Growing structures is not as easy as it would seem.[39] Quick growing willows have been used to grow building structures, they provide support or protection.[39] A young group of German architects are in the process of such a structure and they are continually monitored and checked.[39] Once the trees are of age to be able to take on load-bearing weight they are tested for stability and strength by a structural engineer.[39] Once this is approved the supporting framework is removed.[39] Projects are limited to the trees' weight loading ability and growth.[39] This is being studied and the load capacity will be proved by testing on prototypes.[2]