Well, the nations are permanently seasons because of ancient artifacts of sorts, so there's all sorts of magic stuff going on. As such, the plants would just kinda grow on their own, no planting needed. If the artifacts are removed (which is the bbeg's goal), things would be normal, and planting would be necessary.

As for a calendar... I suppose that the people from before the artifacts could have left a calendar. Which could work into some belief that each nation is favored by the gods at certain times of the year, since most people wouldn't know that the seasons used to change.