Canadian lettuce is mostly grown in organic soils in Quebec. Lettuce is highly sensitive to tip burn, a physiological disorder that can lead to significant marketable yield losses. Tip burn losses have been linked to multiple causes, such as inadequate water and Ca fluxes during transpiration, which in turn are linked to the water fluxes in the root zone to and within the plant. The aim of this study was to investigate the dynamic relationship between water fluxes in the root zone and the occurrence of tip burn in romaine lettuce (Lactuca sativa L.) grown in muck soils. Water fluxes in the root zone were modeled with HYDRUS-2D using measured soil properties and weather conditions. The model results showed that mean unsaturated hydraulic conductivity in the root zone was sufficient to meet transpiration demands down to a matric potential of −31.14 kPa at a depth of 15 cm. Simulations based on field observations revealed that as soon as a root water uptake deficit of 0.14 mm h−1 was reached, tip burn developed rapidly. Hence, the simulations confirmed that below this matric potential threshold, hydraulic conductivity <1.6 × 10−4 mm h−1 and water flow velocity in the root zone <4 × 10−3 mm h−1 became insufficient for plant needs, leading to tip burn occurrence in lettuce, consistent with field observations. Understanding the water root-zone fluxes to anticipate water stress and reduce tip burn damage in Romaine lettuce is a major improvement for growers, giving them a longer time window to initiate irrigation.