Woody Plant Seed Manual (2008)

Rosa L

Susan E. Meyer

Chilling is the treatment most often applied to remove rose seed dormancy, and the achenes of most species will germinate eventually if chilled for long enough periods. For some species, periods of cold stratification corresponding to a single winter in the field are sufficient, as in prairie, multiflora, and wichura roses (table 3). Achenes of these species may show increased dormancy if the chilling period is preceded or interrupted by periods of incubation at warmer temperatures (Semeniuk and Stewart 1962; Stewart and Semeniuk 1965). Interruption of chilling with warm incubation resulted in secondary dormancy induction only if the temperature of warm incubation was too high. If the seeds were held below this ‘compensating’ temperature, no change in dormancy resulted, and the seeds could accumulate the effects of chilling across warm interruptions. Seeds whose chilling requirements had just barely been met germinated best at relatively low incubation temperatures, whereas those that had been in chilling for longer than necessary either eventually germinated in chilling or could germinate at a wide range of temperatures, including those above the compensating temperature. Semeniuk and others (1963) showed that, for prairie rose, the effect of the warm pretreatment above the compensating temperature was to induce secondary dormancy at the embryo level. Interestingly, this dormancy could be alleviated only by chilling whole achenes; chilling the embryos did not alleviate their dormancy.

Other species, such as prickly, Nootka, and Woods roses, show much increased germination percentages in response to chilling periods corresponding to a single winter if the chilling period is preceded by a period of warm incubation (table 3). This requirement for warm incubation before chilling would effectively postpone seedling emergence in the field until the second spring after seed production (Densmore and Zasada 1977). The temperature and duration of the warm treatment is sometimes important. In rugosa rose, a warm pretreatment of 60 days at 20 °C before 90 days of chilling at 3 °C increased germination over chilling alone, but longer periods resulted in decreased germination (Svejda 1968). The effect of warm pretreatment on chilling response has been formally documented for only a few rose species, but it is likely that high-viability lots of any species that show minimal germination after 6 months of chilling would be benefitted by a warm pretreatment.