Minecraft wiki entry for Tree_farming, associated text: Underground saplings rely on torch light to grow. Various patterns of saplings and torches can be used to achieve varying degrees of space efficiency. Since saplings only require light level 9 to grow, a single torch starting at light level 14 can sufficiently light 60 saplings. However, this torch-efficient model comes at the cost of stability. Trees can grow and block the torch light to other saplings. Underground tree farms should stay clear of magma blocks because a bug relating to the South/East rule may let magma affect any leaf/wood blocks occupying the same corner [verify].