The mod pacakages I'm using allow for planting oaks. I have used a partial oak wall, two layers does indeed help with the larger trolls (stars make the creatures larger). So if i get a raid with a 8 star troll in my meadows/BF city (i straddle a biome here) those large suckers cant get through my oaks. Trick on it is to keep the ring far enough back they donbt start crazy swininging or have the troll tossers knock your stuff out from range.

Vanilla game...no option, you cant plant an oak (sad face).