During the winter months, when horticulture staff are busy covering plants in advance of a hard freeze, a visitor might stop and ask, “How badly is it going to frost tonight?” Another might inquire about the frost hardiness of a particular plant that we are covering. I hear far more references to the term frost than I do freeze. But which is correct? It may be a cultural or regional choice to use one word or the other, but growing up on a working farm in Virginia, I learned at an early age that frost and freeze have very different meanings.
When we cover sensitive plants here at the Arboretum, we are attempting to protect them from a range of temperatures at or below 32 degrees F., the freezing point of water. Most of our plants are hardy to temperatures in the upper 20s for a short period of time (a couple of hours or so). However, when freezing temperatures are predicted to slip into the lower 20s or upper teens, we find ourselves going full throttle to cover and protect a greater number of plants.
Frost is different. It’s what I scrape off my windshield. Frost is usually not harmful to most of the plants here at the Arboretum, though there are a few exceptions. For frost to form, two things have to happen. First, the dew point must be reached. This is the temperature at which the air is saturated with moisture and water vapor condenses, forming the dew that we see on plants and other objects. Second, the surface where the moisture condenses must be 32 degrees or below. Ironically, because of radiational cooling (see sidebar), frost can actually form when the surrounding air is above 32 degrees. Except in instances of cold temperatures following winter rain, we do not see much frost here at the Arboretum.
A dramatic sounding term like killing frost is probably a carry-over from home gardens or farms in areas of higher humidity, where frost is associated with the freezing temperatures that signal the end of the growing season for warm weather crops. Even Arboretum horticulture staff commonly interchange the words incorrectly. We will often discuss past frost events, apply frost blankets to protect sensitive plants, and write up a frost report that describes the extent of damaged plants. But in all of these cases, freeze is really the word that we should be using.
Severely cold events, especially those that combine freezing temperatures over a long period of time, are very costly to the Arboretum. This is particularly true with many of our collections of succulent plants, such as our South African succulents and Aloe collection, which is why these plants may be covered for a few days or so in duration. It is unfortunate that we have had three separate occasions of hard freezes within the past six years. From both a professional and personal perspective, it is very emotional, agonizing, and extremely frustrating to deal with these situations. But, it has become part of what can be expected in most years between December and early March.
Whether you call it frost or freeze, the physical damage to gardens from severe freezing temperatures is extremely difficult to remedy. Arboretum staff and volunteers miraculously pull together to protect and preserve everything here to the best of our ability. Mother Nature seems, at times, to seek out and destroy, but she will not win the battle every time.