Chill Hours: What Are They, How do I Count Them, and Why do My Fruit Trees Care?
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 HoursHere'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 TreesAs 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