Introduction
Winter weather plays an important role in growing many deciduous fruit crops. Apples, peaches, cherries, pears, and other trees may need a certain amount of cool weather before normal spring growth begins. This is called winter chill.
Gardeners often search for chill hours by zip code, average chill hours by zip code, or ask, how many chill hours in my area? A local estimate can help you choose better-suited trees, but it is only one part of orchard planning.
Use the Peaceful Valley Bare Root Orchard Planner to estimate local conditions and compare trees.
Find Your Estimated Winter Chill
Enter the ZIP code where you plan to plant. We’ll check Peaceful Valley’s stored database for your USDA growing zone and estimated winter chill.
Use the ZIP lookup on a plant product page to check estimated winter chill.
Historical estimates help you compare varieties. They are not a forecast or a measurement at your exact planting site.
What Are Chill Hours for Fruit Trees?
If you are wondering what are chill hours, they are one way to estimate how much useful cool weather a dormant plant receives.
So, what is a chill hour for fruit trees? The answer depends on the model being used. One traditional model counts temperatures between 32 F and 45°F. Another counts hours below 45 degrees. Other models calculate chill differently.
UC Davis explains these differences in its guide to chilling hours, units, and portions.
This is why chilling hours for fruit trees should not be treated as one exact measurement under every system. A tree's chill hour requirements describe the winter conditions needed to complete dormancy properly. After enough chilling and later warmth, buds can break dormancy and begin spring growth.
Why Fruit Tree Chill Hours Matter
What does chill hours mean for fruit trees in your garden? Each cultivar can have its own chilling need. Meeting the chilling requirements for fruit trees supports normal budbreak, flowering, and fruit production.
If a tree does not accumulate chilling that is suitable for its cultivar, budbreak and flowering may be delayed or uneven. This is especially important after a warm winter.
The required amount varies widely, even among apple varieties and other trees of the same species. Gardeners in mild climates can explore low-chill bare root trees.
How to Estimate Winter Chill
To check chill hours in my area, start with a local chill tool or a representative weather station. Historical temperature records can be used to estimate chilling hour accumulations.
Different models may give different totals. The simple Chill Hours models count hours within a chosen temperature range. The Utah model gives different values to different temperatures, including negative values for some warm conditions. These methods should not be treated as interchangeable.
Your accumulated chill hours can also change from one winter to another. Local elevation, terrain, and microclimates may affect chill accumulation.
Grow Organic's chill hours zip code estimate uses historical modeled weather data. It is a planning estimate rather than an exact measurement for your yard. If you are searching for chilling hours by zip code, use the result as a guide and compare it with local Extension information when needed.
Chill Hours Are Different From USDA Zones
What does chill hours mean compared with a USDA growing zone? They measure different things.
USDA zones are based on average annual extreme minimum winter temperatures. Chill models estimate useful cool conditions during dormancy. A location can therefore be cold-hardy enough for a tree but still be a poor match for its chilling requirement.
Check your growing zone, review the USDA growing zones, and learn how to find your USDA zone.
Then use the Bare Root Tree Selection Guide before browsing bare root trees. Also consider sunlight, drainage, mature size, pollination, and the right location for your bare root tree.
Conclusion
Understanding what chilling hours are and what are chilling hours for fruit trees can make tree selection easier. Compare your estimated local chill with the variety's listed requirement, but remember that models and winters vary. A cold winter does not necessarily mean more useful chill for every fruit tree.
Choose suitable chill varieties, check hardiness separately, and do not assume more cold weather always means more useful chill. Freezing temperatures and chill hours are not the same measurement.
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.