Tucked away in a forgotten box I've an tiny old textbook from graduate school days decades ago that says the best willow for propagation purposes are found above 8000 feet. Most of the world does not reach that altitude and many use their local willow and claim success. (I once shared this tidbit and someone scolded me. They used willow from the sea coast and it worked therefore my book was wrong. Forums do that sort of thing to people.)

I would not spend money on cuttings from the grocer but that is just me.

I believe that if you consult the literature it will disclose that the active component in all Salix(Willow) tissue is Indoleacetic acid (IAA). The concentration is extremely low and thus successful rooting is very chancy.A common rooting substance which is very easy to find is Rootone. It is Naphthaleneacetic acid which has perhaps 1/4 the activity of the Indoleacetic acid. Most propagators use Indolebutanoic acid (IBA) which is relatively inexpensive and as effective as IAA.It is quite simple to locate a source online. Willow tea generally is simply a gross waste of time.
Best of Luck!!!

Thanks taxonomist for taking the risk on this one. If one goes googling for ~ rooting "willow tea" site:.edu ~ one will only see about 14 ghits and three or four of these have folklore in the title.

I've used it, but whether all the cuttings rooted due to the willow tea or just my good luck is probably debatable:)
My grandmother was leaving the house she'd lived in for 60 years and I wanted cuttings of all of her plants and I didn't have rooting compound. I easily took 30 to 40 cuttings and they all rooted except the shrimp plant. Try it, if it doesn't work, the only thing you've lost is a bit of time. I have no idea of the variety of willow, it was just from the local herb store. I steeped the willow until it looked like a nice rich cup of black tea.
Kathleen

Kathleen..what did you do after you make the willow tea. Can you be more specific how it works as a hormone agent? Is there something special beside pouring the boiling water over the willow stems/sticks and letting it steep for a certain amount of time? Do you slit the stems any particular way?

I collected a few dozen links at GW's 'willow water' links. - Tips & Techniques Forum - GardenWeb.