Generally found underground and in caves, Tarma Root is a plant used in crafting. In rare cases, it can also be found on the surface in Extreme Hills or Stone Beach biomes. Unlike mushrooms, Tarma Root will not spread or multiply if left in a dark area.
Tarma Root's two main uses are in the creation of Air Essence , and as an ingredient in spells which include Zone or Channel shapes . The creation of an Air Essence requires two Tarma Root, while spells require one Tarma Root for each Zone and Channel shape used.
Tarma roots can be grown with bone meal in an area with no light at all. Although extremely rare. The best method of obtaining Tarma roots is to explore caves on any level, they can grow on blocks other than grass
Following an update tarma roots can no longer be grown on dirt or grass, but rather must be grown on stone in complete darkness, this can be done using a grow spell directly on stone blocks or by surrounding a single block of grass with stone and using bonemeal on the grass block (Tall grass must be destroyed each time).
From the Arcane Compendium
"Lighter than it looks, Tarma Root grows in caves and other dark places. It is an ingredient in spells, and air essences."