Hi Jonathan, you'll attempt to harvest them whenever you see one of the three Nature's Essence icons. This is best outlined on the Elowen's Grove placard which states:
When the heroes gain access to Nature’s Essence, one hero may sacrifice the Harvest Cost and roll the Fate die. If the roll result is equal to or lower than the Harvest difficulty, the group gains 1 Nature’s Essence. If the roll fails, another hero may attempt, but the cost increases by 1.
Harvesting heroes may add 1 Energy to the Harvest Cost to reduce their roll result by 1, or add 1 Health and 1 Energy to the Harvest Cost to reduce their roll result by 3. This may only be done once per attempt.
Hope that helps! Thanks for the question!