An employee here at the assisted living facility said that the hormone is present throughout the willow, but younger and fresh shoots are the best and most potent.
You can boil it and make the tea, but for an even stronger concoction, you can cut up some willow, put it in a container that is air tight, fill the container with warm water until the willow is submerged, seal the container shut, and leave it shut for two weeks (I think it was two weeks, at least - I'll ask her again and make sure).
The water has to be warm when you pour it in to start the process, but not necessarily boiling. I've filled up two old jam glasses today - I'll make sure to report as I go.