From Dust

This goes hand in hand with Memories found. By completing each map to 100% vegetation, you are unlocking Memories of the Tribe. Anyway, in order to reach 100% vegetation coverage on a map, you will have to spread earth around the map. Vegetation growth starts around the villages and expands outward from there. As I said under Hell, no earth, no vegetation. Absorb as much earth as you can with and place it around your village to allow for the expansion of the plants. As long as there is earth and water nearby, the plants and trees will expand automatically.

On the bottom left of the screen is what I’ll call the growth-meter. It charts your current level of coverage by vegetation on the current map. You will receive a notification when you reach 100% coverage.

From the Resume Story menu where you can level select, any level that does not have a little leaf icon next to it does not have 100% coverage. You can go back to play it after you have completed it and continue on from where you left off in order to reach full coverage. If for some reason your progress does not save this way, you can restart the level without affecting your progress through the rest of the game.