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.
cbalmnd
January 24, 2013Do you think there is any benefit to the leaves remaining on the tree. Such as increased carbohydrate production in Nov/Dec/Jan., creating stronger buds for next season?…
David Doll
January 25, 2013There probably isn’t much to gain by keeping the leaves on the tree. The processes within the tree tend to be reduced, and abscission layers often exist between the leaf petiole and point of attachment, even though the leaf is still attached.
In many cases there may be more risk than gain. The leaves on the tree serve as an overwintering location for rust (and maybe alternaria), providing a inoculum source for early infection. This is most likely why we observed rust in early May last year.
Leigh Teitz
February 21, 2013Hi David,
Do you have any thoughts on using foliar potassium nitrate to initiate or advance bud break? It is used on Australian orchards based on work done in other stone fruit and deciduous tree crops. I’m not sure if there has been any specific reasearch doen in almonds.
Regards
Leigh
David Doll
February 23, 2013Leigh, I am unsure as well. I will ask around and if I find anything, I will let you know. I do know a full irrigation profile and an application of dormant oil appears to affect the timing of bud break.
David Doll
February 23, 2013Hi Leigh:
It could be done, I think, but results from other crops suggest outcomes would be inconsistent from year to year making this approach, generally, a poor management strategy, from my experience. I have worked with a range of nitrogen materials (CAN17, KNO3, N-sure) in European plums (prunes) as well as oil to advance bloom. They all work, but the results are not consistent from year to year so these materials are challenging as management tools. Also, high rates of N applied prebloom when temperatures are high (60+oF) will burn buds and eliminate bloom. I am not aware of any such work with almonds.
I would select varieties carefully and, if problems developed, use pollen inserts instead of trying to manipulate bloom with spray materials. Almond production is all about maximizing fruit (nut) set and the nitrogen materials are basically controlled phyto when used to manipulate bloom. Oil works, but can also produce phyto if the trees are dry (as David pointed out).
Sincerely,
Franz
Leigh Teitz
February 25, 2013Thanks David & Franz for the feedback.
Regards
Leigh