Plant Answers  >  Cold Protection of plants

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The weather forecasters in this area certainly pay homage to the old saying that "only a fool or a newcomer will try to predict Texas weather." I fear that these prognosticators of the climate will have frost on their eyebrows before they will sound the protection alert to home gardeners. To remedy this situation, I have decided to tell you when the first frost will occur!

After careful examination of the thickness of animal fur, the sharpness of porcupine spines and quantities of mesquite pods correlated meticulously with moon signs, sun spots and migratory habits of the gray?backed pillbug, I have decided that the first killing frost for South Central Texas WILL occur in November. A really hard freeze will occur in December. Freeze denotes the condition where air temperature of surface objects (i.e. plants) falls below 32 degrees F. (0 degrees C). Frost is simply the occurrence of condensate freezing on exposed surfaces. For frost to be deposited, the surface temperature of an object must drop below the dew point of the surrounding air. A freeze can occur in a dry air mass without frost being deposited.

Advective freezes occur when a cold air mass blows in; this is the typical "Norther". The temperature of this air is below freezing at ground level with windy conditions, which makes protection of tender plant material difficult. A homeowner may cover plants with straw or similar material, or with a stout container like a trash can. The degree of protection would depend on the insulating capacity of the cover. If you have followed the recommended practice of caging tomatoes, you may cover these cages with large plastic garbage bags to protect the plants within the cages. The cage will act as the supporting structure.

If you do not have cages, you might want to erect a supporting structure. Be certain these cages or structures are well secured or anchored since high winds usually accompany the bad northers. If cages covered with garbage bags should blow over during the night, not only will the plant freeze, but it also will be pulled out of the ground by the roots. In this instance, the remedy for protection may be worse than the frost damage. Be careful not to remove your covers too early on the morning following the frost. If your plants have gotten slightly frosted during the night, a slow "thawing" can prevent extensive damage. Also, rapid exposure to cold temperatures is rough on plants just as it is on folks. Take your time!

A radiation freeze is a different matter. All objects (houses, soil, plants, etc.) radiate heat in all directions; the amount of heat radiated is proportional to the object's temperature. Conversely, all objects absorb heat given off by their surroundings. In the case at hand, a plant exchanges radiant heat with the sky. During the day, the input from the sun is greater than the radiant loss from the plants; therefore, the plant and its surroundings heat up. After sundown the energy flow reverses and the plant loses heat to the sky.

On clear nights, the radiative cooling of plants can be substantial; in fact, a plant can get considerably cooler than the air surrounding it. Even if the day has been mild, enough radiative cooling can take place at night to freeze plant tissues. Significant freeze damage can occur even when the air temperature never gets to 32 degrees whether or not frost is deposited depends on the moisture level of the air. Classic radiation freeze conditions are clear sky and calm, dry air. Cloudy conditions reduce the amount of radiative loss to the sky. Any appreciable wind movement keeps plant tissue from cooling below the temperature of the surrounding air.

One natural control of frost injury is elimination of bacteria on the leaf surface of plants. Dr. Steven Lindow, University of California?Berkeley, has found that three species of bacteria are active ice nuclei (catalysts for ice formation) at temperatures only slightly below freezing and initiate the ice formation that is required for frost damage. Furthermore, Lindow has found that these bacteria live on leaf surfaces of healthy plants in large numbers, and the amount of frost damage increases as numbers of ice nucleation?active bacteria increase on a given plant. A reduction in bacteria population results in a corresponding decrease in frost injury. What this means is that an application of a bactericide spray, such as streptomycin (sold as Agri?Strep or Fertilome Fireblight spray) will provide cold protection to some plants to temperatures as low as 27 degrees F. Researchers have found that the best spray solution to use is 100 ppm of streptomycin (approximately one tablespoon) and two tablespoons of table salt per gallon. Spray to runoff or thoroughly wet the foliage at least 24 to 36 hours before the frost occurs. A treatment should be effective for several weeks. The use of bactericide?fungicide product will also control foliage diseases so its worth a try. Keep the streptomycin; it will be useful to prevent fireblight on apples and pears next spring.

Another thing that you might try is an anti?transpirant spray such as Wiltproof or Cloud Cover. These reportedly provide a certain degree of protection. Remember to be scientific in your application. This means that certain plants should be sprayed with streptomycin and salt, others with the anti?transpirant, some with both and some with nothing. Then after the freeze we can evaluate the effectiveness of the treatments. I have mentioned that the streptomycin can be used this spring to prevent fireblight; the anti?transpirant can be used even sooner to keep your Christmas tree fresher for a longer period of time. So both products have an alternate use.

When should the frost protection be used and on which vegetables? I wholeheartedly rely on our weatherman and subtract a degree factor of eight for safety. For instance, if your favorite weatherman predicts a low of 36 degrees, you had better cover the plants. This applies especially to those living in the northern areas of our city. If the weatherman is right and the temperature only falls to 36, you have merely experienced a trial run. If Parsons is right and the temperature dips to below freezing, you owe the vegetable specialist a Christmas tomato.

Vegetables which are frost tolerant include asparagus, beets, broccoli, Brussels sprouts, cabbage, carrot, cauliflower, celery, chard (Swiss), chicory, Chinese cabbage, chive, collard, endive, garlic, horse?radish, kale, kohlrabi, lettuce, mustard, onion, pak?choi, parsley, parsnip, pea, potato, radish, rhubarb, rutabaga, salsify, shallot, spinach, and turnip.

Vegetables which are damaged by frost include: bean, bean (lima), chayote, corn, cowpea (Southern pea), cucumber, eggplant, muskmelon, New Zealand spinach, okra, pepper, pumpkin, soybean, summer squash, sweet potato, tomato, watermelon, winter squash.

So prepare yourself ?the cold is upon us!


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