The utilization of treated wastewater can enhance the crops' irrigation efficacy by offering an extra source of water and nutrients for agricultural purposes. This methodology helps alleviate the pressure on conventional water resources and can be a sustainable strategy to address the challenges of water scarcity. However, it is essential to ensure that wastewater is properly treated to meet quality and safety standards before its application to agricultural crops. This study focuses on exploring the reuse of wastewater from microalgae production and its impact on Pelargonium × hortorum growth during two seasons (autumn and spring). The established treatments were as follows: tap water (control 1); 100% IW—inlet wastewater (control 2); 100% OW—outlet from the reactor; 75% OW + 25% W—75% outlet from the reactor and 25% tap water; and 50% OW + 50%W—50% outlet from the reactor and 50% tap water. Irrigation with wastewater in autumn did not have a significant negative effect (p > 0.05) on plant height, plant diameter, leaf dry weight, roots, or total dry weight, and it was comparable to the control in all applied percentages. On the other hand, wastewater irrigation during spring had a meaningful positive (p < 0.05) impact on plant growth compared to the control. These wastewater resources have a high concentration of nutrients, making them a valuable source of essential and/or beneficial elements. The levels of nutrients such as NO3− range from 144.08 ppm to 82.10 ppm, PO43− ranges from 14.14 ppm to 7.11 ppm, and K+ ranges from 36.83 ppm to 29.71 ppm. Therefore, the obtained results support the viability and effectiveness of using wastewater after microalgae production in agriculture to reduce water demand, mitigate water pollution, and substitute chemical fertilizer input, contributing to more sustainable agricultural practices. These results, with more detailed evaluations, would be applicable to other related plant species and are even applicable to the commercial production sectors. [ABSTRACT FROM AUTHOR]