Forced Defoliation

Anyone out there have any experience with forcing defoliation with zinc sulfate and/or urea to encourage chill accumulation in the early season in low chill areas? We are going to be doing some experimentation this year on various peaches. The outcome we would like to see is a tighter/more compact bloom (and better set) in certain varieties that are borderline for our area (Phoenix)

Some of our problem as I see it is the lack of a early frost like so many traditional peach growing areas. Trees take lot of the cues to go dormant from that first frost. The issue here is that we usually dont see a frost until sometime in December, but none the less starting in November we start hit temperature ranges that accumulate chill hours. So are these chill hours lost on wood that isnt fully dormant? I believe so. Will chemical defoliation force them to uptake these otherwise lost chill hours? Now thats the question.

Last year, I started some Allegheny Chinquapins from nuts. This was my first experience with these and I did not vernalize the nuts after germination and before top-growth. I started them in rootmaker 18s under lights indoors. After 12-16 weeks they needed to be transplanted to larger containers but I didn’t have room for a couple hundred 1 gal containers.

So, I tried to force dormancy. I cut leaves in have and slowly removed them over time putting the seedlings into a cold room.

They all died. In hindsight, I think the problem was my timing. With seedlings only 16 weeks old, they did not properly harden before I tried to force dormancy.

This year, after planting the nuts in 18s and allowing the root radicle to grow a bit, I put the tray in a large ziplock storage bag and put it in my fridge to vernalize before top growth began.

My experience with forcing dormancy was not good, but it was a last ditch effort rather than part of a well thought out plan.

My greenhouse has been on chill cycle for 2.5 weeks. It went from 95F and in full leaf to chilling in one day. So far about 200 Utah hrs. Now I don’t know for sure that those hrs are fully effective. Shoot I don’t know that they are effective at all. But the leaves are turning color and beginning to fall. Full defoliation of all trees doesn’t usually happen until mid Dec. I’m planning to end chilling soon thereafter.

My thinking is chilling can occur on trees in full leaf. It can’t hurt to defoliate now. Just not sure it will help.

The chill models begin accumulating hrs Nov 1 or even before. The people running those models don’t seem to care if tree has lost it’s leaves.

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Interesting that in your frost free greenhouse that you dont see total defoliation until Mid December also. Im going to have to do some looking around to see if any science has been done on when chill accumulation starts.

It’s clear you don’t need frost before chilling can occur. My greenhouse hasn’t had frost in 11 yrs. And light frost doesn’t cause defoliation on my pome trees and even some stone fruits inside or out. I think it’s pretty clear that chilling can begin on trees still in leaf.

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“Entry into and exit from bud dormancy are often gradual transitions rather than abrupt events. Some researchers have represented these phases as sine wave oscillations, with measurable reference points (e.g. peak growth rate in summer and maximum dormancy in midwinter) which enable comparison of data from different sites (Fuchigami and Nee 1987).”

So thats a fascinating thought. Does a chill hour gained early in this transition equal the same as one later on when more fully in dormancy?

Grower experience says chilling is more effective in fall than spring. I’ve read that many times. Past January in low chill climates and it’s too late. In CA, Nov and Dec are prime chilling. This would be especially true in the warmer areas similar to Phoenix.

""So, why do peach trees in
subtropical regions need defoliation
in the late fall or early winter? The two
main reasons are to allow the fruit buds
to be receptive to cool temperatures and
chill unit accumulation, and as a tool to
manipulate bloom time the following
calendar year. “”

Thats from the article I posted originally. At least in the instance of Florida’s low chill, science there does seem to have some belief that defoliation DOES help push wood into dormancy and help chill accum.

And also from that article

"Previous research has
shown that if leaves have not detached
from the shoot, chill units are not as
effective as if the shoot was defoliated. "

Well then knock those leaves off. I’m going to get 800-1000 Utah hours so don’t see it as necessary. I’ll probably be traveling for Xmas so chilling will likely continue until nearly Jan 1. That would be 10 weeks between 37 and 64. Experience tells me that’s what I can do. Have held 64 max thru 78-81F on numerous days so far. Not exactly Phoenix weather.

will probably stick to Zn SO4 application

in the tropics, foliar potassium nitrate and/or urea are used on mangos, to simulate a change in season and be more productive per unit time(unlike bananas, mangos are typically seasonal, following rainy season patterns and undergoing dormancy without losing their leaves). Trouble with nitrogen-based foliar applications is that it could be outright lethal to mangos, and known to shorten the lifespan/productive years of the trees-sometimes bearing one bumper crop, then dying thereafter.

maybe i am missing something, but why would urea even be necessary? Nitrogen supposedly makes the trees more resistant to defoliation.

So, are you cooling your greenhouse to artificially introduce chilling Steven?

