Part 26 of a series by Eugene Howell
In the previous article we began the discussion of bonsai dormancy. We covered the definition and the fact that not all species go dormant, and not even each plant within a particular species, goes dormant at the same time. We also learned of the interest scientists have in the subject of dormancy. We finished the first part by asking the question, “So what causes dormancy and when can it typically be expected to occur?” We will answer that question in today’s article.
The first part of the question is easier to answer than is the latter part. Typically, dormancy is caused by one or more of the following four events; shorter days, cooler temperatures, hotter temperatures or drought. Two of these sound like an oxymoron, but they are not. Not all plants respond to the same events in initiating dormancy. Some plants need shorter daylight hours and/or lower temperatures while others need hotter/drier conditions. In both cases the plant is protecting itself from adverse weather conditions that might otherwise injure it. In the first case the plant is protecting itself from the cold temperatures that frequent winter, and in the second case the plant is protecting itself from the desiccating conditions that occur in late summer and early fall in some parts of the world.
Most deciduous trees that go dormant during winter clearly signal the fact by the change of leaf color, becoming brilliant red, orange, and yellow in many trees. And eventually the shedding of its leaves. These events take place because there is a layer of cells at the base of each petiole (leaf stem) called the “abscission layer”, which gradually becomes “corky” as fall descends. The cells in this layer swell and harden, thus gradually cutting off the veins that pass through it to carry water into the leaves and sugars away from it. Once these circulatory system is cut off, the chlorophyll in the leaves disintegrates and the colors, which have been there all along but were completely masked by the bright green chlorophyll, begin to show through, thus causing the leaves to change color. When the abscission layer has fully hardened, the cells at the top of the layer begin to disintegrate and the entire leaf and petiole are blown off the tree by even a gentle breeze.
In Florida, winter dormancy periods extend anytime between December and February. Although the trees may be dormant before any real chilly weather arrives, due primarily to shortened days, the trees will certainly be dormant after the first lengthy spell of cold temperatures in the 30’s. One species that goes dormant before cold weather arrives is the Bald Cypress. Following a couple of nights in the 30’s, temperate trees can safely be root pruned.
One caution about bonsai dormancy in Florida: the period of dormancy may be very short, perhaps only 10 to 14 days, so once you decide that a tree has gone dormant, don’t delay doing the work needed.
One might ask if dormancy can be avoided by keeping the bonsai in an artificial environment, thereby speeding up the rate of development of the bonsai. The answer is yes, but you pay the price in the end if the bonsai is a temperate tree. By placing it in an environment with artificial light and warm temperatures year round, the tree will remain active during the entire year. This creates a problem in that temperate plants require a rest period and they need this rest every winter in order to be healthy. By keeping one in an artificial environment to prevent dormancy, you’ll find that flower and fruit production are harmed and the tree will decline in health.
In conclusion, pay attention to your bonsai during fall and winter and take notes on when they go in and out of dormancy. Personal knowledge and familiarity of your bonsai’s habits will help you to create better bonsai.
Photo credit: Winter Silhouette by Walter Pall