Running a search to see if I could find the answer to your question brought up this from a bonsai thread stating shelf lives of powdered hormones is "short". For another brand, it suggests 12 - 18 months. It would probably depend not only on the particular active ingredient, but how it was stored. Good luck. KIm

Here is a link that might be useful: Scroll down to comment about shelf life

Many thanks. Your a dedicated researcher.