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A row of Ficus nitida trees damaged by frost, note brown branch tips

November is Frost Damage Month in Arizona

It’s hard to believe that late in Oct of 2003 we faced 100° days, and now 3 weeks later, we’re bracing for frost season! For the past 20 years the coldest temperatures in Phoenix have occurred around Thanksgiving when frost damage is usually most severe.

There are many misconceptions about how frost actually damages a plant. It is incorrectly assumed that ice forms inside cells causing them to rupture and burst open when they freeze. This never happens. When people first examined frost damaged plant cells under the microscope in the 1930’s, they never found any burst cells. Cells from damaged tissue were always dehydrated and wilted but their cellular membranes were intact and never torn open. The so called, “Rupture Theory”, was proven false in the 1930’s but the notion continues to this day.

It just so happens that freezing damage is a wilting process that is identical to dehydration damage. But how does wilting occur as plants are frozen?

The surprising clue came from a citrus orchard in Israel where farmers were spraying their trees with tetracycline to control a bacterial disease. Half way through the orchard they ran out of spray and couldn’t finish spraying the crop. Later that night the temperature dropped to 25° F and severe frost damage occurred on the trees that weren’t sprayed. But to the farmers’ astonishment the trees sprayed with the antibiotic did not have any frost damage. How could an antibiotic be a frost protectant? It did not make sense. What was the link?

After much study it was discovered that the antibiotic prevented ice crystals from forming on leaf surfaces. How? To the surprise of everyone it was shown that the antibiotic suppressed bacteria that caused ice crystals to form. These ice-nucleation bacteria triggered ice crystals to form that caused the frost damage. If ice didn’t form, there was no frost damage, in spite of the cold temperatures being the same.

The key to surviving freezing damage was to maintain water in a liquid state, even if temperatures dropped below 32° F, the known freezing point of water. Water that stays liquid, even when its temperature drops below 32° F, is called super cooling. As the Israeli’s found out, anything that promotes super cooling may protect plants against freezing injury.

How is super cooling achieved in nature? One thing is leaf cleanliness. Ice crystals form on dusty or dirty leaves more quickly than on clean leaves. This occurred in the Israeli orchard. Another treatment is to spray leaf surfaces with a sugar or glycerol solution. The sugar dissolves in dew on leaf surfaces and lowers the freezing point of the dew by several degrees. Under the right conditions this is just enough to protect the leaves from freezing damage.

Plants that are naturally frost tolerant use this strategy by making excess sugars or proteins to lower their freezing point. Freezing tolerant plants also tend to have smaller leaves and produce smaller cells as day length’s shorten. The small cells increase surface area and making it more difficult for ice to form. This explains why tropical plants have difficulty protecting themselves from frost. Unlike temperate zone plants, tropical plants can’t distinguish long from short days. 

Tropical plants tend keep their leaves and produce large cells year round. Gardeners can control cell size with fertilizer and water applications. Frequent fertilizer and water will promote large cell size that is more apt to freeze. Thus, the traditional practice of “hardening plants” to protect from frost injury is based on sound scientific principles. Cutting back on water and fertilizer increases sugar content that reduces ice crystal formation.

Hardening is one approach to protecting plants from freezing temperatures. The other is to modify the microclimate around the plants. How? By adding heat when temperatures are low. How is this done? Homeowners do this by covering their plants at night to trap heat and prevent it from escaping to the surrounding air. Blankets, towels, sheets, plastic tarps all work. Another procedure is to string light bulbs in the trees. High wattage bulbs generate more heat. This should only be done on the night frost is threatening, not before or after. Infared heat lamps may also work from a greater distance than incandescent bulbs. Fluorescent bulbs do not work well because they emit less heat. Remember that the radiant energy from light travels in a straight line and that trees in shadows will not be protected. Be sure that light surrounds the tree or else the shadowed side could be damaged.  Farmers have traditionally added heat to save their orchards by burning smudge pots or spraying huge amounts of water to their trees during the actual frost. Homeowners should avoid sprinkler irrigation of trees where light bulbs have been strung up. The electrocution hazard is real. Farmers also occasionally use wind machines or helicopters to mix the hot air that sits above the cold air during a temperature inversion. Of course this procedure is impractical for residential homeowners.

The extent of frost damage on trees is dependent on the temperature reached, and the duration of the low temperature. Conditions that favor large versus small ice crystal formation cause the most severe frost damage. For example, four hours at 28° F can be more damaging than one hour at 23° F. Also, freezing damage depends on the plant with each species having its own “killing” temperature. The most sensitive are tropical evergreen plants with large leaves. These can be killed by temperatures as high as 40° by a process known as chilling injury. For most tropical plants, however, the killing temperature is between 32-28° F. The next most sensitive groups are the broad-leaf, semi-tropical evergreens. These include eucalyptus, citrus and Ficus that are killed by temperatures between 32-18° F. The succulents, that include cactus, agave and yucca’s have killing temperatures between 28-5° F, depending on species and their acclimatization. The next group on the list are deciduous trees and shrubs with killing temperatures between 20° F down to -40° F depending on species and their acclimatization. The most frost hearty group are some of the temperate zone conifers, which can survive bone chilling temperatures down to -70° F in the most extreme alpine and arctic conditions.

Although we in Arizona don’t have to face frosts every year they can cause severe and costly damage to trees that large trees that managed to escape damage during the mild years.

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