Here are a few tips on how to prevent freezing of tender plants:
1. In the 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 the more tender tropicals like hibiscus with leaves or wrap them with paper.
3. Consider a plastic frame and light bulb around special plants like 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. Special plants in pots that are subject to freeze can be brought into the house or garage when freezes threaten.
What happens when plants freeze?
Water in the plant, outside the cell walls, is relatively pure and freezes when it reaches 32°F, while water inside the cells of plants contains dissolved salts, sugars, enzymes, and other substances which act as "anti-freeze." 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.
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 yet undamaged cell. If the rate of thaw is rapid, the swelling of the shrunken cell may 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.
