Maybe the foliage just slowly blends into a different sprite over seasons, such as yellower and shorter as it gets colder, like the northern grass sprites appear further south for a while. If they programed the plants to behave to certain weathers, then all that is left is changing the appearance to a sprite that already exists. Maybe while a player "waits" or enters an indoor area they change. It's possible, but who knows. Erosion will definately not take place during the game.
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