Many works point to identification of the principal environmental factor determining the vegetative growth and yield in horticultural crops mulched by cover materials. However, almost nothing is known about changes in this area caused by drought. Answering this question was the predominant aim of our work. We found that the relatively strong negative correlation between the cummulative shoot length (CSL) of watermelon plants (Citrullus lanatus (Thunb.) Matsum. & Nakai) and the root-zone temperature in 5 cm depth (TRZ5) which was exhibited under sufficient water supply, switched to a positive dependency on the relative water content (RWC) in the youngest expanded leaf of the main shoot when under dehydration. At the beginning of the dehydration cycle the highest CSL values were exhibited by plants on white polyethylene (PE) mulch, whereas water witholding favoured red PE foil and black non-woven textile. These results are discussed in respect to plant protection against heat stress on the one hand, and, on the other hand, against water stress.