The influence of vernalization temperature and duration and devernalization treatments on subsequent growth and flower development of Campanula `Birch Hybrid' and Leucanthemum ×superbum Bergman ex J. Ingram `Snow Cap' was determined. In the vernalization experiment, plants of `Birch Hybrid' were vernalized at 0, 2.5, 5, 7.5, or 10 °C for 2, 4, 6, or 8 weeks. `Snow Cap' was vernalized at 0, 2.5, 5, or 10 °C for 2, 4, 6, or 8 weeks. In another devernalization experiment, plants of both species were moved to a high temperature (30/10 °C, day/night) growth chamber for 2 or 4 days at various times during or after the 6-week vernalization period. A 6-week vernalization was necessary to obtain 100% flowering in `Birch Hybrid', and 8 weeks of vernalization decreased time to flower by 7 to 10 days compared with 6-week vernalization. Exposure to high temperature for 2 days during or immediately after vernalization did not devernalize `Birch Hybrid' plants, while a 4-day exposure decreased flowering percentage in some treatments and delayed flowering by 7 to 10 days. There were no significant differences in flowering characteristics of `Snow Cap' plants vernalized at 0 to 5 °C for 4 to 8 weeks. A 2-week vernalization at 0, 2.5, 5, or 10 °C or 4 to 8 week vernalization at 10 °C delayed flowering by 5 to 10 days compared with those vernalized at 0 to 5 °C for 4 to 8 weeks. Exposure to high temperature for 2 d did not devernalize `Snow Cap' plants regardless of exposure times, but a 4-day exposure delayed flowering by 4 to 5 days in some treatments. Combined, the data indicate that `Birch Hybrid' has an obligate 6-week vernalization requirement and `Snow Cap' has a facultative 4-week vernalization requirement that can be fulfilled in the 0 to 10 °C range. Exposure to temperatures of 30 °C (9 h·d-1) for 12 out of 42 days did not devernalize either species but in some cases caused a small delay in time to flower.
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