Looking for a reason to be glad for the cold weather in winter’s stretch? Consider the needs of fruit trees. Fruit trees need to spend a certain amount of time during their dormant winter period at cool temperatures in order to satisfy their chill requirement.
Simply defined, the accumulation of chill units (CU) is a cumulative measure of the number of hours trees spend between 32 and 45 degrees Fahrenheit. Liken this process to a reset of the tree’s biological clock. This clock counts down the time needed to change the nutrients stored in the roots into a form that can flow up the trunk as the weather warms and support flowering and growth. Time spent at winter temperatures above 60 degrees and below 32 degrees counts against the number of accumulated chill units.
Getting enough optimum chill time ensures the tree will successfully break dormancy, flower, and set fruit. The wild weather fluctuations of 2012 brought the warmest March on record (there were 9 days above 80 degrees), which signaled to the trees that it was time to start growing. April’s subsequent sharp drops to freezing temperatures caused tissue injury and poor flowering, leading to a significant loss of 2012’s fruit crop.
Trees are able to withstand cold temperatures when they are dormant as they are now. Chill requirements vary between different pome fruits. Apple, pear, and quince varieties each have their own climate-specific needs. Low-chill apples, while productive in California, won’t produce well in our colder northern climate because they bloom too early.
The Regenstein Fruit & Vegetable Garden’s 34 apple varieties have chill requirements ranging from 600 to 1200 CUs. Chicago’s weather historically can meet those requirements, barring extreme fluctuation like last year. Our current cool weather is right on track and looking positive for growers.
Knowing a fruit tree’s chill requirement is a tool for choosing the right plants for your garden. Come to the garden for a quiet early spring walk through the orchards, perhaps finding inspiration to plant fruit trees in your own garden this spring. In the meantime, please be reassured that the trees and fruit growers are happy with this consistent wintry weather.