We are now officially into fall, which means it is time to start thinking about how to protect our ornamental plants from cold damage.
Daylight hours are getting shorter and nighttime temperatures are dropping. Plants are starting to move in to a state of dormancy, or rest, as their growth hormones slow down.
As our typical first frost dates of mid-October approach, it is important to mitigate the effects of cold damage in order to help our gardens survive the winter.
Many different environmental conditions contribute to garden cold damage. Cold, gusty winds can easily break off branches from trees and shrubs. Cold air is also drier than warm air, and cold winds can ‘wick' moisture from plants. Evergreens will often show damage from these types of winds with ‘burns' on the leaf tips or margins. Defoliation also may occur under these circumstances.
Frost and freezes can cause ice to form inside plant cells, causing stem, leaf and bud damage. Buds and leaves are often left with a mushy, brown appearance. Flowering plants will often fail to bloom the following year when affected by cold weather.
Cold injury often occurs before trees and shrubs have reached full dormancy. Freezes may cause the conductive tissues, located just below the bark, to die off. The section of the canopy above the damaged tissues will die back due to lack of water and nutrients.
Frost cracks occur during the colder months, but are often not noticed until the following year. These occur when trunk tissues expand when exposed to warm sunlight and contract during a cold night. The outer bark cools faster than the inner trunk, causing the bark to split. Young, smooth-barked trees are most susceptible.
Container plants are most susceptible to cold damage. Their root systems are above ground and are not insulated by the ground soil. Also, they are often placed in microclimates, such as patios, which can exaggerate cold temperatures.
Now that we know some of the ways cold damages plants, here are some preventative measures we can take to reduce low temperature injuries.
Choose plants that are known to be cold hardy. Fertilize at the correct time for the specific plant.
Fertilization late in the season will encourage new, tender growth, which is most susceptible to cold damage.
Find the cold microclimates on your property, usually located on the north and northwest sides. Use plants that can withstand very cold temperatures in these spots. Mulching around your plants will help minimize heat and water loss from the soil.
Avoid heavy pruning in the cooler months. Pruning encourages new plant growth, which is highly susceptible to frost and freeze injury.
Move container plants inside the house or to a shed if a frost or freeze is likely. If this is not an option, try to insulate the pot from the outside. Wrapping blankets, burlap sacks or straw around the pots can help reduce heat loss.
You may want to build temporary windbreaks, like fences, to protect from chilly winds. Plants may also be covered with blankets or plastic coverings. Make sure the covering reaches the ground and does not touch the leaves.
When using plastic, be sure to remove the covering during the day, as plastic will hold solar radiation and damage the plant.
Clark MacAllister is the Dawson County extension agent. For more information, call (706)265-2442.