UC Davis

Models to predict bloom timing in deciduous trees have, to date, focused entirely on the effects of temperature, including the interaction between both chill and heat accumulation during eco-dormancy. Recent research has confirmed however that drought stress during either of these dormancy periods can substantially delay bloom. A one-year experiment was performed using potted almond (Prunus dulcis) trees exposed to different levels of drought stress at different dormancy stages to extend these results. Our results confirmed that winter drought stress caused a delay in the development of flower buds that led to a delay in the start of bloom. It also confirmed that this delay, expressed as growing degree hours (GDH) past the completion of chilling for the start of bloom, was a linear function of integrated stress, expressed as bardays, over the entire period from the beginning of chill accumulation, to approximately the beginning of bud swelling. However, stress at or after bud swell was associated with a much longer delay in flowering than expected based on this relationship. Since this is the period of GDH accumulation, and it has previously been suggested that a stress factor during this time to account for increased GDH requirements for bloom (i.e., a decrease in effective GDH accumulation), we were able to estimate this factor to be about a 4% reduction in GDH accumulation per 100 bardays of water stress. We did not find a stress-related reduction in the number of buds that opened into flowers, but did find a statistically significant increase in fruit set for trees delayed due to stress at or after bud swell, presumably as a result of more favorable temperature conditions for pollination/fertilization. These results demonstrate the importance of monitoring SWP during the winter to support decision making for the management of winter/early spring irrigation.