Minecraft wiki entry for Nether_Wart, associated text: When planted on soul sand, nether wart grows through four stages, though the middle two stages use the same texture (the hitbox of stage 3 is three pixels taller). In Java Edition, the exact age can be seen using the debug screen, and the ages range from 0 to 3. Each random tick, nether wart has a 10% chance of growing one stage. At default random tick speed, each nether wart grows one age step approximately every 13653 game ticks (11.3775 minutes) on average, and fully grows from planting to harvest every 40960 game ticks (34.133333333333 minutes) on average. The growth rate is not affected by light or any other environmental factors. Bone meal cannot be used on the nether wart.