Developing a Robust, Predictive Model for Sweet Cherry (Prunus avium L.) Flowering, Comparing Eastern Oregon and Mesic Nordic Climates

Phenological obsevations are sensitive tools for identifying plant responses to climatic changes. Flowering dates for ‘Bing’ sweet cherries have been recorded in Milton Freewater, Oregon and in Ullensvang, western Norway for the several decades. In Norway, over the last decade the onset of the phenophases of sweet cherry during the spring tended to be earlier in Ullensvang, western Norway.  The effects of air temperature during the winter and spring months during two 5-year periods, 1996-2000 (P1) and 2003-2007 (P2) in relation to the start of flowering (first bloom) of early maturing ‘Burlat’ and mid-season ‘Van’ were studied. Average winter temperatures (January – February) were similar (3.3⁰C) in both these two 5-year periods however, average March and April temperatures were slightly warmer (3.2⁰ and 4.0⁰C) and (6.9⁰ and 7.3⁰C), respectively. Average temperatures during the first half of May were similar for both 5-year periods (10.1⁰C). Timing of flowering phenophases were statistically different between P1 and P2 for both cultivars. Mean data for ‘Burlat’ and ‘Van’ first bloom were 8 days earlier during P2, May 2 for ‘Burlat’ and May 1 for ‘Van’. Indeed, during both P1 and P2, for both cultivars the onset of first bloom was 5 days earlier and 8 days later than the long-term average dates respectively. Full bloom occurred 3 days after first bloom and flowering ended 14 days after first bloom. Clearly, increased temperatures of the spring months, March through April sped up flower development significantly. Growing degree-days (GDD), single sine based, 2⁰C base supported this difference; a mean of 221 GDD accumulated during March-April on average during P2 vs. only 197 during P1. A model of sweet cherry 'Van' bloom dates had lower error rates, using a 2⁰C base, 254 GDD to bloom, and starting date of March 1, whereas a recent bloom model developed for sour cherry in Michigan, USA used a 4⁰C base temperature. (Ref: Zavalloni, C, J.A. Andresen, J.A. Flore. 2006. Phenological models of flower bud stages and fruit growth of 'Montmorency' sour cherry based on growing degree-day accumulation. J. Amer. Soc. Hort. Sci. 131:601-607.)