Chill Hours

Everyone who is interested in growing their own fruit will eventually be faced with the issue of “chill hours”, or chill units (CU) – the terms are interchangeable.  Although CUs are sometimes stated in simple terms, any venture into studying CUs will quickly reveal that the subject is more complicated than it first appears.

The complications come from the fact that researchers are still trying to understand the process of dormancy and the part that CUs play in the equation.  To further confuse matters there are several different models used to calculate chill, and each model has its own definition of what constitutes a CU.

A simplified explanation
Dormancy is a state of suspended growth & development.  It is triggered by lengthening nights and decreasing temperatures during fall and early winter.  Physiological changes take place as a plant enters dormancy that allow the cells to survive without damage even when they are exposed to very low temperatures.  We can think of this as nature’s anti-freeze system.

To negate the effect of these physiological changes and begin new growth, the plant must be exposed to temperatures between 32º F and 45º F for a fairly specific period of time.  The length of time required varies from plant to plant depending on the climate to which they have become adapted through evolution, selection, or breeding.

Once this length of time has passed, the plant is technically released from dormancy and can resume normal growth once cultural conditions have become favorable.  Soil temperature is the driving factor.  The soil temperature only needs to rise above 45º F for 3 – 4 days to initiate new growth.  If soil temperatures remain low, the plant may remain suspended in a dormant state.

How CU’s affect variety selection
Most stone fruits (peaches, plums, cherries) and pome fruits (apples, pears, quince) have a minimum chill requirement.  Even some bush fruits, such as blueberries and blackberries, have specific chill requirements.  It is very important to know your Chill Hours Average and to select only those varieties that are expected to grow and produce fruit in that range.  Keep in mind that these are averages and that yearly extremes may temporarily affect production.

If a variety requires significantly more CUs than your area can expect, it may never bloom or set fruit.  If a variety requires significantly fewer chill hours than your area can expect, it may break dormancy and begin to bloom too early after a series of mild days.

If a hard freeze occurs during this early bloom, it will damage or kill the flowers and no fruit will be set. Damage may also occur to new leaves and tender tissue. You should keep frost cloth in several weights on hand when growing fruits in the lower CU ranges.  If killing frosts or freezing weather threaten a tree that has broken dormancy, the frost cloth may protect the blooms sufficiently to set fruit.

You can often locate micro-climates within your yard that vary by as much as 7º F – 10º F.  This may permit you to push your Chill Hours Average one way or the other in these areas.

How to determine your Chill Hours Average
There is an ongoing debate about CU definitions and which model to use.  We are going to leave that debate to others and use one of the oldest and most commonly accepted models to determine total average chill hours in our area:

A Chill Unit is an hour of air temperature between 32ºF and 45ºF, minus all hours above 60ºF.

It is generally accepted that temperatures below 32ºF do not contribute to CUs and that temperatures above 60ºF detract from CUs.  Therefore an hour is subtracted for every hour above 60º F and hours below 32º F are not counted.

The total number of CUs accumulated in an area during an average winter determines the Chill Hours Average for that area.  Chill Hours do not have to be continuous.  They are an accumulation of hours within these temperatures.

Some averages in our area:

 Gulf & Bay Area  ≤300
 Hobby Area  ≤300
 Inner City  ≤300
 Pasadena-South Bay  350 – 450
 Harris County (other than above)  400 – 600
 Fort Bend County  400 – 600
 Cypress-Bear Creek  ≤600
 Counties north of Harris  600 – 900

Finding out how many chill units you have accumulated
While the above chart gives you a guideline to averages, chill units can vary from year to year.  Some local weather stations track and report chill.  This website assists you getting a closer estimate of the chill hours you have actually accumulated in a given season: Get Chill.

There is a two step process.  First, click on the WunderMap link and find a weather station closest to you.  Click on that station, and find the station ID.  Copy that ID number, go back to Get Chill, type or drop the station ID into the box, and click “calculate chill”.  In a few seconds of searching, you will get the current number for that station.  Not all stations record chill.  You may have to repeat this for other stations in your area.

