Terraria Community Forums

Hmm, not sure what you mean - the Truffle farm uses mushroom grass seeds in the spawn region. Are you referring to the dirt used underneath to plug all the holes? If so the biome created by the blocks doesn't actually impact the spawns, since the spawns depend only on the surface, not the biome.

In fact, so long as you have 80 jungle biome blocks (jungle biome has the best spawn rate) the jungle biome will override all other biomes including the desert biome.