Chilling effect on three highbush blueberry cultivars

In recent years, blueberry production has increased in areas with mild winter climates where chilling is one of the major problems concerning plant productivity. Breeders and nurseries normally provide just approximate values of the chilling requirements for specific cultivars. Thus, growers in different areas have to cope with a lack of knowledge about cultivar behavior in their region. Portugal has a diverse climate with 400 chilling hours in the south of the country and more than 1200 hours in the north. In order to evaluate the effect of chilling hours on three blueberry cultivars ('Legacy', 'Duke' and 'Elizabeth'), a trial was set up with five chilling treatments (955, 1,123, 1,267, 1,417 and 1,520 chilling hours) and a control (natural cold, 501 chilling hours). Plants were grown in substrate for three years until the setup of the experiment. Depending on the cultivar, plant production varied with chilling treatments. 'Legacy' had a negative response to chilling and the control treatment resulted in the highest yield (5.0 kg plant^-1). 'Duke' presented an exponential response to chilling with the highest value at 1200 chilling hours. 'Elizabeth' showed a positive response towards chilling but with no clear tendency. Since plants from all treatments left the cold chamber on the same date (March 11), growing degree days were equal for all treatments and no effect on phenological dates was registered. 'Legacy' was the first cultivar to begin harvest (May 29) and 'Elizabeth' the last (June 25). With this experiment we can state that 'Legacy' can be grown in the south, 'Duke' in places where plants can achieve 1200 chilling hours and 'Elizabeth' in places with at least 1000 chilling hours, in order to achieve high yields.