Rhizotrons allow the examination of spatial and temporal in situ root development. Permanent rhizotron installations provide 2-D images of whole root profiles, but their immobility limits the number of soil-plant systems that can be studied. Our objectives were to develop a portable rhizotron and color scanning system for studying the development of whole root systems. Potato root development was monitored in an irrigated experiment at Othello, WA. Covered, rectangular hollow boxes with a transparent glass face were installed perpendicular to planted potato rows, and a seed piece was planted in the soil adjacent to the glass. Rooting in the hill furrow topography was measured at 2 to 4 week intervals. Images of roots growing along the glass face are captured with five scans with a portable, color scanner and a portable computer. Image thresholding discriminated roots from soil using primary color values, color intensity differences, color proportions, or overall intensity. Seasonal patterns of computed root lengths by image analysis were comparable to manual tracing. Primary roots extended to 15 cm from the seed piece prior to shoot emergence, 21 days after planting. Lateral roots began to develop shortly thereafter. Potato roots extended to depths of 60 cm by 4 to 6 weeks after planting, and maximum root density in the hill and furrow was observed by tuber initiation to early tuber bulking. Temporal and spatial trends were similar to previous results using destructive sampling. The method has promise for studying the root growth and development of field-grown plants.