Describing the Progression of Almond Bloom Using Accumulated Heat Units

1. The duration of the blossoming period and fraction of open blossoms over time are defined as a function of growing degree-days for five almond (Prunus dulcis) cultivars. Separate base temperature estimates and equations describing the progression of bloom are derived for each cultivar. 2. Earlier blooming cultivars had lower base temperatures for bloom progression than cultivars blooming later. 3. Equations that were derived from 1987 and 1988 data predicted the progression of bloom for each cultivar in 1989 and 1990 with greater accuracy during the pre-peak bloom than in the post-peak bloom period. 4. The use of base temperature and bloom progression equations, in combination with chill unit requirements to break rest and with heat unit requirements to start bloom, can provide a basis for choosing cultivar combinations that will enhance cross-pollination and nut set.