Questions about dormancy in California almonds
I see what seems like a lot of leaves still left in some orchards around the Sacramento Valley, even after a wet and windy end to 2012. Aldrich trees look to have the most leaves remaining. This observation got me thinking about questions I’ve heard regarding leaves, dormancy, chilling and bloom timing/duration. These questions include:
- Are trees with leaves remaining in January less dormant or somehow different than trees that are defoliated or naturally bare by January?
- Doesn’t spraying trees with zinc in the fall make them dormant faster? They don’t have leaves…
- Do trees with some leaves left in the canopy in January accumulate less chilling than trees with no leaves in December?
I can’t find answers to these questions in past almond research reports from California. I’m not sure anyone has done the research to answer these questions for almond (good Extension research project, Franz) but I’ve done a bit of digging in my references and old notes from grad school and here are my stabs at answering these questions. Next fall I’ll do some simple experiments in an effort to be able to answer these questions from experience. In the meantime, here’s what the experts say…
Deciduous trees have evolved mechanisms (winter dormancy and cold hardiness) to avoid damage during prolonged cold (winter). We will focus on dormancy here, as cold hardiness is rarely an issue in almonds in California.
Dormancy is the lack of bud growth (extension) and the presence of leaves doesn’t constitute dormancy. Bud growth is controlled either by 1) processes and materials within the buds themselves or 2) environmental conditions outside the bud that limit process within the bud (low temperature, water stress, etc.). For example, during the summer, water stressed buds don’t expand or grow until the water stress is relieved. Winter dormancy is a combination of two stages – endodormancy and ecodormancy.
Endodormancy is triggered by shortening day length and/or lower temperatures in the fall, not the loss of leaves. In late October, buds of well watered, fruit or nut tree shoots don’t grow – are dormant — if topped despite the presence of leaves. Endodormancy is controlled by unknown compounds or combinations of compounds within the buds. After the accumulation of a certain amount of hours of cool temperatures above freezing — after the chilling requirement is met — endodormancy ends, or, more probably fades out over a certain period of time. Commercial apples trees native to very cold regions of the world have a high chilling requirement (up to 2,000 hours below 45oF) and almonds have one of the lowest chilling requirements (300-600 hours under 45oF). [Note: There are several different models for chilling measurement. I’ll stick with the traditional units of hours under 45oF. This works well in CA, even though the Dynamic Model, developed in Israel, is the most biologically accurate model currently available.]