In the years 2008–2013 the field experiments with 10 cultivars of willow were conducted in the field that belonged to Koszalin University of Technology. The nitrogen fertilizer was applied at various doses (0, 60, 120 and 180 kg/ha N). The results revealed that the withering of willow rootstocks was highly impacted by the time of re-growing of shoots (years) and the frequency of harvesting. The doses of fertilizers and cultivars were the factors of secondary importance. Until the fourth year of vegetation the average percentage of withering of willow rootstocks amounted to 5.3%. The increase in the number of dead rootstocks occurred during the fifth and sixth year of vegetation. The cultivars were classified according to their liability to withering as follows: very strong – Start (42.4%), strong – Sprint (31.9%), medium – 1054 (26.8%), Jorr (25.0%), Olof (24.1%), 1047 (23.9%), 1047D (23.7%) and Turbo (20.6%). The varieties Tordis and Ekotur showed the weak tendency for withering (13.6%, 9.2%, respectively). The increase of the fertilizer dose, in relation to the plots without fertilization, resulted in intensification of willow rootstocks withering. In the plots with 1 harvesting during the first 4-year rotation, on avarage 5.6% of rootstocks withered while in plots with the harvesting applied twice the withering amounted to 14.1%. Moreover, it was analysed how individual cultivars responded (rootstock withering) to increased harvesting frequency. The studied cultivars were classified as follows: very strongly responding – Sprint and Start, strongly responding – 1047D, 1054 and 1047, with moderate response – Jorr, Turbo and with no response – Olof, Tordis and Ekotur.
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