I received a request from Bill Cox to write something about frosts and freezes since it has been on the mind of a lot of growers this week due to our return to cold conditions after a very warm February. Here is some information I put together for the Garden Professors blog in fall 2022. There is also a good discussion from Michigan State University Extension at the link here.
Frost versus freeze
One of the questions I often get is what is the difference between frost and freeze? The National Weather Service (NWS) puts out both frost and freeze warnings but has different criteria for each. For a frost warning, the predicted temperature may not even get down to 32 F (0 C), but may hover in the mid 30’s. For a freeze warning the predicted temperature is expected to get down to 32 F or below and for a hard or killing freeze it usually gets down to 28 F or lower. Once the area has gotten down to 28 F or lower, the NWS usually stops issuing additional freeze warnings since at that point all but the most cold-hardy plants have completed their growing season and are either dead or dormant. In spring, they start the frost/freeze warnings again once it has been determined that there are vulnerable crops that are out of dormancy that could be impacted by the cold conditions. All of Georgia is now considered vulnerable so they are issuing warnings for the entire state.
How does frost form if the temperature does not get down to freezing?
To understand frost formation and when warnings are issued it helps to know both how frost forms and how temperatures vary near the ground. Frost crystals form on surfaces that get down to freezing and have something on the surface that is conducive to seeding crystal formation. This can happen even when the air temperature is above freezing in conditions of light wind and clear skies that allow surfaces to cool to freezing temperatures by emitting heat radiation out to space at night when there is no incoming solar radiation. Conditions for this can occur with temperatures anywhere in the 30s with a reasonable amount of water vapor in the air and as long as the surface (a metal car body, an asphalt roof, or a blade of grass) can cool to the freezing point. At that point, anywhere on that surface that has an appropriate scratch, particle, or other imperfection can serve as a place for ice crystals to form and start to grow. These are called nucleation sites and allow the initial formation of an ice crystal upon which more ice can grow into delicate but visible frost.
Frost will not form if the humidity is too low because there is not enough moisture to produce visible crystals. Often frost does not damage the plants a lot because most of the frozen water is confined to the surface of the plant and does not affect the interior cell walls, although there may certainly be some damage where the ice forms. Large formations of ice crystals can sometimes form on trees or fences if the conditions are right; this is called hoar frost.
Frost forecasts are also provided with the understanding that the NWS is forecasting temperature values for their thermometer heights of about 2 meters or 6 feet high, since that is how they verify the accuracy of their forecasts. In light winds and clear skies the temperature at the ground level is often colder than the temperature at the thermometer height due to cold air sinking so the ground in your garden may be colder than the forecast would predict. Frost is also more likely to form on elevated surfaces that don’t have contact with the ground, since soil temperature keeps the ground surface warmer in Fall than later in the year due to residual heat from the summer warmth. Blueberry farmers that I work with tell me that you can sometimes see quite a difference in frost damage to their bushes from top to bottom due to the different temperatures that the plant may experience at different heights above the ground. Bridges often have signs that they freeze first for the same reason.
Freeze damage to garden plants
The NWS issues freeze forecasts when the temperature is expected to get down to or below 32 F. The damage that the freeze does to plants depends on how long the temperature drops below freezing and how susceptible the plant is to cold temperatures. If the temperature barely gets down to 32 F for a short period damage is likely to be minimal since the water inside the plant cells did not have sufficient time to freeze. But if it lasts longer the water in the cells freezes and, as you undoubtedly know, ice expands and breaks the cell walls causing irreversible damage to plant leaves and stems that lead to their death. There is a useful table of how different garden vegetables respond to cold temperatures in the 2020 blog on spring frosts, which underlines why some vegetables like spinach and cauliflower do better as late-season vegetables than tomatoes and melons.
This week (March 15, 2023)
With our shift back to a colder weather pattern, the chance of frost has gone up across Georgia, especially in the northern half of the state. On the morning of March 14, the frost was an advective frost, with cold air moving into the state from the northwest. On the 15th (this morning), the frost was a radiation freeze due to clear skies and calm winds with low humidity. We may see more frost on the morning of the 16th in some locations as well. In addition, another shot of cold air is likely this weekend around the 19th to 20th, so we are not out of frost season just yet.
Two publications that you might also find useful in thinking about frost and freezing conditions: