## 4 Discussion
'Mme Plantier' did not experience frost damage every year, unlike most varieties of the Alba group, e.g., 'Celestial', 'Hurdals', and 'Maiden's Blush' [38]; the Gallica group [44]; or the Spinossisim group, e.g., 'Aicha', 'Elegans', 'Fruhlingsduff', 'Fruhlingsgold', 'Poppius', and 'Stanwell Perpetual' [40]; as well as most old and modern cultivars of Rugosa-Hybrids, e.g., 'Agnes', 'Belle Poitevine', 'F.J. Grootendorst', 'Frau Dagmar Hastrup', and 'Rugeaux du Japon' [42; 43] that were observed in the same climate conditions in PAS Botanical Garden CBDC in Powsin. Strong frost damage after winter was observed in cultivars of Hybrid Wehurana: 'Alberite Barbier', 'Alberite', 'Chaplin's Pink Climber', 'Orange Triumph clg', and 'Zophirine Droubin' (Table 3). Their winter hardness can be compared with ground cover roses, but they bloom continuously throughout the season and also in spring, quickly regenerating frosted shoots [39; 41]. Older plants experienced lower frost damage than the younger plants, which was also visible in their level of abundance of flowering (Tables 9-13). The abundance of flowering was always lower on the scale after substantial winter damage and the necessity of strong pruning (Figure 6). In the case of repeat-flowering rose cultivars, ornamental value depended on regenerative abilities after frost damage and also on the height to which pruning was necessary [39; 41]. In once-flowering roses, the shoots were damaged to the ground surface (Table 5), and low pruned shrubs did not flower, or there were only a few flowers on old parts of the shoots of 'Dr Robert Huey', 'Duc de Constantin', 'Gerbe Rose', and 'Paul's Scarlet Climber' after winter seasons between 2000-2006 and 2009-2013 (Table 2). Moreover, the length of the flowering period in several rose climber ('Alberite', 'Gerbe Rose', 'Mme Plantier', 'Orange Triumph', and 'Paul's Scarlet Climber') tended to shorten between 2015 and 2017 compared to the pre-2015 period. Such a response may support the hypothesis [57] that climate warming may cause significant reduction of the length of flowering period in plants. The term of the beginning of blooming seems to be less sensitive to climate warming [57]. However, complete frost damage (without signs of regeneration) was not recorded, and the rose plants pruned to the ground grew intensively until autumn (Figure 6). Woody plants of the temperate zone can survive the winter thanks to bud dormancy and cold hardness [20; 58], a process which could be disrupted by climate changes, as in the case of _Vitis_[20; 59]. Winter hardness can be connected with the prolongation of the growing season [50] as well as hardening and dehardening [20], because the decrease of low minimum temperature was only correlated with higher frost damage in December and March (Table 4). Low temperatures in April caused increased frost damage in 'Alberntine', 'Chaplin's Pink Climber', and 'Zephirine Drouhin'. In contrast, the frost damage increased for more frost resistant climbers ('[PERSON], '[PERSON]', and 'Mme Plantier') when the temperature decreased in March and then rapidly increased in April (Figure 4, Table 3). Moreover, low rainfall in late autumn and early spring (Figure 5) can exacerbate frost damage of green, primarily one-year-old shoots due to the phenomenon of physiological drought [60]. Climate warming, accompanied by temporal amendments in average temperature and other variables, exerts an effect on phenology [25; 61] by influencing temperature values [53] and photoperiods [62]. Previous research in _Vaccinum membrananceum_ showed that greater advances in phenology are likely to be in northern and higher altitude regions of North America [63]. Spring phenology stages, such as leaf unfolding and blooming, have been shown to mainly advance, some by a few weeks, with median increase of 4-5 days per degree Celsius. Autumn traits, e.g., leaf coloring or leaf drop, have typically become delayed, though with more changeability than spring events. Amendments in terms of summer phenological stages have been stirred [18].