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Publication By: Patrick Malcolm

Cold hardiness Importance in Growing plants and Trees.

The lowest cold temperatures during the mid winter onset can vary somewhat from one year to the next, but over a period of many years the average temperatures are remarkably stable, however, once every 50 years or 100 years a drastic drop in temperatures may occur. An example of the sudden, drastic temperature drop occurred in South Georgia in January of 1983, when it dipped to zero degrees F., and no one living could ever remember extreme temperatures that low, when the average low winter temperature here is in the 20's F.

Most climate maps show low and high temperatures that are averaged over a number of years, however, a random event, like a drastic temperature drop, like that in South Georgia can happen about every 100 years that caused it to appear that particular winter, when the thermometer was 0 (zero) degrees F., and that South Georgia was in zone 6, whereas, it normally is in zone 8 or 9, when the temperature is normally in the high 20's degrees F.

The effect of a drastic temperature drop to zero degrees, such as described above that is measured on plants depends on how many hours that the low temperatures last, before it warms up. Naturally, if the freeze lasts many hours then the freeze will penetrate deeper into the soil and more and more root tissue will be damaged or killed. The time of the day or night is also a consideration when the low temperature is reach, because of the effect of the sunshine can warm up the plants.

In order to understand the effect of freezing temperatures on plants, one must think about the nature of plant cells with each being surrounded by an enclosed cell wall that is filled with a complex liquid mixture called “Cytosol”. Within the cytosol liquid, mostly water, is dissolved a number of different chemical ions such as Calcium (Ca), Potassium(K), enzymes, protein complexes, etc. Therefore, one can see that the cell will vary in intracellular contents from one another in infinite and indefinable numbers. A plant's 'mass of cells' is made up with numerous cells that are connected together with many other cells stacked on top of one another. If one of these cells with its interior liquid is filled with the complex liquid mixture, ('Cytosol') is compared with an automobile radiator filled with water – and both the 'mass of cells' and the radiator are exposed to below freezing temperatures, it is evident that the fluid (water) will freeze at different thermometer , depending on what soluble products are contained within the water. Thus, by adding to the concentration of the antifreeze to the radiator water, will delay or prevent the freezing and bursting of the radiator. The same example holds for the plant cells, meaning that the different concentrations of chemicals within the plant cells is variable, so that it could lead to the survivability or the death of the plant like the bursting of the radiator depending on the concentration of the (Cytosol') Every plant would show a different tolerance to the freezing temperatures.

The survival of the plant can depend on the environmental factors, and the rate of plant growth during both the fall growing season and the spring growing season. During the fall the plant should slow down in the growth rate. For instance it is well known that the application of Nitrogen based fertilizers should be reduced or eliminated, because the nitrogen causes the plant to increase its rate of growth, thus, the cells are extended and more water is absorbed, that makes the tender growth much more vulnerable to cold damage or morbidity. When potassium “K” fertilizers such as KCL is used, the K ion acts like antifreeze and the plants become “hardened off” and are much more resistant to the cold damage. The alkaline effect of the potassium “K” is also effective in raising the pH of the cells to above “7” which increases cold resistance. Adjusting a growing plant to the approaching fall temperatures to force it gradually into a dormant stage can determine whether or not the plant will survive until the following season, so that the plant is not stressed and weakened by random temperature drops that sometimes happen suddenly. When the cells of plants are swollen or turgid with more than 70% of water, the plant becomes very susceptible to freezing a bursting.

During the spring the growth of plants can suddenly begin by a premature warmup, sometimes called an “Indian Summer”, and if a sudden and drastic temperature drop occurs the plant can be severely impacted. In the year, 1950, an orchard of Schley and Success pecan trees that were growing in South Georgia had prematurely leafed out in February, with some trees showing twig growth of one foot or more, and on March 20th the temperature dropped to 20 degrees. All the new growth was killed on the pecan trees took several years to recover to production. Fruit growers and peach growers are very familiar on how early spring freezes can kill the flowers that might have produced peaches.

In the Northern States where snow is common during the winter, the thick layers of snow flakes actually insulate the plant from extreme temperatures, mainly due to the air spaces between the snow flakes, however, ice, can prevent freeze damage for short periods of time, but eventually after a few hours, freezing can occur.

Grafted fruit trees, shade trees and cloned plants will all behave similarly when drastic temperatures occur, because all of the genetic material is identical in each named cultivar. Some exceptions to this rule do occur, for instance, the rootstock on the grafted plants all are grown from seedlings (generally) and each root stock has a different genetic code from the other and thus, a different resistance to the freezing temperatures/ Rootstocks should be selected that have a greater resistance to the freezing that the top grafted scion. Cloned plant cultivars all have identical genetic cytoplasm and identical freeze resistance.

In the terrible freeze in South Georgia in 1983, when the temperatures plummeted to zero degrees F., and some large Pindo Palm trees were frozen, but many that were planted in the same location that were similarly sized trees showed no leaf damage and survived. Palm trees, like the Windmill palm, Washingtonia palm and the Sabal palms that are offered for sale on the commercial market are normally seed grown and each of these palm trees may show a slightly different tolerance of cold temperatures, except those that clump and clone themselves like the Pygmy Date Palm tree (clone) and the Needle palm tree that will show identical cold hardiness as the mother palm tree.

Sago palm trees also behave in a similar pattern during deep freeze winters, Most sago palm tree are commercially grown as tropicals and are grown from the seed, and therefore, react with individual differences very subtly to extreme frost and some seedling plants will show no damage and others will be killed by the same temperatures., Cloned sago palms will react identically to extreme frosty conditions. In South Georgia, zone 8, Sago palm trees will often show leaf damage when the temperatures drop into the low 20's, sometimes partial leaflet damage, sometimes entire damage with brown leaves that do not regrow until June.

There are other factors that may influence the tolerance of freezing in plants, such as planting the tree, etc. on the North side of a building or plant it in some shade.

Nematodes (eel worms) that are found in the South will damage the roots of a plant and stress will reduce the cold hardiness. Beetles, worms, and woodpeckers can stress trees, along with even fire ants.