is normally the case with spruce saplings and I tested it also on a superflat world, where there is indubitably plenty of room.

I think this patch for

WorldGenTaiga2

(the small spruce tree generator) is causing the issue.

 

The original line checks that the neighbouring block is not air and is not leaves. The replacement line checks that the neighbouring block is air and is not leaves.

Well, spruce trees generate fine in the world, both 1x1 and 2x2. I'm not sure if that code is or is not part of the world generator, or if gets called only when a spruce tree grows. I'm not really in position to comment on that.

 

Just to clarify, the bug happens only when a player plants a single spruce sapling - it never grows into a 1x1 spruce tree regardless of how much time passes or how much bonemeal is used.

 

 

EDIT: The issue has been solved in one of the recent updates.