Frost vs Freeze

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It’s that time of year again where we see frost on the grass in the mornings and are worried about our tender plants freezing! Even though the words “frost” and “freeze” are used interchangeably, they do mean totally different things. So what’s the difference?

A frost occurs when ice crystals form at the surface of plants, which can damage the leaves and blossoms of cold- sensitive species. For frost to form, plant-level temperatures must reach the freezing point of water which is 32 degrees Fahrenheit. Frost damage usually results in burnt leaf or flower edges, and maybe some leaf and bloom drop from the newer plant tissues. The National Weather service issues frost advisories when low temperatures are forecasted to be between 33-36 degrees. Our average first frost date for Alexander County is around October 21st and our last spring frost is usually around April 21st. These dates aren’t set in stone of course because the weather doesn’t read a calendar, but they do offer a basis as to what time of year frosts come and go.

Now a freeze is generally more damaging than a frost, as it occurs when the water in the plant cells freezes and expands. Freeze damage usually ends up killing the plant because it damages their vascular system (cell walls); they wilt, turn brown, die back, etc. A freeze also requires air temperatures of 32 degrees or less, and when weather stations are seeing these temperatures, plant-level freezes are likely. The first freeze will generally come one to two weeks after the first frost, but again, the weather will do what it wants.

To protect some of your more tender plants, you should water plants at their base prior to freezing temperatures. This ensures that their roots are well insulated. Drought stricken roots are more susceptible to freeze and frost damage. You can also cover your plants with a frost cloth, cardboard, or a sheet all the way to the ground. Lastly, adding mulch to the base of your plants will help regulate the soil temperature by adding another layer of protection and warmth.

You can leave your summer herbaceous perennials out to be killed by the frost, because they should die back in the winter anyways. After the frost/freeze kills or browns the foliage, you can cut these leaves and other plant parts back. Up until now, these plants have been using those leaves to gain energy in their bulbs or crowns, to be used for next year.

I hope this clears up a little about a frost vs a freeze and what needs to be done in these instances!