There are lots of solutions if scarcity of resources is not a problem. Building massive temporary greenhouses is likely limited by the economics: importing any

There are lots of solutions if scarcity of resources is not a problem. Building massive temporary greenhouses is likely limited by the economics: importing any materials, constructing them to be able to survive strong winds, irrigation, limiting life cycle CO2 emissions, and other issues unique to greenhouses. Since they grow saplings in greenhouses and take those out into the world, greenhouse space is likely economically constrained. It seems optimal to start a bunch of trees, and send them on their way, and keep the process going. They currently have a ban on imports of live trees, and lack restrictions on sheep grazing (learned per other comments on this post). These things alone might be less costly than constructing lots of temporary structures.