Overview
Greenhouse Effect is a secondary quest in The Witcher 3: Wild Hunt, set in the southeast corner of the valley around Kaer Morhen. To get there, head south from the keep, cross the river, climb the hill on the left past an old distillery, and keep going east as far as you can. It lies east of where Geralt meets Eskel in To Bait a Forktail….
The broken spell
Witcher Senses show that the strange weather is caused by a decaying heat spell, once a magical greenhouse for growing exotic herbs. The mage’s notes are hidden behind an illusory rock that the Eye of Nehaleni reveals. According to them, two of the three obelisks must be hit with Igni and the third with Aard.
That fixes the spell, but a chort that liked the broken weather attacks. Once it is dead, the greenhouse starts working again.
The harvest
You do not actually need to wait days. Walk a little way down the hill and back, and the site is covered with herbs: crow’s eye, fool’s parsley, ginatia, han fiber, honeysuckle, hornwort, longrube, mandrake, pringrape, ribleaf, verbena, white myrtle and wolfsbane.