This one just takes a little perseverance!
When you walk up the stairs to Skyhold's throne room the first door on your left will lead out to a lovely garden with nothing but 3 meager pots to plants seeds in, after completing the quest in which you collect seeds.
Alas there is a chance to collect materials to requisition a garden upgrade, adding 3 more pots to your collection.
The part that makes this easy? The plants refill every time you leave skyhold and come back, Happy Grinding!
P.S. Make sure to tie this into your upgrade potions/tonics/grenades 30 times.