We have had many questions about freezes and how to care for tender plants when temperatures are expected to drop below freezing. Here are a few tips:
1. In absence of a good rain, water well the day or night before a freeze. The moist soil and higher humidity around the plants will moderate temperature extremes.
2. Mulch with leaves or wrap with paper the more tender tropicals, such as hibiscus.
3. Consider a plastic frame and light bulb around special plants, such as tender citrus.
4. Do not wash frost off plants the morning after a freeze. This action raises the temperature too quickly and usually damages cell tissue.
5. Set tender-potted plants in protected areas on the ground. Do not leave them in exposed windy locations on concrete.
6. The special plants in pots that are subject to freeze can be brought into the house or garage when freezes threaten.
I have been asked to try to explain what happens when plants freeze. Here goes: Water in the plant, outside the cell walls, is relatively pure and freezes when it reaches 32 degrees F, while water inside the cells of plants contains dissolved salts, sugars, enzymes, and other substances that act as "antifreeze." As ice crystals form between cells, the water inside the cells is drawn out through the cell walls. This causes the cells to get smaller. The resulting pressure and stress may cause the walls to break. If the temperature drop is sudden and extreme, ice crystals may form inside the cells of some species, rupturing the cell protoplasm and killing the cell in this way.
A more common means of plant dieback or death occurs when the water between the cells begins to thaw. This causes a flow of water back into the not-yet-damaged cell. If the rate of thaw is rapid, the swelling of the shrunken cell could be so sudden that the cell walls may rupture. This explains why plants subjected to bright morning sunlight after a heavy freeze will show freeze damage. Loquat, gardenias, pittosporum, and even ligustrum may die back.
Some plants are less affected by freezes because of their "hardening" ability. This normally occurs during autumn periods of repeated exposure to slowly increasing coldness. This hardening may involve the formation of cutin (the shiny material on leaves), the suberin of corky bark, the cellulose of all plant cells, and the lignin found in woody plants. In addition, there is a gradual loss of water so that hardened tissue is drier - less of water so that hardened tissue is drier - less ice crystals to form.
