Degree Days and Chilling Hours

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Wonder why sometimes the vegetables in the garden don't grow or produce as well as they should have? Besides the usual influences of our general weather conditions like too much or too little soil moisture, another factor is something called growing degree days. This is based on heat units collected as the spring, summer and fall weather moves along. We usually start to add up those heat units on temperatures above 50 degrees. Our vegetable plants grow and develop into flowering and fruiting plants. A comparable we see in the seed catalogs is the phrase short season or long season in the vegetable variety description. A short season crop will need fewer growing degree days to mature. Watermelon is an example of a long season crop for most of us here in N. Illinois and if planted too late in the season, we may never see a fruit or if we get one it may not mature in time. This happens as a result of not getting enough heat units. N. Illinois has about 160 to 170 frost free days while S. Illinois has about 190 to 200. It is easy to why watermelon s are easier to grow in the southern half of Illinois than here, more good weather to hit that "days to harvest" figure or collect enough growing degree days.

Now for something different. Our mild winter for the most part anyway has had horticulturists talking about "Chilling Hours". This is all about our temperate plants getting enough cold weather to know it is time to come out of winter dormancy and begin to grow come spring. Chilling hours are recorded at weather reporting stations along with the rest of the weather data. Chill hours are recorded for temperatures between 32 and 45 degrees. Temperatures below 32 degrees are not counted as plants are completely dormant. Any wintertime temperatures above 60 degrees are subtracted from the totals. Our temperate landscape plants having evolved over the millennia have their own way of keeping track and know when it is safe to break winter dormancy. This explains why some fruit trees are damaged by a late frost or freeze, they have already received enough chilling hours earlier and are attempting to grow. Fortunately for the plants native to our area, their chilling hour requirements are higher so they break dormancy without the risk of having flower buds or leaf buds damaged by late frosts. Examples for fruit trees depending on varieties are:

Chill Hours Required to Break Dormancy Using the 45oF Model


Apple 800 to 1750

Pear and sour cherry 600 to 1500

Plum (Japanese) 600 to 1600

Cherry (sweet) 500 to 1450

Peach 375 to 1200

Apricot 300 to 1000

So go about your gardening and fruit growing like everything is normal, because it is.

About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.