Tuesday, January 7, 2014

5 Types of Cold Damage to Landscape Plants


Winter Leaf Scorch,
U DE, R. Mulrooney
Plants vary tremendously in their tolerance to cold temperatures. The U.S.D.A. publishes a Hardiness Zone Map that is based on decades of data and indicates Atlanta is in Zone 8A. This is used as a guide when choosing plants. Crabapple recommends the selection of plants that are reliably cold hardy in our area, and then excellent maintenance during the growing season, as the two best ways to guard against cold damage. Factors that affect plant selection include minimum and maximum daily temperatures, difference between day and night temperatures, and average daytime and nighttime temperatures. Location/site/microclimate in the landscape also plays a role.

Winter Burn or Desiccation Injury
Atlanta is filled with needle leaf evergreens (pine) and broad leaf evergreens such as camellias, tea olives, holly, anise and boxwood that keep the winter landscape green. Unfortunately, the broad leaf evergreens are more prone to winter burn or leaf desiccation injury than dormant, deciduous trees and shrubs.

Winter Leaf Burn,
photo Glen Jacobsen
Winter burn occurs when the absorption of water by the roots cannot keep up with the amount of moisture lost by the foliage (transpiration). When the soil is frozen the roots cannot absorb water or translocate it up to the leaves to replace what is lost, and this results in leaf desiccation. This occurs on sunny and windy days, on days that are exceedingly cold (or hot), as well as when the soil is frozen and the plant cannot absorb water, or in a drought situation. Injury appears as brown leaf margins or needle tips. Usually the leaves are injured but dormant buds survive.

  • A 3-inch layer of organic mulch (pine straw, woodchips, or leaves) helps to insulate the ground and moderate the soil moisture level.   
  • The microclimate of a garden plays a big role in actual garden temperature. Changes in elevation, cold air drainage, sun exposure, and thermal heat mass (surrounding buildings, pavement or rocks) will make some gardens significantly warmer or cooler than the temperatures recorded for the area. Air drainage cold air sinks to the lowest spot. 
  • Knowledgeable advance siting is important at the planting stage for broadleaf evergreens, and  includes such considerations as protection from winter wind and/or the direct rays of early morning sun. Crabapple accomplishes this by planting in the airy shade of tall pines, or on the west side of a building, wall or hedge.  
  • Landscape plants should be well-watered prior to the cold blast. Luckily, there is plenty of moisture in metro-Atlanta soil due to our recent hard rains. 
  • Anti-desiccants (brand names include Wilt Proof or Moisturin) can be sprayed on broadleaf evergreens to reduce the amount of water lost through leaves. However, leaves lose water through the same pores that take in carbon dioxide for photosynthesis, so the overuse of antidesiccants can be toxic to plants.

Freezing injury
Freeze injury to Camellia
blossoms, Geri Laufer
When temperatures get too low, plants are susceptible to freezing injury. Flower buds are frequently affected, for example peach, dogwood, magnolia, forsythia or hydrangea flowers may not bloom well after an extremely cold winter. 

Container plants are more vulnerable to polar cold snaps than in-ground landscape plants and will benefit from some protection. Containerized nursery stock have small, above ground root balls that freeze easily, and newly planted bare uoot or balled-and-burlapped plants with their reduced root systems, are also very susceptible.

One method of protection: remove branches from discarded Christmas trees (plentiful this week) and lay over the tops of nursery flats or pots. Then bury the whole area in fallen leaves. (Do not cover with plastic, because direct sun hitting the plastic can steam-cook the crops.)  

As for woody ornamentals planted in decorative accent containers, wooden, foam or plastic planters usually come through freezes just fine, but clay pottery can break if subjected to freezing temperatures. Move them to an unheated garage for a day or two to get over the worst of the weather.

Winter injury, thin bark split
U of OH, "split happens"
Freeze cracks/sun scald
Sometimes it isn't really the cold temperatures that cause problems, but rapid fluctuations between warmth and cold as temperatures climb and plunge. Bark on some trees may split on the southwest side due to rapid temperature changes. Thin-barked trees such as cherries and maples are most likely to crack. Shade protection or wrapping with tree wrap may give protection against frost cracks. If thin-bark trees have split, call Crabapple LandscapExperts to shape and clean the wounds so they can heal over properly.

Frost injury
Frost injury typically occurs in low-lying frost pockets, especially at the beginning or end of the cold season, when summer annuals and vegetables can get that “wilted lettuce look” or camellia petals or early-blooming magnolia flowers are turned brown in a late cold snap. Delicate petals and new leaves are the most susceptible to frost damage. One way to avoid frost damage is to select late-flowering cultivars that bloom in the spring instead of the early ones.

Winter Frost Heave,
Missouri Botanical Garden
Soil heaving
Soil heaving results from alternate freezing and thawing of the soil and is rarely a problem in metro-Atlanta. Shallow perennial plant roots are pushed out of the soil leaving them exposed to cold and drying winds. Good root establishment helps to protect from heaving, but some plants (mums, strawberries) have characteristically shallow roots and are prone to heaving. 


Replant perennials that have been tossed out of the ground and water-in. Reduce heaving with an application of organic mulch, such as 3 inches of pine straw, wood chips or shredded leaves. Snow cover (also rare in metro-Atlanta) also acts as an insulation for perennials and often prevents heaving. 

January 4, 2014
History of Cold Temperatures in Hotlanta
Arctic Blast
10 coldest temps recorded at Atlanta's Hartsfield-Jackson Airport:
1. -8 degrees (January, 1985)
2. 0 (January, 1982)
3. 1 (December, 1985)
4. 5 (December, 1989)
5. 5 (January, 1994)
(tie)6. 6 (January, 1986 and February, 1996)
8. 8 (January, 2003)
9. 9 (January, 1985)
10. 10 (January, 1996)

Call your Crabapple LandscapExperts for help with winter damage to the landscapes you manage. 

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