Back to Kaer Morhen
Greenhouse Effect is a secondary quest in Kaer Morhen and is very easy to miss. You can find it by exploring the area to the south west of the keep. Parts of a road are still visible on the map.
When you get close to the area Geralt will notice a strange weather anomaly. Use your Witcher senses to look around. Geralt will figure out that the odd weather is a result of an old heat generating spell that was used to create the effect of a greenhouse. If you can find the mage’s notes you might be able to reverse the spell.
You’ll need to use The Eye of Nehaleni on the illusionary rock. You must then use Aard on one of the obelisks and Igni on the other two. When you do this a Chort will attack.
With the Chort killed and the obelisks repaired the greenhouse should be working as intened. You can return here in a couple of days to pick up a large crop of various herbs.