There seems to be some confusion when speaking with gardeners about chill hours and chill unit requirements for their fruit trees, I want to see if we can unravel the mystery together to assist the gardeners in their selection of trees and fruit for their gardens or orchards. The simple definition of chilling hours is the number of hours below 45F accumulated by the tree during the winter to overcome dormancy. Now with that said, there are a couple of ways to calculate the total number of hours. The time span used for the calculations can be whatever span you want to calculate for, you can select just the winter solstice, or base it on statistical models from previous years. For the Lima area, the average date that you observe a 45 degree day or colder is mid November extending through the end of February. This information can be found at http://www.climate-charts.com/USA-Stations/OH/OH334551.php for Lima and other cities around the state.
The first is to take the total number of hours less than 45F, but greater than 32F. This will provide a number which is smaller than other calculation methods, so it is a conservative method of calculation.
The second method of calculating chill hours if to just take the total number of hours less than 45F, this will typically increase the total chill hour total above method one because you will be adding the additional hours that are below 32F.
The third and most complicated and possibly the most accurate method is as follows, this method is called the Utah model:
1 hour of chill below 34F is worth nothing
1 hour at or between 35 and 36F is worth 1/2 a chill hour
1 chill hour is given at 37 to 48F
49 to 54F is again worth just 1/2 a chill hour
55 to 60F is again worth no chill hours
Anything above 60F is considered a negative chill hour
The more that I look at the calculations; I can see why everyone is confused. I have listed only a three of the possible ways to obtain chill hours or units and this is making my head hurt. Is this level of detail necessary for the average gardener, I do not think so. As long as you are conservative in your calculations, you should be covered by using a simple chart such as the one from University of Maryland, the link is provided below.
If you want to be a little more aggressive and calculate using one of the methods shown above, then you may look for a specific chill hour requirements based on the tree or fruit you plan to use. Charts for apple, pear, and all stone fruit are shown below. The charts include chill hour requirements based on individual fruit varieties and also listed by hardiness zones.
Although calculating the chill hours for the previous year can be helpful, a simpler way is to look at a chart for the type of tree that you want to plant for the zone that it can be grown in. Lima is zone 6, so a tree that can be grown up to zone 4, 5, or 6 will work beautifully within the Lima area. But a tree that is listed as zone 7 will not able to handle the cold winters we have. The zone 7 tree will either die back to the roots during the winter or die completely. Use the charts shown above to pick trees that will be hardy for your zone and area, then you should not have to worry about counting chill hours and units. Remembering that the charts above do not take any micro climate information into account, such as your tree will be sitting in front of a large dark colored stone rock pile which will absorb heat and release it slowly over night. This type of micro climate change could be worth adding one hardiness zone rating just by itself. So use the methods above as a guide, but use your skills of observation to make a final determination regarding the specifics of your site. The time to start ordering your fruit trees for spring delivery is now, so get started with your pen and paper.
With personal and professional regards - Vince