You can get the "Nymphs' New Roots" quest from Herio after completing "[Calpheon] To the Delphe Knights Castle". You need to complete the main questline on the toon to receive the quest.
Once, you talk to Herio during the quest, he will give you a boss scroll for "Root Nymph King". You can repeat this quest.
The quest itself is not giving you the knowledge as a reward. It's only an opportunity to get the boss scroll. After that it's up to your luck getting the knowledge, once you kill the boss.