Due to it's relatively high sell value it's always good to pick it, especially early in the game. Especially that the Elder druid in the Druids' grove buys it, also sells it.
It is actually quite abundant in the Swamp, especially in the north-east, around the Nonhuman camp.
As early as Chapter II in the Swamp at the Nonhuman camp a haughty Scoia'tael dressed as a huntress asks Geralt if he knows about it, a golden opportunity for Geralt to give her a dressing down for her bigotry... or not. He can also ask her about it, not that she will tell him, but she does give him Lara's Gift. If Geralt talks to her again, he can choose to give her flower, and gets a response that seems to be a dropped plot/translation bug. She will mention "burmarigold", which does not exist in the game and does not result in any journal entries. Strangely, after resting for an hour, the same conversation can be repeated, and if Geralt chooses the same responses (give her flower, etc.), she will tell you about "Green mold", which also does not seem to result in journal entries.
Upon accepting the quest Echoes of Yesterday from Yaevinn in the Druids' grove during Chapter III, he will tell you a story about Lara Dorren and mention feainnewedd now grows where she "breathed her last". This conversation does not unlock feainnewedd in the journal.