During the fall months, the gradual decrease in temperatures and day length increases a plants ability to withstand cold temperatures, but a sudden drop to below-freezing temperatures from a period of relatively mild weather can damage plants that might normally suffer little or no damage.
The most important factors in how much damage a plant receives from cold are how hardy it is and how cold it gets, but there are other factors that also play a role in how much cold injury occurs.
Make sure good care is given to landscape plants during the growing season. Plants that are doing poorly because of inadequate nutrition or that have been weakened from disease or insect damage are more susceptible to cold injury. Pruning and fertilizing trees, shrubs and groundcovers should be avoided late in the season as this can stimulate a flush of tender growth which is not cold-hardy.
The tender bark on the south facing sides of deciduous trees and shrubs warms much more quickly than on a northern side, causing sap to flow even on cold days. If temperatures drop suddenly, the sap rapidly refreezes causing sunscald – the plant cells rupture and the bark splits open. Cold winds can also cause alternate freezing and thawing of the bark and sap. This type of damage can be prevented by covering the trunks in the fall with wrap or burlap.
Evergreens are not as affected as deciduous trees by sunscald trunk damage due to the shade provided by their own foliage, but they are very susceptible to desiccation caused by the drying effect of the wind and sun when water loss exceeds the uptake of water by the roots. The use of anti-desiccants will give some protection from desiccation, as will protecting vulnerable plants with a windbreak of stakes and burlap.
Covering plants protects more from frost than from extreme cold. If they are not too large, individual plants can be protected with cardboard or Styrofoam boxes, or cloth such as sheets or landscape fabric. Any covering should extend to the ground and not come in contact with plant foliage. Foliage in contact with covers can be injured due to the heat transfer from the foliage to the colder cover. Remove covers as soon as the temperatures begin to rise to prevent damage from trapped heat.
After sudden freezes, do not prune for several days. It can take a few days for the damage to become evident.
In southern and western areas with moderate winter conditions, watering plants both below and above ground before a freeze can help protect plants. A well-watered soil will absorb more solar radiation than dry soil and will reradiate heat during the night. Be cautious, as prolonged saturated soil conditions damage the root systems of plants.
The foliage of ornamental plants can be protected by sprinkling the plants with water. Sprinkling utilizes latent heat released when water changes from a liquid to a solid state. Sprinkling must begin as freezing temperatures are reached and continue until thawing is completed. The water must be evenly applied to maintain a film of liquid water on the foliage.
In northern regions, mulch can help protect plants from freeze-thaw cycles. It will insulate the ground to reduce the heaving of plant crowns caused by the freeze-thaw cycle and moderate the soil temperature to prevent root damage. Mulch also holds snow acting as an insulator.
Proper plant selection, good landscape maintenance practices all year long, and a quick response when the weather forecast turns nasty can help prevent sudden weather changes during the fall and winter months from seriously affecting your customer's landscape plants.
The author is a landscape account manager for Four Star Greenhouse/Proven Winners in Carleton, Mich.