Microalgae are gaining attention as a sustainable and efficient alternative in modern agriculture due to their biostimulant and biofertilizer effects, which promote plant growth and improve plant resistance to abiotic stress factors. Their effectiveness vary according to factors such as concentration and growth stage. This study evaluates the potential of Chlorella vulgaris (C. vulgaris) to stimulate various plant parameters, including growth, biomass, leaf development, and flowering. The trial was carried out in a greenhouse, using Pelargonium × hortorum plants grown on coconut fiber substrate. A nested factorial design was applied, with treatments including a control (irrigation water only) and four experimental groups with microalgae applied at concentrations of 0.1 g L−1 and 1 g L−1, harvested in the stationary or exponential growth phases. The experiment was repeated in autumn and spring to assess seasonal effects. Variables such as plant height, diameter, number of leaves and flowers, as well as the fresh and dry weight of the different plant organs were measured. In addition, leaf area was determined. The results indicate that higher application rates (1 g L−1) increased nitrate, phosphate, and potassium levels, highlighting the role of C. vulgaris in improving plant nutrition. The application of C. vulgaris at 1 g L−1, especially in the stationary growth phase, promoted growth and advanced flowering in Pelargonium × hortorum. In conclusion, C. vulgaris shows significant potential as an agricultural biofertilizer, promoting the growth of Pelargonium × hortorum
