![]() ![]() MSU is an affirmative-action, equal-opportunity employer, committed to achieving excellence through a diverse workforce and inclusive culture that encourages all people to reach their full potential. “ Freeze damage depends on tree fruit stage of development”.“ Fall color show and winter dormancy in woody plants”.“ Winter cold hardiness in Michigan fruit crops”. ![]() “ Forcing cuttings to determine the end of dormancy in fruits and other plants”.The plants actually begin growing before we notice the buds swelling. Growth first becomes apparent when buds swell and then green tissue emerges from the bud. There is usually a slow progression of development when the plant begins to grow as the temperatures slowly rise. Once the plants start to grow, they lose the ability to readjust to colder temperatures. Warmer temperatures into the mid-40s will cause them to begin growth. The plants are dormant only because of cold or cool weather. As long as the plants are in endo-dormancy they have the ability to acclimate to colder temperatures and withstand winter cold.Īfter chilling is completed the plants are no longer in endo-dormancy. Maximum cold hardiness occurs when plants have been subject to cold, subfreezing temperatures for several days or more. As long as the temperatures are below freezing, the plants are ready for really cold temperatures. As long as the trees, bushes or vines are in endo-dormancy, they have the ability to acclimate to cold weather we call this cold hardiness. The relatively warm winter we are experiencing in 2013 is allowing many plants to accumulate their chilling earlier than normal.Īnother important thing happens during endo-dormancy. Some are long and cold, others are warmer. Of course, there are big differences between winters. We normally complete endo-dormancy here in Michigan in January, early January in the south and late January in the north. ![]() Most of the fruit crops we grow in Michigan have chilling requirements of 700 to 1,300 chilling units. Plants from the middle temperate regions like Michigan have relatively long chilling requirements that keep them dormant through the long stretches when the temperatures cycle above and below freezing in a Michigan winter. Likewise winters in the south are short and mild. In the far north, it is below freezing for a long time and the spring warm-up is relatively short. Plants from the south or far north have relatively short chilling requirements. These peach buds appear dormant, but have completed their chillingrequirement and are waiting for warmer weather to begin growth. Photo credit: Mark Longstroth, MSUE Temperatures just above freezing and above 50 F are less effective and temperatures above 60 F often have a negative effect on chilling. Temperatures between 40 and 50 degrees Fahrenheit (5 to 10 degrees Celsius) are most effective. Chilling and endo-dormancy normally prevent plants from beginning growth during warm spells in the middle of the winter. If warm weather occurs before the plant completes its chilling requirement, no growth occurs. Hours below freezing have no effect on chilling, but will increase cold hardiness. Many people think the plant is tracking hours below freezing. The number of hours required for chilling varies for different plants from less than 500 to 1,500 hours or more. Chilling units are hours of time spent above freezing. (See the Michigan State University Extension article, “ Fall color show and winter dormancy in woody plants.”)Īs the plant enters endo-dormancy, it tracks chilling units to track the passage of the winter. Short days and freezing temperatures in the fall induce endo-dormancy in the plant. ![]() The other is called eco-dormancy and occurs when the plant is ready to grow but the environmental conditions are not right, usually too cold. In endo-dormancy, something inside the plants is inhibiting growth. In endo-dormancy, the plant will not grow even under good, warm, growing conditions. There are actually two types of dormancy during the winter. Since most plants do not grow during the winter, we say they are dormant. ![]()
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