Shannon Henderson, ANR Educator, UGA Extension Forsyth County
Oh, how wonderful the cool weather is! The brisk mornings roll into perfect afternoons, fire pits are crackling and marshmallows, chocolate bars, and graham crackers are flying off the shelf. Unfortunately, with the onset of this blissful sweater weather, there comes the threat of frost damage to our landscape.
During the fall months, perennial plants prepare for dormancy; they stop producing new leaves and store energy in their wood and roots. If the plants haven’t completed this process before temperatures drop to 32 degrees Fahrenheit or lower, the water in the plant cells freezes, expands, and ruptures the cell walls. This is what causes the characteristic mushy, drooping injury in plants. Cold or windy conditions can also cause leaf browning and scorch, twig death, and in extreme cases bark-splitting. Usually, it’s obvious that your plants suffered cold damage, but occasionally the effects of a freeze show up weeks or months after the weather that caused the injury.
So, what can you do to protect your plants from such a sad fate? As with most gardening issues there are preventative measures… and more preventative measures. The best way to avoid frost damage is to plant species that are hardy in the USDA hardiness zone where you live. Forsyth County is in USDA hardiness zone 7. Also, be aware of the areas within your landscape that are less sheltered. Plants placed in full sun are out in the open to provide their required light, but on clear cold nights, that exposure can threaten these sun-loving plants due to radiational freezes that strip heat from the ground. Windy areas are also more exposed and thus more likely to cause damage.
Plants in containers have additional concerns. Container plants cool down faster than other plants because they don’t have the benefit of ground heat. No matter how large your flowerpot is, it can’t insulate plant roots the same way that the ground does. The best practice is to move potted plants into the house, garage, or shed in freezing weather. If that isn’t possible, cluster them together in a protected area and apply mulch all around and between the plant pots, then wrap blankets, burlap, or plastic around the gathering.
But what if it’s too late? If you planted the wrong thing in the wrong place, how can you save it from frost and freeze damage? The best thing you can do for a tender plant is to dig it up and to put it in a planter to store inside during winter. If that isn’t an option, this is what you need to do: maintain proper soil moisture, avoid nitrogen fertilizers, and mulch your plants as temperatures cool off.
It may seem counterintuitive to water a plant to avoid freeze injury, but a drought-stressed plant will be weakened and thus more susceptible to other types of damage. Nitrogen fertilizers encourage the plant to produce new leaves, the exact opposite of what plants should be doing in fall and winter, so avoid using nitrogen fertilizers while plants are slowing down and moving into dormancy. Lastly, at night the ground radiates the heat it has collected during the day, so mulch will trap heat from the ground, and it will retain soil moisture.
When the forecast predicts a freeze, cover your plants with cloth or cardboard. You want the material to span the space above the plant and contact the ground all around the plant; this will help trap the ground heat and hold it around the plant. It is also common practice to create a frame to support the covering; this way the fabric or cardboard is not damaging branches. Plastic will work to cover plants and keep the heat in, but you will need to remove the plastic soon after daybreak. The reason for this is that plastic will heat quickly in sunlight, and while you intend to protect your plants from the cold, they could end up baking under the plastic, instead.