How are weeping willows harmed in their environment?

There's quite a few. You can put the Weeping Willow into shock if the ground isn't wet enough while planting. They also obtain black spots around their "body". If your Weeping Willow is still small just cut the black spots off. Leaving your Weeping Willow exposed to such diseases will start the Death Clock. I guess it depends on where they are, if they get put into shock and if the ground has livable soil.