Make it so, that fresh pipeweed leaves being left in stockpile will become dried or moldy with 50% chance over period of time.
Let's say, every 12 hours 10% of stockpile will make a decision to become moldy or dried.
So, if you want you can get full amount of dried leaves using herbalist tables. Also you can store fresh leaves in containers, if you need. But if you dont care about quality and drying amount - you will be able to use stockpiles.