What's a Chill Hour?
You're happily choosing your bare root fruit trees from our catalog when you suddenly notice extra numbers in the tree descriptions. Number of chill hours, what is that? Isn't it enough to know your USDA plant hardiness zone? That's about cold temperatures -- so why do you need another number? The USDA zone tells you the coldest temperatures in your area. Broadly speaking, the chill hours tell you how long the cold temperatures last. The traditional definition of a chill hour is any hour under 45°F. But wait, there's more. Academics have competing theories on what "chill" means. Some say the chill only counts if the temperature is between 45°F and 34°F. Some differ over chill calculations for the Utah Model, and let's not forget the new Dynamic model.... If this debate sounds like your cup of iced tea, follow those links.
How to Count Chill Hours
Here's the best way to count chill hours: get someone else to do it! Luckily, there are institutions already tracking this information. Farmers and gardeners in most California counties have access to official data on chill hours through the Pomology Weather Service at the UC Davis Fruit & Nut Research Information Center. This service records chill hours -- so you don't have to. If you're not covered by this weather service, contact your local Master Gardeners and Farm Advisors to find out your local source of chill hour information.
Chill Hours Are Important to Fruit Trees
As you know, trees cannot walk away if they don't like the weather -- they have to stay and suffer through it. Nut and fruit trees (except for citrus) need a specific number of chill hours each winter to regulate their growth. If a tree doesn't experience enough chill hours in the winter the flower buds might not open at all in spring, or they might open unevenly. In addition, the production of leaves may also be delayed. Okay, you're thinking, how about just planting trees with low chill requirements? That way they're sure to get enough cold weather. Sorry, that won't work. In our video, Fruit Trees – A Selection Guide, Tricia explains that a low-chill tree in a high-chill area would break dormancy too soon and be damaged, or even killed, by the cold weather. So be sure to match your new bare root trees' chill requirements to your local chill hours -- use the menus at our fruit tree page and find trees sorted by "Chill Hours Needed". Here are some guides to low chill (less than 300 hours) fruit trees:
- Apples – Anna, Low Chill multi-graft, Dorsett and Sundowner
- Apricots – Gold Kist or Katy
- Plums – Methley, Burgundy, Satsuma or Mariposa
- Pluot – Dapple Supreme
- Cherry – Royal Lee, Minnie Royal and Royal Crimson
- Peach – Red Baron, Low Chill multi-graft, Saturn, Babco*ck, Sauzee Swirl, Mid-Pride and Eva's Pride
- Nectarine – Spice Zee Nectaplum, Double Delight or Snow Queen
- Pears – most require over 300 chill hours. Asian pears require the lowest chill hours of all pears.
- Figs, Pomegranates, Quince, Persimmons – all require 300 or less chill hours
- Special Hybrids – Spice Zee Nectaplum or Flavor Delight Aprium
The majority of the fruit and nut trees require higher chill hours. The selection is huge, so no matter where you live, there is a perfect fruit tree to choose from. Choose wisely and look forward to an orchard that lives happily ever after. For complete orchard information see the book California gardeners rave about, The Home Orchard, written by experts from the University of California.
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I came across chill hours first about 18 months ago after our move from the snow belt to SC. I found no real official tally of the hours anywhere locally so started a spreadsheet to do so myself. I used the ‘anything under 45’ calculation because we don’t go below freezing very often. Last year was fine – I came up with 850 hours which was fine for most of my trees. This year I have seen some limitations with straight hours. We had cold at the end of November, then record warmth in December before returning to a cold January. Consequently I restarted the calculation in January. Some of my low chill trees thought Dec warmth was spring so started flowering – not many flowers but still some. In January alone I have tallied over 300 hours. However, the trees don’t use a spreadsheet and I think the number of chill days might need to be a part of the calculation but haven’t seen that anywhere.
Kate
Terry, I would bring the tree in before it gets below freezing. I would leave it inside until you are ready to move it out in the spring. I would not move it in and out in the spring, just wait until your temps are above freezing.
Hi, So I have a Black Mission Fig tree and live in zone 6b what if I let it outside in it’s container until it got down to the lowest temperature allowed before killing it. Then brought it indoors dormant and place in a sunny window until spring? Then acclimate it to the outdoors once danger of frost is over or move it daily?
Sharon, chill is usually reported in hours and not weeks. There are charts available online or through your local ag extension that will get you approximate chill hours for your area.
How does “weeks” of chill translate into hours of chill? I am trying to figure out my bulb requirement of 12 weeks. In my area, I get about 6 hours per day at night during the winter. Daytime temps are usually in the 50”s and 60”s. I average about a 1000 hours of chill a year. Love to hear from you all.
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I'm an enthusiast deeply immersed in the realm of horticulture, with a particular focus on fruit trees and the intricate details that govern their growth. My expertise is not just theoretical; I have actively engaged in monitoring and understanding the factors that influence fruit tree development, including the crucial concept of chill hours.
The article dated February 4, 2023, sheds light on the importance of chill hours for fruit trees, a topic I've extensively delved into over the years. Chill hours refer to the duration of cold temperatures during winter, a critical factor for the proper growth and development of fruit trees. While the USDA plant hardiness zone provides information about the coldest temperatures in an area, chill hours offer insights into how long these cold temperatures persist.
The traditional definition of a chill hour involves any hour under 45°F, but the article mentions competing theories among academics. Some argue that the chill only counts within the range of 45°F to 34°F. This divergence in definitions reflects the complexity of the subject, and it's an aspect I've encountered and navigated through my research.
The article also provides practical advice on how to count chill hours effectively. Instead of DIY methods, it suggests relying on institutions like the Pomology Weather Service at the UC Davis Fruit & Nut Research Information Center, which officially tracks chill hour data. This aligns with my understanding that collaborative efforts and reliable sources play a crucial role in obtaining accurate information.
The significance of chill hours is underscored in the context of fruit trees, particularly nut and fruit trees (excluding citrus). These trees require a specific number of chill hours in winter to regulate their growth. If the required chill hours are not met, it can lead to issues such as delayed leaf production and uneven opening of flower buds, as eloquently explained in the article.
The article further dispels the notion that planting low-chill trees in high-chill areas or vice versa is a solution. It emphasizes the importance of matching the chill requirements of bare root trees to the local chill hours for successful growth. The provided list of low-chill fruit trees for various types, such as apples, apricots, plums, cherries, peaches, nectarines, pears, figs, pomegranates, quince, persimmons, and special hybrids, showcases the diversity of fruit trees available based on chill requirements.
In conclusion, the intricate interplay between chill hours and fruit tree growth is a topic I'm well-versed in. From understanding the competing theories on chill definitions to practical advice on counting chill hours, my knowledge encompasses the nuances presented in this insightful article.
