In order to improve the ability of the Cortaderia selloana seedlings to withstand the shortage of irrigation water, a pot experiment was conducted at the Experimental Farm of Ornamental Plants and Landscape Gardening, Res. Dept., Hort. Res. Inst., ARC, Giza, Egypt during 2017/2018 and 2018/2019 seasons. Seedlings were irrigated with three irrigation levels at 100% (control), 75% and 50% of pot capacity (PC) and sprayed with ascorbic acid (AsA) at concentrations of 0, 100, 150 and 200 ppm. The results showed that the shortage of irrigation water led to a gradual decrease in all vegetative, flowering and root characteristics as well as the contents of total carbohydrates and chlorophylls, this was accompanied with more accumulation of proline in leaves. Regarding foliar spraying with ascorbic acid, there was an increment in vegetative, floral and root characteristics due to increasing AsA concentrations until reaching the maximum growth rate at a concentration of 200 ppm. There were no significant differences between the effect of AsA at 150 or 200 ppm in most cases, as it led to increase leaf carbohydrates, total chlorophyll, and proline, and it also increased the leaf content of total carbohydrates, total chlorophylls and proline. The interaction between ascorbic acid and levels of irrigation water treatments cleared that AsA application at 150 or 200 ppm ameliorated the adverse effects of lack of irrigation water on shoots, flowers and root growth. The best results of growth and flowering were obtained with AsA at 150 and 200 ppm for plants irrigated with 100% then 75% PC with no significant differences in most cases, while the interaction between irrigation level 50% and ascorbic acid at 200 ppm led to the highest accumulation of proline in leaves. In addition, the interaction between PC at 100% (control) and AsA at 200 ppm achieved the best values of total carbohydrates and total chlorophylls in leaves. Accordingly, it could be recommended to irrigate Cortaderia selloana seedlings grown in pots with 75% of the pot capacity once every three days, with spray plants with ascorbic acid solution at 150 ppm every three weeks intervals, as this treatment was economical to save 25% of the applied irrigation water with maintaining high quality plants.