Trees Breaking Dormancy

It has been a mild winter. The past few weeks have especially started to feel like spring. I have heard people around the Bottineau area say that the weather recently has felt like mid-late March. If some of the trees in our area could talk, they would probably say the same thing.

Perhaps like you, I have noticed that some of the trees around my home have swollen flower buds already, particularly the aspen. That is unusually early. They will likely get zapped before the winter is over.

During a mild winter like this, one might wonder what prevents buds from breaking dormancy during warm spells in December for example. What actually is involved with trees maintaining dormancy? And what factors lead to the breaking of dormancy? Actually there is a highly choreographed dance of environmental cues, hormones, and other factors that trigger, maintain, and break dormancy. To understand this, however, we need to go back to last fall.

The shortening days of fall, not the dropping temperatures, are the main environmental cue in triggering dormancy. As the days shorten, changes in hormone production start the process. One of the major hormones in stimulating dormancy is abscisic acid which is involved with the shedding of leaves, and is also a strong inhibitor of bud growth.

Over the course of natural selection and evolution, deciduous trees have become synchronized with the local conditions. As one would expect, trees in higher latitudes generally require a longer cold period than trees of lower latitudes. It seems odd, but a cold period is actually required before the buds can begin to grow anew in the spring. One of the factors involved with the cold treatment is that the cold temperatures break down the growth inhibiting abscisic acid.

Increasing day length certainly plays a part in breaking dormancy, as do warmer temperatures, moisture, and the nutritional status of the plant. As you might expect, there is a delicate dance of physiological changes in the production and activity of enzymes, hormones, and other substances in response to changing environmental conditions. One example is that the spring conditions stimulate the production of growth promoting hormones such as gibberellins.

Some of our trees may get nipped this spring, but they should be okay: A little setback, perhaps. And remember spring is officially less than a month away!

Chuck Lura

Natural North Dakota is supported by NDSU Central Grasslands Research Extension Center and Minot State University-Bottineau, and by the members of Prairie Public. Thanks to Sunny 101.9 in Bottineau for their recording services.