3 April 2007
From: Bill Fountain, Ext Spec for Horticulture
Re: Cold damage to woody ornamentals and What to Expect (current cold snap)
Situation: There has been a flood of calls into the diagnostic lab and my office regarding various evergreens with brown foliage. Among these are evergreen hollies, boxwoods and especially the evergreen magnolias. On some the foliage is brown only at the tip; others have foliage that is completely brown. Most have green buds and stems and may have even begun putting out new growth.
Cause: Fall 2006 was warmer than normal with the weather turning colder than normal in February 2007. Many of these cold days were also characterized by windy weather and periods of bright sun. Sun warms the foliage causing transpiration to occur even though it is winter; wind and low humidity increase the rate of water loss. Unfortunately soil moisture is often unavailable because it is either frozen or the stems are frozen preventing the movement of water up through the plant. Winter desiccation of the foliage was especially serious on newly installed evergreens because of limited root systems and/or failure to irrigate the soil ball. Watering the surrounding soil does not help if that water does not actually enter the soil ball.
What to do: The best general advice is probably to do nothing. Leaf tissue that is brown now is dead and will never green up. Watering after the fact will not restore the green color but may help the plant recover. Applying nitrogen base fertilizer now is just going to exasperate the problem. Nitrogen will cause a reduction in cold hardiness and we are going to see more cold. If fertilization now does not cause problems before the last of the cold has passed; it may cause the plant to produce more shoot growth this spring than the limited root system of newly transplanted material can support if the summer is hot and dry. If the urge to apply fertilizer to the landscape too strong, people should wait until all chance of cold has passed and apply a low rate (1 perhaps 2 pounds of nitrogen per 1,000 square feet) and use a slow release form of nitrogen. Phosphorus and potassium rarely if ever benefit woody ornamentals.
Pruning can be done if twigs are dead. Sheering is going to impact the natural form of many plants. Selective pruning of only dead branches will leave the plant with a more natural habit. If only the leaves are brown these dead leaves will be shed and new foliage will be produced as spring growth begins.
What to Expect (current cold snap): Plants that have begun to produce new growth may be nipped back over the next few nights as temperatures drop below freezing. If they are dry it would be best to water them as soon as possible. Covering plants at night so that they look like a lollypop on a stick is not going to help. It is best to cover the entire plant with a sheet so that all of the plant is in ground contact. Heat radiating from the ground will be trapped and aids in keeping tissues from freezing. Using a wet sheet (or spraying the sheet with water once it is on the plant) so that the sheet freezes makes the sheet more airtight. Do not cover plants with plastic. Plastic traps the warming rays of the early morning sun and causes additional problems. Damage to unprotected plants that have begun to grow will turn brown almost immediately and often develops a shepherd’s crook.
Plants in flower are more likely to be damaged than those that have already set fruit. For crabapples and other fruits, check the flower / temperature chart in the ID-21 Disease and Insect Control Programs
for Homegrown Fruit in Kentucky Including Organic Alternatives, 2005 pages 18-19 http://www.ca.uky.edu/agc/pubs/id/id21/id21.pdf
to match the expected temperature in your area with the bud stage.
William M. Fountain, PhD
Extension Professor of Arboriculture
N-318 Agri Science North
Univ of Kentucky Dept of Horticulture
Lexington KY 40546-0091