Stimulatory effects of an exogenously applied seaweed extract on the morphological and physiological growth and yield in juvenile Amaryllis belladonna L. Bulbs

Amaryllis belladonna L. is a hysteranthous bulbous species indigenous to the Cape Floristic Region of South Africa. The species’ attractiveness, adaptability, and low-maintenance needs have drawn international interest to its desirable uses in ornamental and landscape applications constrained by the observably slow rate of natural multiplication to reach flowering. A 24-week study was performed to determine the stimulatory effects of a seaweed extract, Kelpak®, on the morphological and physiological responses of A. belladonna bulbs cultivated under greenhouse conditions. Juvenile bulbs from five successive age groups were used to evaluate the consistency of observed responses. Treatments consisted of a 0% untreated control and three Kelpak® concentration dilutions at 0.2%, 0.4%, and 1% (v/v) administered to five age groups of A. belladonna bulbs as a monthly soil drench. The results showed that even at low concentrations, Kelpak® treatments improved the phyto-stimulatory responses of both the bulb aerial and, more substantially, the below-ground storage organs in a concentration-dependent manner. While treatments enhanced the morpho-physiological responses, the consistency of bulb age differed. Higher morphological yields were associated with older bulbs; however, bulbs of A. belladonna in years 1 and 2 were deemed the most receptive in circumference, weight coefficients, and chlorophyll content. However, to maximize the efficacy and proliferation rate of the species in a reduced timeframe, a 1% Kelpak® dilution applied at an early developmental stage within the first two years is most beneficial and a priority to elicit rapid, uniform, and healthy bulb growth and development.