The research described in this paper was conducted at the University of Debrecen, Farm and Regional Research Institute in its Botanical and Exhibition Garden, Hungary. The aim was to investigate the influence of plant biostimulants on the morphological parameters of three lettuce varieties (May King, Kobak and Great Lakes). Willow bark extr
...act and Bistep were used through irrigation and spray onto the plant leaves with the amount of 5% Willow and 0,5% Bistep. Vegetative parameters like head structure (1….10), head weight (g head-1), head closing (1….10), number of leaves, head diameter (cm), internal stem size (cm) and root weight (g head-1) were measured in 2019 and 2020. According to our results, vegetative parameters are significantly affected by plant varieties and treatments. In the experiment of 2019, important vegetative parameters were influenced by the Willow extract, while in the season of 2020 the most influenced parameters were under the impact of Bistep biostimulants followed by Willow+Bistep. However, the most favourable vegetative growth was recorded in the Kobak variety in the season of 2019 followed by May King and Great Lakes varieties in the season of 2020. On the other hand, the interaction effect of plant varieties and plant biostimulants on the vegetative parameters were varied from seasons. For example, for the season 2019, significantly the highest vegetative value was recorded for the interaction of May King variety treated with Willow extract, whereas in 2020, significantly the most influenced vegetative parameters were under the influence of the interaction of Bistep and May King variety followed by the interaction of Willow+Bistep and Kobak variety.