Among methods for stimulation of plant growth an essential role can be played by seaweed biostimulants. The aim of the study was to assess the response of onion to the method of application of the seaweed biostimulant Kelpak SL. Doses of the seaweed extract applied from the three-leaf stage amounted to 3, 3 + 2, 3 + 2 + 2 dm³ ha^-1, and from the four-leaf stage 2, 2 + 2, 2 + 2 + 2 dm³ ha^-1, for a single, double and triple application, respectively. The biostimulant applied from the three-leaf stage increased the chlorophyll index after double or triple application, whereas applied from the four-leaf stage, also after a single application. The highest increases in the fresh weight yield of bulbs as well as fresh weight of roots resulted from the triple application of the biostimulant from the three- or four-leaf stages. Each dm³ of the biostimulant caused an increase in the fresh weight yield of bulbs by 0.76 t ha^-1, and each additional application resulted in an increase in yield by 1.76 t ha^-1.

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