A fence connecting to an adjacent block placed by a player or moved in by a piston causes the observer to trigger, a tree growing besides a fence does not, even though it connects.
- Place a sapling
- Place a fence next to it
- Place an observer facing the fence
- Place a redstone lamp or a piston behind the observer
- Bonemeal the sapling
→ When the tree grows, the observer will not trigger the lamp / the piston