Hello,

I’ve picked up a willow tree branch ripped by a storm.
Since they root fast, I’ve gave it a shot to wire it in an “S” curve.
In time, the branch darkened: first at the top, later in multiple places.

Did I bend it too much and killed it? Or was the branch sick?

Did you cut it at the top? I have 4, one that is 5+ years old and weeping. Only times I have had issues was related to over or under watering. In nature their roots spread far and shallow looking for water, so if you had it in a small pot and it dried out for an extended period of a few days, that could have done it.