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In the Garden with Extension Educator Larry Zilliox, May 5, 2008
A recent article peaked my interest in the Yard and Garden News on the University of Minnesota Extension website by Charlie Rohwer and Chris Currey where graduate students in the Department of Horticulture posed this question "How do plants know when it is spring?" The article seemed appropriate to the recent late season snowstorm and the below normal temperatures that followed.
In the fall, we hope our plants go dormant, so they do not continue growing and freeze delicate tissue. In the spring, this process must be reversed once favorable temperatures return. However, why don't the plants start growing in those years when we get a week of above freezing temperatures known as the January thaw? There must be other factors controlling when the plants know it is appropriate to start growing beside just temperature. One controlling feature we know about is called chilling factor that releases the plant from dormancy. In the case of apples, they need 600 plus hours of chilling factor, an accumulation of hours with the temperature between 35 and 45 degrees. A reason you don't find apples growing in warm climates.
Another factor contributing to the release of dormancy is daylight, more correctly, the length of night. Here in Minnesota this is critical to many plants because we get wide swings in temperatures, such as the January thaw. A great example was last year when we had 70 degree temperatures the last week of March, followed by a week when it never got above freezing with several nights with single digit temperatures. In my own yard, I lost two apple trees that must have come out of dormancy during the warm week while another apple tree 20 feet away survived. Normally we would expect our temperatures to start moderating by late March along with those warm April showers, but Mother Nature didn't cooperate last year and gave us that extreme cold snap even though the day length was appropriate.
Another interesting phenomenon that intrigues me is what triggers seeds to germinate? Why doesn't a seed start germinating when it falls to the ground? We know that some seeds can lie in the ground for twenty or more years before it germinates. Some seeds, such as the morning glory, have a coat that prevents the absorption of water until the coat is degraded. Walnuts need to have the freezing and thawing crack the outer shell before the embryo can start growing. Some desert plants have a chemical inhibitor that needs to be washed out of the seed by rain before they will germinate. Also infra red light is needed by other seeds to germinate. The seeds brought to the surface by the heaving of the soil due to freezing and thawing action initiates the germination process.
You thought it was April showers that brought May flowers, but the world is a complicated place. That is why I find it so fascinating to be involved in scientific discovery.
Posted by mgweb on May 19, 2008 5:12 PM in Gardening Columns