Flowering is of paramount importance in horticulture as it is the first step for sexual reproduction. The process of transition from the vegetative to flowering phase is influenced by various factors including nitrogen (N, dominated by ammonium and nitrate). Numerous studies have elucidated that low N supply stimulated flowering, while a U-shaped flowering curve was conferred corresponding to the availability of NO3 -. However, substantial controversy remains for the flowering priorities of N forms. The present study aimed to evaluate and compare the influence between NH4 + and NO3 - as N sources on flowering of salvia. The flower development was monitored after salvia plants were subjected to one of three NH4 +:NO3 - regimes (0:100, 50:50, and 100:0). Additionally, levels of carbohydrates (total soluble sugar and starch) individually in leaves and petals were assessed at the end of the cultivation. Consequently, despite salvia plants cultured with 100:0 NH4 +:NO3 - ultimately developed NH4 + toxicity symptoms in leaves, they showed early flowering concomitantly with greater number of flowers. A sole NO3 - nutrition benefited more shoot growth, while retarding flower induction. Injured leaves with remarkably reduced starch content resulted in gradual improvement of total soluble sugar level in response to an increasing external NH4 + supply from 50 to 100%. In contrast, intact petals had progressively diminishing endogenous carbohydrate (starch and soluble sugar) stock. These data indicate that salvia plants withdraw the leaf starch to detoxify the NH4 + toxicity, but consume petal carbohydrates for flower initiation when high NH4 + ratios are used as the N source.