Dosage-Specific Effects of Commercial Seaweed Extracts on Seedling Growth, Biomass Accumulation and Root Growth Traits of Capsicum annuum

Commercial seaweed extracts (SWEs) are widely used as plant biostimulants; however, information on formulation-specific optimum dosages for greenhouse-grown Capsicum annuum seedlings is limited. This study evaluated the effects of five commercial SWE formulations applied at five dosages (1–5 mL L−1) on seedling growth, biomass accumulation, and root growth under greenhouse conditions. Morphological traits, biomass characteristics, and growth efficiency indices were assessed using univariate and multivariate statistical approaches to identify formulation-specific optimum application rates. Commercial SWE application significantly (p < 0.05) increased chlorophyll content, shoot and root growth, and fresh biomass, although the magnitude of the response varied among the formulations and application rates. The optimum dosages identified by the Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) were ASCO at 2 mL L−1, AQUA at 3 mL L−1, BIO at 2 mL L−1, KAT at 3 mL L−1, and SAGA at 3 mL L−1. Principal component and Pearson correlation analyses further demonstrated coordinated improvements across multiple growth traits. These findings demonstrate that optimum seedling performance depends on formulation-specific rather than universal application rates. The identified optimum dosages provide practical guidance for selecting commercial seaweed biostimulant application rates in greenhouse C. annuum nurseries.