Did the Freeze get your plants?

As you know, Sunday and Monday night of this week our temperatures dropped below freezing.  One challenge you can always add to the list of gardening challenges is a late freeze. It doesn’t happen every year, but occasionally we have a late freeze come in that can cause damage to landscape plants, vegetable plants and trees. Some things can be done to warm season crops to minimize freeze injury, such as watering well before a freeze, covering plants with row covers or other material (you must drape the covers to include the soil around the plant) and/or mulch plants and beds to help insulate the soil. These strategies can help with mild freezing temperatures or frost events, but sometimes temperatures drop low enough that we have no control over Mother Nature and what happens to our plants.

We all know that freezing temperatures can kill or damage plants but what is really taking place? All plants have water inside the tissue cells. When we have freezing temperatures, the moisture inside the plant tissue freezes, creating ice crystals. The crystals pierce the plant tissue and damage the cell walls. As the ice begins to melt, the fluid in the cells leak out and the tissue will turn to mush as it begins to dry out. The first sign of freeze damage will be plant leaves turning dark. They will appear almost black, and then turn to a brown color and dry up.

When we talk about frost on plants, things can get a little more confusing. First damage begins on the exterior of the plant. As the air temperature begins to drop, the plants begin to radiate heat and create water vapors on the outside of the plant. As the temperature drops, the water vapors freeze, forming frost on the exterior of the plant. Often times I am asked, “Can you have a frost without having a freeze?” The answer to that is yes and no. When temperatures begin to drop, solid surfaces will lose heat faster than air. An example is your vehicle windshield. Often times you may see frost on it and not on anything else. This is because as the air temperature began to drop, the windshield started radiating its heat away, causing the temperature on the glass to drop faster than the air temperature, and therefore frosting up. Plants will do the same thing. Since the plants loose heat as the air temperatures drop, the plant surface can be a few degrees lower than the air temperature around it. This means that air temperatures can drop close to freezing and the plants themselves are below freezing, creating frost to form on the plant surface without the air temperature truly dropping below 32 degrees Fahrenheit. So what does all that mean? When you have frost on your plants, there was a freeze at the plant surface, but the actual air temperature may not have dropped below freezing. To help avoid frost, you can place anything over the plant that will help radiate the heat back down towards the plant, such as row covers, buckets, milk jugs, etc. This will force heat back towards the plant helping reduce the chance of freezing on the plant surface. Next time you have a frost, look around you probably won’t see frost forming under your porch, under trees and shrubs and other protected places.

Frost damage will usually occur on still, clear nights when temperatures drop near or just below freezing for a short period of time. Freeze damage is usually more extensive and caused by lower freezing temperatures for longer periods of time.