This quest can be obtained by observing an unnatural weather effect in the southeast corner of the Kaer Morhen valley. The site is not easily visible on the map, although fragments of the road which lead to it can be seen. It can be reached by heading south from Kaer Morhen, across the river, up the hill to the left (passing through an old distillery), and then eastward as far as possible. It is east of the site where you meet Eskel in the quest To Bait a Forktail... (which has you exit the area to the south instead).
Once you have found the site, you need to look around your witcher senses. The strange weather is arising from the decay of an old heat-generating spell, which served as a magical "greenhouse" for the production of exotic herbs -- it can be fixed if you find the original mage's notes, which requires using The Eye of Nehaleni on an illusionary rock. You must then hit two of the three obelisks with the Igni sign and the remaining one with the Aard sign. However, a chort which appreciated the broken weather then appears and attacks. With the chort dead and the weather anomaly resolved, the "greenhouse" resumes its original function, and you can complete the quest a few days later by returning to the site (which now has a substantial crop of herbs, including Crow's eye, Fool's parsley leaves, Ginatia petals, Han fiber, Honeysuckle, Hornwort, Longrube, Mandrake, Pringrape, Ribleaf, Verbena, White myrtle petals, and Wolfsbane).
- Investigate the weather anomaly using your Witcher Senses.
- Read the mage's letter.
- Use the obelisks to restore the place's proper microclimate.
- Kill the chort.
- Return to the site of the anomaly in a few days.
- It turned out even the area around Kaer Morhen, a place which - one would think - Geralt knows by heart, can hide a great many interesting and undiscovered phenomena. Not far from the keep, Geralt came across an area where the weather differed remarkably from that of its surroundings. He decided to investigate what sort of forces were at work.
- After a brief investigation, the witcher was able to conclude that the weather anomaly was the doing of one of the resident mages of Kaer Morhen, who had created a sort of outdoor greenhouse in which to cultivate the herbs used for the Trial of the Grasses. Though you might not know this, Geralt is quite the handyman, something which came in handy (pun intended) at this time. He banged and nudged the broken magical apparatus and returned it to working order in no time. Before long, rare herbs were growing outside Kaer Morhen once again.