The leaves changing color and falling is a sign that colder weather is getting closer and closer. Trees are entering into dormancy. Dormancy is an important process to allow the tree to survive through the winter. Let’s talk a bit about how dormancy works in trees and why they need to do it.
Dormant trees will stop growing above the ground. Dormancy is partly brought on by temperature change, but even more so by the change in day length. The college word for this phenomenon is ‘photosensitive’, meaning the trees change in response to day length. Leaves will begin to change color and fall off. Leaves begin to change color because chlorophyll begins to break down. Chlorophyll is a green pigment that plants use to turn light energy to chemical energy. There are other pigments in the leaves too. As the chlorophyll breaks down the other pigments are left, resulting in the wide array of leaf color that we see. Evergreen trees like pine trees of hollies will have a needle drop in the fall and again in the spring.
As we move into winter trees will enter what’s called endo-dormancy. In endo-dormancy an unsatisfied chill hours requirement will keep plants from waking back up. Different plants have a different number of chill hours, or hours spent below 45 degrees Fahrenheit. Chill hours are supposed to keep plants from waking back up too soon. In 2019, we had a couple of days in February that were over 70 degrees. While my pale legs might like to see some sun, February is not time for plants to begin waking up, as we have more freezes that are coming. Hopefully, those plants haven’t met their chill hour requirement, so even though I’m wearing shorts, the trees are still dormant.
Endo-dormancy also makes trees cold hardy. Trees will behave in a couple of different ways to make themselves cold hardy. One strategy that trees use is to keep all their water inside their cells. Frozen water expands. If all that water froze and expanded the cells would burst. To combat this trees move minerals and hormones in to mix with the water. Mixing water drops the freezing point, so that plants’ cells don’t freeze and burst. Another strategy is to move the water out of the cells so that it can freeze safely in the intracellular space.
When spring rolls around temperatures start to rise and trees will start to bud out because they have satisfied their chill hours requirement. However, if plants start to bud out and then we have another freeze, that can kill off the buds and cause damage to the trees. We saw that happen this year with a couple of freezes in April and then one on May 10.
Evergreens will continue to need water throughout the winter. If the soil freezes for long periods, the roots can’t take up the water. Deciduous trees, trees that lose their leaves, will not need as much water. Roots in trees will continue to grow as long as soil temperatures are over 40 degrees. Soil temperatures are warmer than air temperatures in the winter because they are insulated.
If you have questions about trees and dormancy contact your County Extension Office or email me at [email protected].