On a somewhat different, but related topic:
In a typical natural environment, is there any energy transported from the leaves downward to the root system for storage? In other words, does the onset of natural fall weather initiate any sort of change to capture any energy source contained in the foliar tissue? I’ve always kinda thought it did, but the more I think about it, I don’t recall ever actually reading it anywhere.
Reading the discussion here and Amadio’s query made me think about this.

Interesting discussion. I don’t have to worry about my chilling of stone fruit here but my tropical plants and cacti require chilling to make it through the winter.

I have a pair of Fuji apples side by side and did experiments on leaf stripping, pulling off the leaves on one in mid-November and letting the other one drop naturally. It had no effect, they both blossomed the same time.


I use shading and evaporative cooling to lower daytime temperature inside the greenhouse. This allows as much as 17F cooler but usually 10-12F cooler midday. Most days I can hold the high temp below 60F which is the temperature at which chilling goes negative. At night I heat to 37-39F which gives a lot of hrs in the ideal 37-45F range. From here until January I can average ~18 Utah hrs per day. Ave outdoor temps will be ~65/33F the next two months.

There probably is a little energy recovery from the leaves but mostly nutrient recovery like NPK. However that wouldn’t be any reason to avoid defoliation in a long season area like Phoenix. Trees there should be pumped full of energy reserves by now. In a short season area on a fruit like Goldrush it might be a different matter. The nutrients can be easily replaced by fertilizing. Sometimes by foliar feeding in fall.

Trees that have been recently fertilized with nitrogen are resistant to going dormant, That was the point of them mentioning that in the original article. But foliar urea along with zinc defloiation seems to have some positive effects in spring budburst according to research.

it depends on what season of the year, but for the most part, energy is transported from the leaves down to the roots. Roots and most lignified stems are parasitic energy-hoarding ‘animals’. And these really are like animals, as the living parts of roots and stems ‘breathe in’ oxygen, and breathe out CO2, as they cannot utilize the energy in carbohydrates without oxygen. Just as CO2 and CO are toxic to animals, the same gases are toxic to roots, being waste products of respiration.
leaves also utilize O2 to utilize the carbs they make, but being the solar panels that they are, they produce more chemical energy from CO2 and sunlight in the form of sugars(which includes wood), hence the carbon in CO2 is locked in, and this makes foliage a net producer of oxygen, and a net absorber of CO2.

as for the season of the year mentioned, roots and stems become the ‘givers’ during spring when leaves have yet to sprout and develop. Think maple sap --or that sweet potato one left on the kitchen counter which started sprouting–it is sugar energy amassed from the foliage from the previous year’s production. When leafing out, the buds must rely on the previous year’s storage to develop, but once the foliage are up and running, the stems/roots reverse roles with the leaves, with the latter again resuming the major role of ‘giver’, and the former resuming the major role as ‘taker’.

only time when roots will reverse roles in late spring, summer, or early fall is if you intentionally remove all green parts of the plant. Or, say, you cut down a tree into a leafless stump. Developing buds will have to tap the energy from stems and roots anew.


personally, will probably just apply nitrogen to the roots, and use zinc as defoliating foliar agent.

btw, not sure if you’ve tried this before-- for extending chilling time/dormancy, lowland farmers in some subtropical countries, they buy fruiting-spur budwood from farmers in higher/colder elevations, and put them in chillers, and graft onto their local trees in spring. Following this line of thought, and using grafting shears, you could just trim the budwood from your trees when they are dormant, put them in the fridge, then regraft onto the same branches in spring, with the added convenience of not having to worry about caliper differences :smile:
obviously not cost-effective nor labor-efficient, but subsequent bumper crops of cherries and high-chill peaches will be a picture easily finding a spot on the front page of your local newspaper.

I was reading through a rather long and detailed PDF on cherry growing practices in Australia when I encountered the section listed below. Note the sentence in bold.

To determine when “chilling” commences the model needs to be run continuously
because changes in the positive and negative values recorded by the model
indicate when the colder temperatures start to have an impact on the peak negative
(hot) value accumulated through Spring - Summer - Autumn.
Temperatures below freezing have no influence on the accumulation of chill units.
Chilling is not regarded as commencing until trees have lost all of their
previous seasons leaves.
Since the development of the Richardson Model several other models have been
developed around the world to address the issue of chill accumulation. One of these
models was developed in Israel (Erez Chill Model) by Dr Amnon Erez. This model
uses the terminology of “Chill Portions” as compared to “Chill Hours” in the
Richardson Model.

i agree. Case in point would be jujubes, which flower and fruit continuously beginning spring up to fall(especially in warm areas).

Come fall, the more cold-hardy varieties will often still be with full fronds of leaves and fruits pending maturity , whereas the other varieties have already dropped their leaves. Definitely not in dormancy if they are still nursing their fruits to maturity…