Spirit Herb

Spirit Herb is a crop added by Roots 2. It is obtained by placing 2 ferns, 1 brown mushroom, 1 red mushroom and 1 nether wart in a Moonlight Circle, as outlined in the Book of Herblore. The Nether Wart must be grown on Soul Sand. When all items are at full growth, a light effect will be seen and the central item will be transformed into Spirit Herb. The crop can then be obtained by right clicking. Spirit Herb doesn't have a seed. It can be propagated by placing the crop itself on Soul Sand.

Note that although the Book of Herblore states that this transformation takes place under moonlight, this doesn't seem to be the case.