Irrigation is a prerequisite for economical onion production under dry conditions. However, its effect on dry matter and nutrient content often remains a concern for growers. A direct sown onion hybrid was grown under open field, rain-fed and irrigated conditions for three years, investigating the effects of air temperature and water supply on some nutritive constituents. Dry matter, storage sugar, total flavonol and total polyphenol content showed strong positive correlation with average air temperature and negative correlation with water supply. However, irrigation had a positive effect on storage sugar and dry matter content. Presumably better water supply during dry periods ensured by irrigation provided the basis for higher photosynthetic production, and hereby more dry matter partitioning and accumulation in the bulb, a storage organ. An unexpected decrease in vitamin C content was experienced in 2011 and 2012, compared to the result of 2010, which was explained by the hot and dry conditions of the pre-harvest irrigation cut-off period. Fibre and ash content was found to be the most stable nutritional characteristics, affected neither by the environmental conditions, nor by the irrigation. Irrigation has proved to be very beneficial for direct sown onion, doubling bulb yield while not affecting the nutritive quality negatively.