Romaine lettuce production is the most important vegetable production in Southern Quebec Histosols, Canada. However, this lettuce is highly sensitive to tip burn, a problem potentially linked to calcium storage and flow within the plant. Therefore, using appropriate irrigation set points appears critical in limiting the importance of this physiological disorder. Field experiments were carried out to investigate possible limitations to waterflow in Histosols which would be causing tip burn in lettuce. At two different dates, irrigated and non-irrigated field plots were followed to evaluate water fluxes during high evapotranspirative demand in relation to tip burn development. This study showed that tip burn symtoms appeared during important ET demand periods of the summer 2010, whenever soil water potential fell below a trigger point between -10 to -15 kPa. This limiting soil water potential was consistent with low unsaturated hydraulic conductivity values at different horizons.  A modeling approach using Hydrus was used to quantify water fluxes from the soil. It indicated that the limited rooting to deeper horizons and the slow water fluxes at soil water potential lower than -15 kPa were contributing factors aggravating the importance of this physiological disorder within these periods.