Precision fertilizer management for germplasm maintenance of queen’s flower (Lagerstroemia speciosa)

Introduction Nitrogen management in container-grown ornamentals must balance plant quality against the risk of nutrient loss to surrounding water systems, a challenge that is especially pronounced in subtropical nursery environments. This study evaluated precision nitrogen strategies for Lagerstroemia speciosa ‘Big Pink,’ an increasingly popular landscaping tree, by integrating leachate monitoring with plant-based optical sensing. Methods We evaluated the effects of six Azalea fertilizer (8-4–8 N-P-K) treatments: T0 (25 g), T1 (15 g + 10 g supplemental fertilizer, SF), T2 (20 g + 10 g SF), T3 (25 g + 10 g SF), T4 (30 g + 10 g SF), and T5 (35 g + 10 g SF), each with five replications, on container-grown Lagerstroemia speciosa ‘Big Pink’ under subtropical nursery conditions in southern Florida. Plant growth (leaf count), chlorophyll-based indices (SPAD, atLEAF), canopy reflectance (NDVI), and leachate chemical properties (Ca 2+ , NO 3 − , K + , Na + , pH, salinity, and EC) were assessed monthly and analyzed across early, mid-, and late growth stages using ANOVA, Tukey’s HSD test, and Pearson correlation analysis. Results The two highest fertilizer treatments, T4 (30 g base rate plus a 10 g supplemental application) and T5 (35 g base rate plus a 10 g supplemental application), consistently produced greater NO 3 − and K + concentrations in leachate across all growth stages, indicating elevated nutrient mobility and leaching risk. Treatment (T5) further lowered substrate pH and raised salinity and EC at the late stage, reflecting salt accumulation in the root zone. Leaf production was maximized under intermediate fertilizer rates (T2-T4), while both insufficient (T0-T1) and excessive (T5) fertilization reduced growth. Chlorophyll indices (SPAD, atLEAF) reliably detected nitrogen deficiency under T0 from the mid-stage onward, whereas NDVI became a responsive indicator only at the late stage as canopy development increased. Correlation analysis showed progressively stronger associations among leachate variables over time, alongside consistent agreement between SPAD and atLEAF across all stages. Conclusion Moderate fertilizer rates (T2-T3) optimized leaf growth and plant nitrogen status in container-grown L . speciosa ‘Big Pink’ while minimizing nutrient losses, supporting their adoption as a precision nitrogen management strategy for subtropical ornamental nursery production.