Minecraft wiki entry for Weeping_Vines, associated text: Weeping vines can be placed on, and grow from, the bottom of a block. The top-most vine block always has a starting age value of 0, which increments every time the block grows. If the bottom-most vine block has an age below 25, it continues to grow provided that there is free space below the vine. When the vine does grow, a new vine block with an incremented age value is placed beneath it.