Do you dream about apple trees laden with fruit, a freezer stocked with home-grown blueberries and frozen pomegranate juice, or cherry pies baked with cherries picked off the tree in your backyard? When the new seed catalogs arrive, do you stare longingly at the pears and figs and think about whether they belong in your front yard or if you should start an orchard? If so, you’ve probably seen the phrase “chill hours” buried in the description.
At its simplest, chill hours, also known as chilling units or chilling hours, measure the hours with temperatures below 45 ⁰F. Another measure uses hours between 32 and 45 ⁰F. The Utah Chill Unit Model and Dynamic Model are both more accurate and more complex.
Think about chill hours in terms of dormancy. During the first stage (acclimation), you’ve already harvested your crop and fall is setting in. Winter arrives just as your trees enter endo-dormancy. During this phase, it racks up chill hours. After it receives enough chill hours, it enters eco-dormancy and breaks bud. As temperatures rise above 40 ⁰F, it starts a new tally: growing-degree days. Once it’s received enough chill hours and enough growing-degree days, it assumes spring’s arrived and starts growing.
The good news is that until your tree receives the necessary chill hours, it won’t bud out. That’s also the bad news.
If you plant cherry trees in Florida, they’ll grow. The low chill varieties might produce a few cherries. Just don’t get your heart set on pounds of cherries picked from the cherry tree you’ve lovingly tended for the last five years. The same applies to growing oranges in Wisconsin. It goes double for the pistachio tree I’ve always wanted.
Find out your chill hours before ordering any fruit or nut trees and, preferably, before planning your orchard.
Finding Your Chill Hours
Call your county extension office. They’ll know the average chill hours for your area and may already have a list of recommended varieties with a proven local track record.
Another option is AgroClimate’s Chill Hours Calculator. Enter your zip code and set the chill hours model to 32-45 ⁰F. Then set the end date for the projected period to a year. (For instance, start 10/01/2018 and end 09/30/2019.) Under graph options, click display historical average and display last season. Now, select the USDA research station nearest you from the map and open the Total Accumulated and Projected tab.
Do you mind a little risk? Do you want the earliest possible fruit even if it means misting your precious apple blossoms with water at 5 AM to protect them from frost? Having done this, I don’t recommend the before sunrise (and morning coffee) water hose routine.
If you don’t mind installing an overhead sprinkler system or waking up before the rooster, choose 90% probability from your state’s Freeze/Frost Occurrence Data sheet. Otherwise, go with 50% or 10%. (The lower the probability, the lower the frost risk.) Scroll down until you find the nearest city to your location and note the frost date.
Now, find your frost date on graph. The green cone shows the estimated minimum and maximum chill hours. Mouse over the top and bottom green line for these numbers.
A large grower may select multiple varieties so his orchards produce even during low-chill and high-chill years. You see this a lot with peach farmers because even 100 chill hours plus or minus the ideal growing condition can impact yields. Gardeners and market farmers should focus on the bottom. Select varieties that need a few hundred chill hours less than your available chill. That way they’ll produce during even during uncommonly warm years. Then narrow them down based on whether they flower early, mid-season, or late. Ideally, you want them to bloom after your frost date.
A Real World Example
My childhood neighbor retired from farming and took up apple growing. He grew Anna’s, Dorset Golds, horse apples, and Fujis. Every spring, he gripped because his low-chill Annas and Dorsets always bloomed before the first frost. He spent many hours outside in the cold with a water hose. That region of Georgia receives about 400-600 chill hours every year. Anna apples flower extremely early and need less than 300 chill hours. His liked to bloom in February. Most years, his Anna and Dorset Gold trees produced gorgeous blooms and frost killed the apples before they grew. His prized Fuji apples featured in many an apple cake.
Anna (apple). (2018). In Wikipedia. Retrieved from https://en.wikipedia.org/w/index.php?title=Anna_(apple)&oldid=848719303
Chill Hours Calculator – AgroClimate. (n.d.). Retrieved October 23, 2018, from http://agroclimate.org/tools/chill-hours-calculator/
Chilling accumulation: its importance and estimation. (n.d.). Retrieved October 24, 2018, from https://aggie-horticulture.tamu.edu/stonefruit/chillacc.html
Chilling Hours / First and Last Frost | Weather. (n.d.). Retrieved October 24, 2018, from https://etweather.tamu.edu/chill/