Originally posted by Ren:

Placing workbenches will prevent trolls from spawning in the vicinity.

I have an enclosed worbench on all 4 sides of my enclosed tree farm. Also one on my farm and the surrounding buildings. They've still spawned right inside within the workbench area. I think the workbench thing only works with blocking randomly spawned mobs. Since this is an event, I assume the spawn rules are different.

I've done the moat trick in the past and it does work, but I've just not wanted to put the time in since my main base is spread out across multiple buildings. I guess that's still my best bet if I want to keep my crops and tree saplings.