We talk about frost a lot around here. Even our name, Frosty Garden, is deeply rooted in the concept. So, let’s deep dive into what frost is, how it happens and what it means for your plants.
It all starts with this mysterious meteorological measurement that you’ve probably heard of, but have no idea what it means. And that’s called the dew point. It’s largely used by pilots to determine the risk of icing or fog conditions that could present risk to their aircraft. But, the gardener can also use it to understand, and sometimes predict, frost.
The dew point is a temperature that air must be cooled to where it will reach 100% relative humidity. When the air temperatures drop below that dew point, that humidity will condensate into water form. From a weather perspective, we see these things as fog, mist or sometimes a morning dew cast over the ground. It’s also why your water bottle sometimes condensates on the outside. When temperatures are cold enough to freeze, these conditions result in frost.
We’ve talked about this many times, but temperatures are rarely the true enemy of plants. But, when frost develops on a plant, any plant that doesn’t have inherent frost tolerance can experience irreversible cellular level damage almost immediately. If severe enough, it’s game over for that plant.
Frost most frequently occurs over the wee hours of the morning, typically at the coldest point overnight. This is why it’s quite dangerous to put your plants outside too early in cold climates. The outdoor grower has zero control over either the air temperature or the dew point. At best, you have frost tolerant plants to work with. If that fails, all you have is frost cloth.
When we talk about the frost tolerance of plants, we’re talking about the degree of frost severity at which that plant can resist cellular damage. For simplicity, growers have boiled these down to two points, a hard and light frost. These translate to cold hardy and frost tolerant plants. Every plant can be measured on this scale.
When you understand how frost forms, you can now appreciate why we only heat our greenhouse to a meager 35 to 40 degrees Fahrenheit. Our objective isn’t to keep our plants nice and toasty. Rather, it’s to raise temperatures to a level where if the air temperatures fall below the dew point, frost cannot develop. (Because it’s too warm!)
When we talk about “average last frost dates,” that’s the average point at where air temperatures can no longer present the risk of frost. Basically, where nature is providing the above mentioned greenhouse conditions. While these dates are averages based on long term climate data, the risk of outdoor frost is real until that risk passes each season.
Hopefully this deep dive into frost has helped further your understanding of why the cold temperature tolerance of plants is vital for us to understand. It helps us navigate our cold climate and fully understand where our risks are, how to avoid them and things we can use to ensure our plant’s success.
