Some of the ingredients would work as an infusion - but some, like the gravel root and Black-Walnut need to be decocted. If any need to be decocted - you have to do that or you won't get the properties out.
I make about 3 cups in a pot (lid on), use what I need and then add more the next day. 3 days is the max I do that before chucking and cleaning.