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?
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 cooler 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 2000 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.]
Once they have the capacity to grow, once endodormancy is over, buds need a certain amount of heat units to begin growing. This dormancy is imposed by external conditions and called ecodormancy by the plant physiologists. I’ll bet, as of January 15, that almonds in the Central Valley have passed through endodormancy and are in ecodormancy. We are currently running between 600-800 hours under 45oF in the Sacramento and San Joaquin Valleys (check for chilling accumulation levels throughout CA at: www.fruitsandnuts.ucdavis.edu/Weather_Services). If endodormancy has passed and the chilling requirement met, buds on a shoot cut from the orchard and placed in water in a warm place will “break”, will expand and grow. The more chilling, the shorter the heat accumulation needed to bloom.
So, after all that, what about the questions posed at the beginning of this post?
- Are trees with leaves remaining in January less dormant or somehow different than trees that are defoliated or naturally bare by January? No, they should be the same.
- Doesn’t spraying trees with zinc in the fall make them dormant faster? They don’t have leaves… They don’t become dormant faster. Endodormancy is triggered by shorter day length and/or cooler temperatures, not the absence of leaves.
- Do trees with some leaves left in the canopy in January accumulate less chilling than trees with no leaves in December? No, chilling accumulation is related to temperatures and conditions in the buds, not the leaves.
Note, removing leaves in the fall or winter, is a part of an integrated disease management program in almonds.