I promised to explain how a pansy can stand the cool weather. Actually nobody is totally positive but we think it takes advantage of three facts.
The first fact is that plants die from ice inside the cells. The ice inside the cell punctures the cell wall. Ice between the cells doesn't kill the cells. Ice on top of the plant surface doesn't kill the cells. Ice on top of a plant can occur when it is not cold enough for ice to form inside the cells. This has happened several times this year at my house. Washing the frost off in these cases will not help the plant cells. They are going to live anyway. If you wash the frost off it could hurt. As long as you are spraying water on it it is okay. When you walk away from it, evaporative cooling will cool off the plant. Evaporative cooling is a big word but it just means what happens when you take a shower outside in the wind. If the plant tolerates the evaporative cooling it will live but so what. It would have lived anyway.
When frozen water occurs on the surface, the term white frost is used. A killing frost occurs when it is cold enough for the ice to form inside the cells. The term black frost describes an event where the low humidity and wind don't allow the frost to form on the outside of the plant, yet the temperature is cold enough to form ice inside the cells. You don't see frost but when it warms up the cell contents dump and the plant turns black.
The second thing happening in a pansy is osmosis. I think this is the same thing that happens when you play too long in the bathtub and your fingers wrinkle. The water has entered your fingers. Then the skin expands and has to wrinkle in stay in place.
Finally, the more impurities there are in a solution, the lower its freezing point. This is the same thing that happens when we put salt on the road. The salt makes the ice impure and lowers the freezing point.
The process of ice going to water or water to ice takes time. It doesn't happen instantaneously. For example, the ice cubes in our drinks last awhile. In a plant the process of water changing to ice also takes awhile.
So let's put all of this together. Ice forms between the plant cells in the pansy. As the pure water freezes, it changes the solution outside the cells. Water moves from inside the cells to outside the cells by osmosis. This water also freezes between the cells. More water moves out. It also freezes. The remaining solution inside the cell has a higher concentration of sugars and other molecules so inside the cell doesn't freeze. With no ice inside the cell the plant lives.
So a garden variety pansy could set on a deer stand at 10 degrees for eight hours with no problem. Why we use this plant name as a synonym for an effeminate male is beyond me.
Article was written by David Goforth Agriculture Extension agent North Carolina Cooperative Extension Cabarrus County Center. Visit my homepage http://cabarrus.ces.ncsu.edu/ or http://cabarrus.ces.ncsu.edu/index.php?page=lawngarden or my blog http://gardeninggurugoforth.blogspot.com/
Contact me at David_Goforth@ncsu.edu. Reviewed 2007.