GardenLine | Porpourri - Miscellaneous | Dormancy In Plants: A Process For Survival

Wood was an instructor with the Department of Horticulture Science. This column is offered as an extension service by the Division of Extension and Community Relations and the Department of Horticulture Science, University of Saskatchewan.

You have all heard of the Garden Of Eden - a place where plants grow and flourish all year long. This land is blessed with warm temperatures, ample water and plenty of sunshine. Ah! What a life for a plant. Then there's Saskatchewan. A land characterized by hot, dry summers, cold, dry winters, and plenty of drying winds. It's a tough place for plants to survive, and those that do must conform to the demands of Mother Nature.

One way that plants have adapted to survive here and in other harsh environments is through a process called dormancy. To explain this phenomena, we will follow a tree through a typical year. Early in spring the days begin to lengthen, the sun rises higher in the sky, and the temperatures begin to rise. These environmental factors trigger a change in the driving mechanism in plants. This driving mechanism consists of the supply of plant growth regulators (mostly hormones). The concentration of each growth regulator changes in response to the favorable growing conditions; as a result, plant growth is stimulated.

As spring departs and summer arrives, the trees are growing rapidly, taking advantage of the long days and optimum sunlight. For most trees, growth ceases by late summer (early August). That is, the tree does not produce any more leaves after this time. Again, this is a response to the constantly changing concentrations of plant growth regulators. From now until fall, the objective of the tree is to prepare for winter. During fall the trees are storing up food and energy reserves which are needed for next spring's growth. Forcing new growth occupies the plant and delays the energy-storage process.

The shortening days and the reduction in sunlight cause another change in the driving mechanism, and this stimulates dormancy in the tree. Dormancy is a period of rest and inactivity. The tree is far from dead; it is just awaiting spring. Changing the conditions around the tree may alter the time of year that the tree goes into dormancy. Heavy watering and fertilizing in fall (September) may stimulate the tree to continue growing, and therefore this is not recommended. New growth produced at this time of year will have little chance to prepare itself for winter, and will most likely die from winterkill.

You may argue that watering in fall is recommended to prevent winter desiccation. Agreed, but the timing of the water is important. Late October watering is recommended to prevent winter desiccation. By this time, the tree should be fully dormant and ready for winter. For less tender plants, mulch the soil around their bases to protect the root system from extreme cold, and from fluctuating spring temperatures. If possible, encourage a good snow cover around the plants. Snow is an excellent insulator.

As winter sets in, the tree has become fully dormant, and will not come out of this state until the changing environmental conditions once again stimulate growth. This type of dormancy also applies to seeds of trees grown in this area. Many hardy trees produce seeds which will not germinate in the fall. If the seeds were to germinate in the fall, many of the young tender seedlings would be killed by the severe winter conditions.

The same growth regulators which bring on dormancy in trees also prevent growth of seedlings in the fall. Other types of trees have mechanisms (such as hard seed coats) in the plum and cherry family which prevent immediate germination. When spring arrives, and environmental conditions favor growth, the seedlings burst from the ground and look forward to a year of good growth - weather permitting!

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.