Strawberry is propagated vegetatively. Nowadays, traditional bare-root transplants are often replaced with plantlets (plug plants) produced from runner tips. The nutrient level and type of growing medium may influence the quality of planting material. Phosphorus and potassium are important elements for plant metabolism. Insufficient supply of plants with these elements negatively influences the rooting and plant vegetative growth. The objective of the study was to evaluate the influence of substrate and monopotassium phosphate (MKP) on rooting and quality of strawberry plantlets grown in the greenhouse soilless system. ‘Elsanta’ mother plants were planted in containers and set on a special rack in the greenhouse. Emerging plantlets were set (clipped with metal clips) in micro-pots filled with peat or coconut substrate. Four concentrations of MKP in the growing medium were applied: 0, 0.22, 0.44, 0.66 g dm^-3. Rooting of plantlets was carried out before cutting them off from the mother plants for a period 7, 10 or 14 days. The investigations have showed a very similar dynamics of root formation both in coconut substrate and in peat. A significant effect of the application of monopotassium phosphate on rooting of strawberry plantlets was observed. The optimal dose of MKP depended on the length of rooting period of plantlets. After 7 days of rooting the highest root weight was recorded in case of plantlets rooted in the growing media supplemented with 0.44 or 0.66 g dm^-3 MKP. After 10 and 14 days of rooting even the lowest dose of MKP (0.22 g dm^-3) resulted in significantly stronger root growth in comparison with not fertilized control plantlets. MKP also significantly influenced the growth of plantlets after transplanting them into bigger containers. The highest quality plantlets were obtained at the dose 0.44 or 0.66 g MKP dm^-3 of substrate.

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