Acta Horticulturae 618: 469-475 (2003)
Heat units may explain variation in duration of bud dormancy in Zantedeschia
C.P. Carrillo Cornejo, K.A. Funnell, D.J. Woolley, B.R. MacKay

Control of the growth cycle and, in particular, dormancy, is required if growers are to produce flowering plants of Zantedeschia at any chosen time of year. In order to improve the predictability and accuracy of duration of dormancy in Zantedeschia, the effect of different planting seasons and cultivars were assessed using a heat unit model. Plants of cultivars 'Black Magic' and 'Treasure' were ex-flasked in July and November 1999, grown in a heated greenhouse, and sampled throughout growth. The changes in production rate of both external and primordial structures were used as potential indicators of the onset of dormancy, as well as emergence data. Results were compared to two previous studies where the duration of dormancy was similarly quantified, but where plants were grown under different environments and seasons. Plants grown out of season (July ex-flask date) in a heated greenhouse readily completed their growth cycle and exhibited some level of bud dormancy from November through to January/February. In contrast, plants ex-flasked in November exhibited dormancy from February through to April/May. When results were compared with previous studies dormancy collectively extended over seven calendar months (November to May), with no overlap in timing between plantings in different seasons. A heat unit model resulted in the wide differences in duration of dormancy largely disappearing. Using a base temperature of 2.1°C, dormancy extended between 1466 and 3476 degree-days. While variation in the range of degree-days was evident between each planting date, season and cultivar, all plantings exhibited dormancy between 2145 and 2747 degree-days. Cessation of leaf appearance was not directly related to the onset of dormancy and, therefore, cannot be recommended as a tool by growers to monitor the onset of dormancy. A heat unit model appears able to predict dormancy in Zantedeschia cultivars grown in differing seasons and growing environments.

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