A sword tree shows up as a gag in an early The Order of the Stick comic. Specifically, Roy's sword was destroyed and he's stuck without a replacement because they sold the other magic swords they found in the dungeon. While in the forest, Roy is forced to use a non-magical greatclub due to lack of alternatives, complaining that "swords don't grow on trees." Naturally, the last panel of the page shows the group narrowly missing an encounter with "the world famous sword-growing tree."
I can't find such a strip in the archive. On the other hand, the greatclub seems to just appear somewhere between Belkar complaining Roy doesn't have a weapon
and the encounter with the bandits
, so maybe there's a strip missing?