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, Babcock, 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
12 comments
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.