What about Zones?
You may find that some fruit tree nurseries do not readily make the required chill hours known on their labels or in their catalogs.  Some list the USDA Zone only.  This is not sufficient information to select a successful fruit variety for your area.

CUs are moderated by certain climate and environmental features such as elevation, large bodies of water, prevailing winds, and exposure.  These features vary widely across each USDA Zone.  Not all Zones with the same USDA number are created equal either.  Zone 9 Texas and Zone 9 California are two completely different climates.

If the CUs are not listed for a variety you are interested in, ask.  If they do not freely offer the information, look elsewhere such as the internet or a reliable garden book designed for your area.

While I do not recommend purchasing a tree if you do not know the CUs, I do recommend experimenting to see what will work in certain micro-climates in your own yard.  You are only limited by your budget, your patience, and your tolerance for failure.  Either way, you will learn from the experience, and will become a better fruit gardener.

If a variety proves to be a total failure, all is still not lost.  If the trunk is growing vigorously and is healthy, graft a variety known to be successful and prune out the unsuccessful experiment as the newly grafted variety grows.

Symptoms of insufficient chill
If you have inherited fruit trees with your property, or have planted a fruit tree in your yard that is not producing, here are symptoms to look for to determine whether insufficient CUs are the problem:

Delayed Foliation – Small tufts of leaves will appear at the tips of the stems in early spring, but the stem will be devoid of leaves for 12 – 20 inches below the tips.  The lower leaf buds will eventually break, but full foliation is delayed, fruit set is reduced, and the tree is generally weakened.

Reduced Fruit Set – Flowering follows the same pattern as delayed foliation.  The delayed and extended flowering can lead to abnormalities in pollination and fruit set is reduced.  Any fruit that does set may remain small and misshapen as they ripen.

Poor Fruit Quality – Some varieties of stone fruits may have an enlarged tip and reduced firmness.  Color may also be affected – the fruit may remain greener and not develop full color.  Other fruits may remain small regardless of thinning or other good cultural practices.

If your unnamed trees exhibit any of these symptoms, it is probable that they require more chill hours than your area provides.  If you suspect that this is the case and your tree is otherwise healthy, you may notneed to remove and replace it.  Seek the advice of a local grafter to see if a more suitable variety can be grafted onto the existing tree.  The grafted variety will benefit from the size and vigor of the existing root ball.

Taking a deeper look at Chill Hours
As mentioned above, there is an ongoing debate about just what defines an hour of chill, and what model to use.  It’s like discussing compost, or soil preparation methods, or which tomato is the tastiest.  Get two gardeners together, and you are sure to strike up a lively debate any time these subjects are brought up.  I think all lively discussion has merit, as long as no one gets bonked in the head with a shovel!

I have tried to digest the hours of research I have done on this subject into a few palatable paragraphs, but I do encourage further reading.  A caveat – it can get tricky out there.  For example, there is a chart out there that uses mean January temperatures to determine CUs.  According to this chart, my area should get 800 hours of chill (our mean January temperature is 51.75).  Trust me, this is never going to happen!  I am lucky, and grateful, to get maybe 300 – 350 in an average winter.

There are three major models used to determine chill, and several minor models.  All authors of these models present their cases about why theirs is more valid than another.  The three you will see most often are:

The 32º F – 45º F model
The ≤ 45º F model
The Utah model

There are others as well, including assorted US charts that seem to draw their numbers from somewhere in Oz.  One shows my area at 800 CU average, and one shows it as 200 CU average, but both are labeled “US Chill Hours Charts” and are proudly displayed on fruit tree nursery websites by well-meaning and sincere people.  Even the official Texas A & M charts vary from reference to reference, and none of them are truly accurate for my area.

You have to always compare what you study to what you observe and experience.  If a tree does not bloom and produce fruit for you, no model, chart, or formula is going to resolve this problem!

Some recommended additional reading:

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