The Karstian Seed is a Key item found in Momodora III.
Melilot gives you this item in trade for the Owl Badge. The Karstian Seed, so she says, is a sort of seeds from her homeland Karst. However, because of the fact that huge areas of Karst are barren, they seldom grow there.
The item can be planted in the Belltower Garden area on the small pyramid, signaled by the pillar of light shining on it. After planting it, the door to the Distortion in Time will appear. The room can be easily reached by warping back to the start of the Belltower Garden and using the yellow jelly to jump left.