If you want to prepare for Yoimiya’s ascension materials, then you will need a ton of Naku Weeds (168 to be precise). Here is my farming routes you can follow to get 49 Naku Weeds every 48 hours (from pickup).
You can also buy 5 Naku Weeds from Aoi (she runs the General Goods shop, Tsukumomono Groceries) in Inazuma City every weekly reset.
See all item locations in the Teyvat Interactive Map.
If you want a video demonstration, hope this guide will help you get Naku Weeds easily.
You can also check this guide if you want to farm Sakura Bloom.