Minecraft wiki entry for Chorus_Flower, associated text: If upward growth does not occur and the flower's age is less than 4, the flower may attempt to branch horizontally. 1–4 branches are attempted on an unbranched plant or 0–3 on a branched plant. For each branch a horizontal direction is chosen at random, and if the block in that direction is an air block with more air blocks on all other horizontal sides and an air block below, it is replaced with a flower with age incremented by 1. If at least one branch succeeds, the original flower block is turned to chorus plant.