Almond (Prunus amygdalus Batsch.) blooming date is determined by the temperatures during the dormancy period, from the onset of endodormancy to just before blooming. In this work we have developed a model, based on several years data, to estimate the mean transition date from endodormancy to ecodormancy in 44 almond cultivars covering the whole range of almond bloom, through the significance of correlation coefficients between the temperatures occurring during dormancy and the date of full bloom. The estimation of this date for each cultivar has allowed the calculation of its chill and heat requirements. It was found that most cultivars have chilling requirements between 400 and 600 chill units, whereas the span of heat requirements was wider, from 5500 to 9300 growing degree hours Celsius. Some cultivars show high chilling requirements and low heat requirements whereas others show opposite requirements. These differences confirm the wide almond adaptability to different climatic conditions and offer the possibility of being utilized in breeding programs. The good fit shown by the application of this model in the prediction of bloom time may sustain its application in chilling and heat requirement estimation in other fruit species if blooming dates and climatic data for several years are available.

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This chapter discusses the taxonomy of almond, including wild almond species, and describes the morphology of the root, canopy, bud and flower, fruit and kernel. Information is also given on the crop reproductive physiology, including flower initiation, flowering, pollination, fertilization, fruit development, and vegetative growth.

Although Global Warming is producing an increase of the medium air temperature with the progress of the years, this negative temperature trend cannot be yet appreciated in some areas, as shown by the analysis of the air temperatures observed in the Ebro Central Valley (Zaragoza, Spain) during the last 24 years, with drastic changes of temperatures during winters and spring, as well as a high variability between years. In these conditions there is an excess of chill offer and most almond cultivars satisfy their chilling requirements very early, during the first two weeks of December, when endodormancy finishes. As a consequence, blooming dates are ruled mainly by the heat requirements. Early blooming almond cultivars showed a higher variation for dates of full bloom because they are more dependent on warm temperatures during January and February, where heat accumulation is slower. Late blooming cultivars showed more stable blooming dates because have their high heat requirements are quickly satisfied by the higher temperatures during March. Cultivars with high cold and heat requirements are desirable for stable late blooming. This characteristic is essential in choosing the cultivars to be planted in a given region in relation to their climatic conditions, especially to avoid frost risks.