by George Buehler

Dormancy is a phase in plant development which allows a plant to survive harsh climatic periods. We normally think of dormancy in terms of plants losing their leaves in preparation of arrival of winter. However, some plants go through a semi-dormant period when they are exposed to extremely high temperatures in the summer. Therefore we can define dormancy as a survival strategy various plants have developed to stay alive during periods of high stress.

Whether the plant goes dormant to escape the heat, to escape the cold, or both, these periods of rest are crucial to the survival of the plant. Some plants like deciduous trees go into total dormancy – drop their leaves and send their sugars to the roots where they are stored. Others, like evergreens, do not have complete dormancy since they do not drop their leaves and they must continue to photosynthesize and respire throughout the winter, but at a much lower level.

Now what happens if a juniper bonsai, for instance, is forced to grow year round? For the first couple years, the tree would grow on unfazed. It might even survive for four or five years; but eventually, without the dormancy rest period, it will decline and ultimately die. Deciduous trees, on the other hand, would not survive more than a year or two without the needed dormancy rest.

What about tropicals, like Ficus? Do they go dormant? It appears that tropicals and subtropical plants have modest dormancy requirements. They have continuous growth at 70 degrees F with little or no effect on their physiology. However during certain times of year, they do grow more slowly or not at all. Thus I guess we could call this non- or slow-growth period semi-dormancy.

Are there environmental affects that cause a plant to enter dormancy? The main environmental trigger for the onset of dormancy is day length. Short nights promote vegetative growth for most temperate woody plants. As the amount of daylight shortens (long nights) in late summer, growth slows and initial dormancy begins. [Some reports state that the main trigger is the length of night or darkness, not the amount of daylight].

Decreasing temperatures also play a role in dormancy development. Some reports indicate that decreasing temperatures play a more important role than the shortening daylight. Dormancy in many temperate woody plants develops more quickly when short days occur in combination with cool temperatures.

Both water supply and mineral nutrition also interact with the onset of dormancy. Water stress will deepen dormancy, and if severe enough, it will result in a resting bud and defoliation in some trees. High fertilization, especially with high levels of nitrogen, can also delay dormancy. In fact, high nitrogen levels should never be applied in late summer or early fall since they may actually cause resumption of growth and a delay in onset of dormancy.

If the bonsai artist wants the deepest dormancy, he will reduce the day length, reduce the temperatures, cut back on fertilization and mildly water stress plants. Light reduction, fertilization and water can easily be manipulated. Reduction of temperatures is more of a problem for most of us, and we normally depend on Mother Nature for this.

I noticed this year that a number of my trees did not appear to go into full dormancy as usual, apparently due to the high fall temperatures. This was especially evident with my elms. Others, like my cherries, Stewartia, and maples, seemed to go into dormancy (lose their leaves) normally even with the high fall temperatures. From this I would conclude that the amount of daylight for some species is more important in the onset of dormancy then for other species – since all of my trees were watered equally and no high nitrogen fertilizer was applied. For others, the temperature seems to be more important.

I found that there appears to be stages of dormancy:

• Pre-dormancy – This is the first phase of dormancy. Its onset is visible as setting of buds in mid to late summer, a change in leaf color, and a subsequent build up of sugars in the root mass. This stage is shortened or lengthened by the amount of cold temperature it receives (i.e. the colder temperature the shorter this stage). In some species – but not all- leaf or flower buds develop during this stage; and, if the growing conditions are made favorable (more daylight, higher temperatures, etc), the leaf or flower buds can be made to start growth again.
• True-dormancy – In true dormancy, there is no growth at all; and, even if the environmental conditions are altered to optimal growing conditions, growth will not resume. This is the stage when leaf fall occurs for deciduous trees.
• Post-dormancy – This stage occurs in late winter or early spring as the temperatures increase. If the temperatures increase enough or are high enough for sustained periods, bud swelling occurs.
• Cold Hardiness – Although not a stage of dormancy, it is a necessary part of dormancy in temperate climates. As the plant begins pre-dormancy, the stem tissues begin increasing levels of sugars and carbohydrates in response to the lowering temperatures in the fall, thus protecting the cells from freezing. Different species have developed different degrees of cold hardiness depending on their genetics. It should be mentioned that roots do not develop the same degree of cold hardiness as the stem tissue and must be protected.

How long of a dormancy period does a tree need? It appears that each species has its own requirement and amounts vary, depending on the source of information. I have seen one reference that stated a Japanese garden juniper required three months at or below 50 degrees. Another site indicates four to six weeks is all that is required. I could find a lot of information on seed dormancy (or cold requirements), and dormancy of in-ground fruit trees, but little on bonsai dormancy requirements. Needless to say, the longer the tree is in full dormancy, the more rest it will have and the better for the tree. As a general rule, a dormant plant needs between 500 and 2,000 hours of temperatures around 35 to 40F.

If the outside temperature goes above 40oF, the tree can enter post dormancy and start a growth cycle. From my experience with both bonsai and garden trees, trees seem to have the ability to start and stop the post dormancy growth periods. Although I don't have scientific data, in looking back at my bonsai database, it appears that when a tree has been in full dormancy for a substantial amount of time, these growth spurts have little or no effect on the tree. If however, the tree has had a short amount of time in full dormancy, the growth spurts started by elevated temperatures cause the next fall's pre-dormancy period to start earlier. I have no way of determining whether this is harming the overall health of a tree.

In our Ohio valley, we know that the temperatures continue to cycle from below freezing to 50oF or more. The effect on our trees is minimized when they have been buried in mulch. Even though the branches of the tree are exposed to higher temperatures, the roots, being protected by the mulch, remain more constant and generally cooler. So we mulch our plants to not only protect the roots from freezing, but to also moderate temperature fluctuations so that the tree stays in dormancy for a longer period of time.

Some research indicates that a tree has a build up of abscisic acid (ABA) in the pre-dormancy period. ABA is a growth regulator and is slowly broken down when the plant cells are exposed to the cold temperatures. Until enough hours have accumulated to remove the inhibitory effect of ABA, the plant will not break bud.