Dormancy and Early Bloom to Start 2014

How is it that our peach trees are already blooming in the south central part of the state?  It seems like we are too early by at least three weeks in many cases.  It makes for a busy holiday season watching our weather to make sure that we stay on top of frost protection.  A current analysis of the chill accumulation across our state doesn’t look good – keeping in mind that last year was also a warm year and winter.  See AgroClimate:

Accumulated Chill Units in Florida, January 1, 2014.

As a grower, what is there to do?  Well, first I think it would be good to start out with an explanation of the types of dormancy that perennial trees go through (Lang et al., 1987). 

Growth cessation and dormancy in peaches is driven by reductions in the photoperiod.  Photoperiod is the amount of light that we get during the day vs. the night period, and as we approach the autumn, the light period decreases and the peach trees respond to this by dropping their leaves (defoliation). 

Ecodormancy is where the tree stops the visible growth process and goes into a reduced state of metabolic activity because of an environmental stress.  This could be water stress (we shut off the water around the 1st of November to hasten this process) or cold temperatures (happens up to the north quite a bit). 

Paradormancy is a type of dormancy where physiological factors outside of the bud affect its growth and development, and this might be the production of auxin (a plant hormone that contributes to apical dominance), or a reduction in photoperiod during the season. 

Endodormancy is a period of dormancy that is induced within the flower or vegetative bud, and typically includes chilling responses and photoperiodic responses.  We see this type of dormancy affect vegetative buds, when we don’t get enough chilling and the resulting response is delayed vegetative bud break, or leafing. 

There are two main types of chill accumulation models that can help growers to track units of chill that are accumulated at night.  One is the standard chill accumulation model and the other is the Utah chill accumulation model

The standard chill accumulation model is one in which 1 hour at a temperature less than 45°F (7.2°C) equals 1 unit of chill accumulation (Weinberger, 1950).  In this model, any temperature, even those below 32°F are considered temperatures at which chill units are accumulated.  However, this doesn’t work that well for peaches, and a researcher in Utah working with ‘Elberta’ and ‘Redhaven’ peaches delved deeper into this subject and devised a second model (Richardson et al., 1974). 

The second model, and the one which we use for peaches, is called the Utah chill model.  In this model, an ideal range for chill accumulation was established for peach buds to come out of “endodormancy”.  For each hour at the following temperature ranges, a chill unit, or portion of a chill unit is accumulated.  These temperatures are:

chill table

In the Utah chill model, you can see that there is an ideal range at which 1 chill unit is accumulated, and then portions of chill units are accumulated at higher and lower temperatures.  So, although we may be above the ideal range (36.5 – 48.4°F), we still have the opportunity to accumulate half-units of chill during our warm nights in Florida.  However, the caveat with this model is: if it is too warm immediately after chilling is accumulated, warm temperatures can negate the chilling from the previous two nights.  Chilling accumulated prior to this warm period and the previous 2 nights remain valid within our working model. 

We can also observe fruit set problems when these warm nighttime temperatures occur during the bloom period with some of our peach varieties, notably, ‘UFOne’ and ‘UFBeauty’ in warm areas of Florida. 

The AgroClimate site which covers chill accumulation ( offers two options when you choose a weather station:  <45°F, and 32-45°F.  Although not quite within the ranges that we need to calculate the proper chill accumulation with half-units, the 32-45°F model does a pretty good job of an estimation.  So, be sure to choose this one when tracking your chill unit accumulation this winter. 

For those of you that use Weather Underground, I found a neat tool that calculates the chill accumulation for a particular station.  It’s called Get Chill Hours! (, and calculates the chill accumulation with three different models.  One caution:  when the model calculates the Utah Chill Model accumulation, it will most likely be negative because of our warm nights interspersed with our nights that accumulate chill.  Thus, the 32-45°F is again a better depiction of our chill accumulation with certain caveats as mentioned in the Utah Chill Model explanation. 

Results of Low Chill Accumulation?

So, now understanding some of the background about dormancy, you can see that in South Central Florida with our low-chill peach varieties, trees in most years appear to not enter into endodormancy, which can hinder the ability of the tree to push vegetative buds when they should.  Observation of trees in commercial orchards indicate that trees enter into ecodormancy in late October and early November when irrigation and fertilizer are reduced in the orchard. 

Warm temperatures during the past three years in December have resulted in trees flushing and blooming approximately December 15th without significant accumulation of chill units.  Fruit set in varieties like ‘UFSun’, ‘Flordaglo’, ‘UFBest’, and ‘TropicBeauty’ has been excellent under these conditions. 

The result of not leafing shortly after bloom or having the delay of the vegetative buds push, is that the tree must rely on stored carbohydrate and nitrogen reserves to produce fruit, and both cell division (which affects fruit size) and the cell enlargement (when sugars and peach flavors get transported and develop in the fruit) can be reduced, ultimately producing a “flat” tasting fruit.  The leaves, through photosynthesis, produce the sugars and flavor compounds needed to give the fruit that “tree-ripe” flavor that we sell to the consumers. 

A second result of low chill unit accumulation is an extended bloom period.  This results in an extended fruit set period, and thus thinning must be done several times during the late winter/early spring, rather than once during the thinning period.  Multiple trips through the orchard results in more labor dollars spent, reducing the orchard’s bottom line. 

What can be done?

All of this is wonderful information, but how will it affect how we grow this year’s crop?  In years with low chill unit accumulation like this one, experience tells us:

  1. The trees will bloom early.  Be sure that your frost protection systems work, and you are ready for any cold fronts that sweep down the state. 
  2. Because the trees are blooming early, if you have >50% bloom, start watering and fertilizing.  Stress during the fruit set and cell division period will result in fewer and smaller fruit. 
  3. Be sure to know what your fruit developmental period (FDP) for your peach variety.  These ranges still hold true for overall fruit development, but cool, rainy weather during the spring will delay any gains in the bloom period that we have achieved.  In addition, if you bloom around January 1st you will most likely harvest ‘UFSun’ around April 1st.  But, if you bloom around January 20th, then you would probably harvest ‘UFSun’ around mid-April – because it is warmer from Jan 20 – April 15th then from January 1 – April 1st.  Warmer temperatures will compress the fruit developmental period. 
  4. If you have fruit on the trees, start your disease and insect protection programs along with your irrigation and fertilization programs. 

In the future, we hope to gain some knowledge about the use of rest-breaking chemicals like hydrogen cyanimide (Dormex® or BudPro®); however, with the products there is a great potential to do significant bud or branch damage with high concentrations and/or application at the wrong time.  In cooperation with a few growers around the state, I’m tracking the results of various applications and rates and will have more to report by the end of this year. 

Cheers and Happy New Year!


Fraisse, C. and A. Whidden.  2010.  Chill accumulation monitoring and forecasting.  Available at:

Lang, G.A., J.D. Early, G.C. Martin, and R.L. Darnell. 1987. Endo-, para-, and ecodormancy:  Physiological terminology and classification for dormancy research. HortScience 22:371-377.

Richardson, E.A., S.D. Seeley, and D.R. Walker. 1974. A model for estimating the completion of rest for ‘Redhaven’ and ‘Elberta’ peach trees. HortScience 9:331-332.

Weinberger, J.H.  1950.  Chilling requirements of peach varieties.  Proc. Amer. Soc. Hort. Sci.  56:122-128.