Describing the progression of almond bloom using accumulated heat units.
The duration of the flowering period and the fraction of open flowers over time were defined as functions of growing degree-days for 5 almond cultivars (Neplus, Nonpareil, Price, Peerless and Mission) growing in a commercial orchard near Dixon, California, USA. Separate base temperature estimates and equations describing the progression of flowering were derived for each cultivar. Earlier flowering cultivars had lower base temperatures for flowering progression than cultivars flowering later. Equations that were derived from 1987 and 1988 data predicted the progression of flowering for each cultivar in 1989 and 1990 with greater accuracy during pre-peak flowering than during post-peak flowering. The use of base temperature and flowering progression equations, in combination with chill unit requirements to break dormancy and with heat unit requirements to initiate flowering, can provide a basis for choosing cultivar combinations that will enhance cross pollination and nut set.