Chill hours for fruit trees
Apples, peaches, cherries, blueberries and many other fruit plants need a certain amount of winter cold before they will leaf out and bloom properly. That cold is counted in chill hours. This guide covers why the trees need it, what goes wrong without enough, how chill is counted, and how to match a variety to your winters. The math is the same as the Chill Hour Calculator.
Dormancy
A peach tree forms next year's leaf and flower buds in summer. As the days shorten and cool in fall, those buds go dormant. Dormant buds can take temperatures far below freezing, and they do not start growing during a warm spell in midwinter.
To come out of dormancy, the buds need a set amount of cold. Once they have had it, they grow when spring warms up. Texas A&M researchers call that cold chilling, and they note that most people agree temperatures below freezing or above 60 °F do not count toward it. Each variety has its own chill requirement, given in hours.
Too little chill
A tree that gets too little chill grows poorly. Texas A&M lists three signs:
UF/IFAS Extension adds that warm spells also matter. If temperatures stay above 60 to 70 °F for long periods, the chilling effect can be lost.
Counting chill
Nobody has pinned down exactly which temperatures count, so there are several ways to count chill. Texas A&M describes the three most common:
| Model | What counts |
|---|---|
| Below 45 | 1 per hour at 45 °F or less |
| 32 to 45 | 1 per hour from 32 to 45 °F |
| Utah | Weighted, see below |
The Utah model gives each hour a weight, and warm hours take chill away:
| Hour at | Chill units |
|---|---|
| Below 35 °F | 0 |
| 35 to 36 | 0.5 |
| 37 to 48 | 1 |
| 49 to 54 | 0.5 |
| 55 to 60 | 0 |
| 61 to 65 | −0.5 |
| Over 65 | −1 |
Utah model, as given by Byrne and Bacon, Texas A&M.
The same day can score very differently under each model. Here is one mild day, a low of 40 and a high of 70:
Try it
Set a winter day's high and low
A hard-freezing day shows the other split. A low of 20 and a high of 45 gives 24 hours below 45 but only about 12 hours from 32 to 45, since the hours below freezing do not count in that model.
Comparing a variety's chill rating with a total counted by a different model. A rating in Utah units and a total in hours below 45 are not the same number. Check which model the nursery or extension service used before deciding a variety will work.
Quick check
A winter day runs from a low of 50 to a high of 75 °F. What does it add to the Utah total?
Answer: It takes chill away, about 9 units. The night only drops into the 0.5 band, and the long afternoon above 60 counts against the total. Under the hour counts, the same day just adds zero.
Chill ratings
Chill requirements vary by crop and even more by variety. Here are typical ranges. The calculator uses the same ones:
| Crop | Hours |
|---|---|
| Fig, pomegranate | 100 to 200 |
| Persimmon | 200 to 400 |
| Strawberry | 200 to 400 |
| Blueberry, southern | 150 to 700 |
| Peach | 200 to 800 |
| Apple | 200 to 1,000 |
| Cherry | 700 to 800 |
| Blueberry, northern | about 800 |
| Plum, European | 800 to 900 |
| Currant, gooseberry | 800 to 1,000 |
UF/IFAS Extension where its table has the crop, otherwise the Midwestern Regional Climate Center's list.
The ranges are wide because breeders have made low-chill varieties for warm places. A 200-hour apple and a 1,000-hour apple are both apples, and only the first fits a winter with 300 hours of chill. Find the variety's own rating on the tag or in the nursery's catalog.
Your winter
To pick a variety, you need to know how much chill your winters give. The calculator counts it from your local weather, starting October 1. For the southern US, Texas A&M also gives a quick estimate from the average temperature of the coldest month or months:
Chill = 4,280 − 68.8 × the average of the December and January mean temperatures
Their example is Williamson County, Texas, in the winter of 2005 to 2006. December averaged 42.6 °F and January 51.5. The average of the two is 47.05, so the winter gave about 4,280 − 3,237 = 1,043 chill units.
Where January averages 48 °F or warmer, the January mean alone works best. In a year when December and January differ by more than 6 degrees, use the two-month form above instead:
Texas A&M's January mean model. They found it accurate in Texas from the Lower Rio Grande Valley to the Red River, and expect it to work across the Southeast. Below a 48 °F January they use the December and January form. Its answer is in chill units, so set it beside an hour rating only as a rough guide.
Plan around a typical winter, not a cold one. A variety rated right at your average will come up short in a warm year.
Summary
- Dormant buds need a set amount of winter cold before they leaf out and bloom well.
- Too little chill means late, sparse leaves, a long late bloom with poor fruit set, and softer fruit.
- Hours below 45, hours from 32 to 45 and Utah units are different counts. Compare a rating only with a total from the same model.
- Under the Utah model, warm afternoons above 60 °F take chill back.
- Choose the variety by its own chill rating, not the crop's range.
Sources
University extension services, the same ones the Chill Hour Calculator uses.
- Byrne, D.H. and Bacon, T., Texas A&M University, "Chilling Accumulation: its Importance and Estimation" ↗. Dormancy, the signs of too little chill, the three models and the Utah weights, the mean temperature model and the Williamson County example.
- Rezazadeh, A., UF/IFAS Extension St. Lucie County (2024), "Chilling Requirement and its Importance for Fruit Trees" ↗. Chill hours by crop, and long spells above 60 to 70 °F.
- Midwestern Regional Climate Center, Purdue University, "About the Chilling Hours Tool" ↗. Chill hours by crop for those UF/IFAS does not list, from Raintree Nursery.
Keep reading
Mulder's chart
The nutrient wheel every grow forum reposts, with a source behind each line. Tap a nutrient to see what it blocks, what it helps, and which lines are folklore.
14 min read GardenDiagnosing plant problems
Before blaming a pest, check the pattern. Which plants are hit, where, and how fast tells you whether the cause is living or not.
6 min read IndexAll Bizon guides
Everything published so far, and what is being written next.
Browse the guides