Few studies have asked at what spatial scale environmentalstimuli regulate plant development and when during the pat-terning process these signals act. We have discovered thatplant roots can sense microscale heterogeneity in water avail-ability across their circumference, which causes dramatic dif-ferences in the patterning of tissues along this axis. Rootbranching is a target of such hydropatterning; lateral rootsonly form on the side of the main root contacting water in soilor agar. We show that hydropatterning is a conserved processin Arabidopsis, maize, and rice and reveal the importance ofauxin biosynthesis and transport in regulating this process