Willow tissue contains a low level of 3-Indoleacetic acid, a rooting hormone. Aspirin is the acetylated form of salicyclic acid which has little or no rooting hormone activity. The readily available commericial preparation, Root-tone, works very nicely for almost all rooting techniques.Almost all cuttings root better if taken in the early AM.Good Luck!

I just recently read that you can dissolve 4 aspirin in 1 gallon of water to use as a willow water substitue in rooting cuttings. Haven't tried it yet myself but I intend to.

It has been 50 years since I studied this stuff but I am not sure that substitution of an acetylated form of salicyclic acid aka aspirin for the salicyclic acid in willow would work nor am I sure that it will not work.

I do know that when I did controlled experiments in a greenhouse that in my own research I never found any benifit to using aspirin.

BTW, FWIW, all those things, I've a textbook boxed away here somewhere that says the best willow are found above 8000 feet. This will not help most users here but just in case you have a cabin up there somewhere I would suggest you try it and report back.

I thought the benefit of aspirin was that it's acidity killed bacteria.

What are you trying to propagate? The necessity of hormones is exaggerated in most cases.