Significance Organ formation is an essential process in plants and animals, driven by cell division and cell identity establishment. Root branching where lateral roots form along the primary root axis increases the root system and aids the capture of water and nutrients. We have discovered that tight control of the cell width is necessary to coordinate asymmetric cell divisions in cells that give rise to a new lateral root organ. Although biomechanical processes have been shown to play a role in plant organogenesis, including lateral root formation, our data give mechanistic insights into cell size control during lateral root initiation.