All the thickets appear/grow randomly on your side. These are grass, branches, stones, wild flowers, bones. According to the real season you can see also the other seasonal items that appear onsite during quests, for example, mushrooms, puddles etc.
I do not have any thickets onsite. Why?
1. The thickets need plenty of space (very important!) to grow. Make sure that you have enough free place so they can grow.
2. Thicket does NOT grow next to man-built and other decoration items in the game.