Japanese Knotweed powder (and other herbs) - do you refrigerate?

My herbalist told me to do that once I open them. They can be stored in the freezer or fridge. This mainly applies if you buy in bulk 1 lb lots as it will take months before you use it all.

Alright, just an update. I emailed 1stchineseherbs about this and here is their response:

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Hello
The Japanese Knotweed should be stored in a cool dry place, but not in the
refrigerator, as it can get condensation.
Some where in a cupboard, not near the stove or dishwasher

I usually put mine into a large mason jar so it is air tight.
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So I guess an airtight jar is all that is needed.