A common question during winter when we are expecting temperatures to really dip into the freezing range, like they are this week is, “Will my plants survive the cold snap?”

This is not as straight forward as one might think.  We live in a region where we often have plants that are a bit more tropical than what our climate actually is planted in our landscapes.  Here in Carteret County we are in the USDA plant hardiness zone 8b, which means our average low temperature is between 15 and 20 degrees Fahrenheit.  As the USDA plant hardiness zone number gets lower the average low temperature also gets lower.  This means plants that can grow in a lower USDA plant hardiness zone, like 7a or 6b, will do fine when we have a below average temperature for a night or two.  However plants that are at the northern end of their USDA plant hardiness zone tolerance may receive some damage when temperatures really drop.

Here is an example: The sago palm (Cycas revoluta) is a beautiful plant that I see in some landscapes in our area.  This plant is labeled as hardy to USDA plant hardiness zone 9a – 12b.  We are just outside of that range. Most years temperatures don’t get cold enough to cause injury to the plant, but occasionally we will experience an extremely cold night or two that gets down below the tolerance of this plant.  When that happens ice will form inside the cells of the plant causing the cells to burst.  When the temperatures warm back up, the leaves of the plant will die and turn brown.  If the temperature gets low enough to freeze the protected growing tip of the plant, then the plant will also die.

Another example is the dwarf palmetto palm (Sabal minor), which is native to our area and is listed as having a USDA plant hardiness zone range of 7a – 10b. When we have an extremely cold night, this plant will be fine, because it has developed the ability to withstand colder temperatures than what we normally experience.

“How do plants protect themselves from the cold temperatures?” That is another good question.  Scientifically speaking, they load up their cells with sugars, salts, and proteins that prevent the moisture in their cells from freezing. A good way to test this is to fill up a water bottle with tap water and another with water that has a significant amount of salt (1 to 2 tablespoons) dissolved in it and put them both in the freezer.  Check them occasionally, about every 60 minutes, to see which one freezes first.  The more salt that is in the water, the longer it should take the water to freeze.

There is a bit more to it than that, but that is the basic idea.  Plants have developed in specific regions to withstand the colder temperatures of those regions.  Plants that are planted on the boarder of their tolerance zones will struggle, while those that are well within their zones will be fine.