Phenology Overview

Phenology is defined as, “the study of the timing of recurring biological events, the causes of their timing with regard to biotic and abiotic forces, and the interrelation among phases of the same or different species.” [1] In the framework of temperate perennial crops, these biological events include production of flower and leaf buds in the summer, dormancy of these buds over winter, flowering and leaf-out in the early spring, pollination of flowers and development of fruit in the spring, and growth of the fruit over the course of the summer.

Each of these stages is vulnerable to changing climatic conditions in their own way. This site focuses on the two areas of phenology studied at UC Davis that we believe will most impact future yields: Dormancy Requirements and Fruit Growth and Yield.

Temperate tree buds need adequate chilling in order to bloom with enough flowers and in a small enough window for farmers to make a profit [2-4]. Research by Eike Luedeling, formerly of UC Davis, now with the World Agroforestry Center, has shown this requirement to be in danger of not being met first in looking at California, and recently for many of the other major temperate fruit cultivation zones around the globe. This site provides access to global chilling projections data so that researchers can focus on their geographical area of interest using our interactive maps or by downloading the dataset.

This chilling reduction work has raised the question “What other factors influence bloom, and can they compensate for reduced chilling?” It is thought, for example, that spring heat can compensate for a certain lack of chilling [5-7]. However, there are no models to our knowledge that accurately integrate chilling and forcing for tree crops, particularly in a Mediterranean climate. Current research by Katherine Jarvis-Shean is working to address this problem.

circle Figure 1. Phenological stages in the production of almonds include winter chilling, spring heat forcing bloom, and nut development and growth.

There are other areas of phenological concern, many of which are crop-specific, under investigation at UC Davis and elsewhere. In the coming months and years as these studies are published we will continue to update this site with their findings and links for further information.


  1. Lieth, H. (1974). Introduction. Phenology and Seasonality Modeling. H. Lieth. New York, Springer-Verlag.
  2. Chandler, W.H. (1925) Fruit Growing. Boston, Houghton Mifflin.
  3. Crane J C, Takeda F. (1979) The unique response of the pistachio tree to inadequate winter chilling. HortScience, 14, 135-137.
  4. Crabbé, J. (1994). Dormancy. Encyclopedia of Agricultural Science. C. J. a. R. Artntzen, E.M. New York, Academic Press. 1: 597-611.
  5. Richardson, E. A., S. D. Seeley, et al. (1975). Pheno Climatography of Spring Peach Bud. Hortscience 10: 236-237.
  6. Couvillon, G. A. and A. Erez (1985). Effect of level and duration of high-temperatures on rest in the peach. Journal of the American Society for Horticultural Science 110: 579-581.
  7. Swartz, H. J. and L. E. J. Powell (1981). The effect of long chilling requirement on time of bud break in apple. Acta Horticulturae 120: 173-178.