As most of us already knew, and are being reminded this week, January is the coldest month of the year. The temperature can drop quickly, as much as 30 to 40 degrees in just a few hours, when cold fronts move through.
Most ornamentals in the Augusta area can acclimate well to such blasts, provided they are cold hardy for the growing zone.
Acclimation or cold hardiness is triggered by two environmental phenomenons: short days and cold temperatures. As days get shorter from fall to winter, plants are signaled to shut down their growing cycles and go into dormancy. With those first killing frosts and freezes, plants go fully dormant and become well acclimated to the cold.
Sometimes, several days of unseasonably warm weather can cause plants to de-acclimate and lose hardiness. A sudden freeze following several warm days can be disastrous to landscape plants, particularly those that are not genetically hardy for the zone where they are being grown.
The term "genetically hardy" refers to the ability of a plant, because of its heredity, to tolerate certain temperature extremes.
In Georgia there are five cold-hardiness zones. Zone 6b is in the mountains and northeast corner, where average minimum winter temperatures are zero to 5 degrees. Last Monday morning in Blairsville, Ga., it was below 10 degrees.
Zone 7a is also in the north Georgia mountains at lower altitudes and a bit farther south, with minimum temperatures of 5-10 degrees.
Zone 7b is in the north-central/piedmont section of the state where minimum temperatures range from 10 to 15 degrees average.
Zone 8a is in the central-south portion of the state, having average minimum temperatures of 15 to 20 degrees.
And finally, zone 8b is along the coastal areas and basically the counties that border the Florida line, where lowest temperatures average 20 to 25 degrees.
In the Augusta area, you are either in hardiness zones 7b or 8a.
Plants adapted to zone 8b winters are marginally hardy in zone 8a and not reliably hardy in zone 7b. Some examples of zone 8a and 8b plants that are often planted in our area are climbing fig, Algerian Ivy, Confederate jasmine, holly fern, Japanese fatsia, pittosporum, mahonia, loquat, oleander, podocarpus, pyracantha, tea olive, sweet viburnum, and Florida anise tree.
As a general rule, if you live in zone 7b, you should take more precautions if you plant any of those shrubs hardy only to zone 8.
One thing you may have noticed in our area is how low winter temperatures can vary so much, regardless of which zone you live in. I have seen as much as 12 to 13 degree differences between Bush Field and Daniel Field morning lows. And I have seen it be much colder at Bush Field than Appling, even though Bush Field is in zone 8a and Appling is in 7b. Bush Field is in a low area, so the cold air settles there. As you can see, the low you get depends on the elevation of your residence.
One of the best ways to protect tender ornamentals from the cold is to grow them in containers that can be carried indoors before a severe freeze.
If you are attempting to grow them in the landscape, plant them in a courtyard or inside a windbreak, and preferably close to the house or some other structure. Make sure the plants are well mulched because the roots are more sensitive to the cold than the tops.
Blankets can provide some protection. Plastic sheeting, 2-mil to 4- mil in thickness, can be used, but it's best to build a wood frame mini-greenhouse around the plant before covering it with plastic. Whatever covering is used, keep it open to the ground to allow warmth from the soil to radiate upward into the canopy of the plant.
You can provide extra protection by placing an incandescent light bulb under the covering to provide warmth.
Of course the best advice would be to plant only those plants adapted to our hardiness zones in the first place. Most reputable garden centers will carry only plants adapted to our area, or they'll at least warn you about the tender ones.
So, unless you are particularly fond of lemons, oranges, bananas or some other tender tropical, or you don't mind covering and uncovering plants repeatedly throughout the winter, select plants adapted to your hardiness zone and the imposed hardships your landscape site has to offer.
Reach Sid Mullis, the director of the University of Georgia Extension Service Office for Richmond County, at (706) 821-2349 or firstname.lastname@example.org.