A Green Wall

, a mission given by Dawa, begins when you receive a letter from him in the mail:

Visit the Worktable to view the recipe for an Insulated Planter Box. Collect enough ingredients to make 6 of them. Once you have built all of them, wait for word from Dawa. He will bring some Poplar Trees, which you can combine with the planter boxes to make Sand Blockers.

Bring the Sand Blockers to the area just outside of Siwa's house, which is on the other side of the Portia Bridge. Interact with the mission area there to place them and complete the mission.