As you walk through your garden after that first hard frost you see some plants with leaves hanging limp like wet laundry on a clothesline or like lettuce that had been sitting out of the fridge for way too long. Other plants look completely normal, unaffected by the escapades of Jack Frost the night before. As you survey this, have you ever wondered why this happens? Why are the dahlias flagging? And why are the sedums acting like nothing’s happened?
Let’s take a look at frost, freezing injury, and why this occurs –
What does frost damage look like?
- It feels like we’re at risk here in our Edmonton area ANYTIME of the year….which is probably pretty close to true. Our climate here boasts an average of 110-115 frost free days. The risk of frost is pretty much gone by June 9 – but not guaranteed, and we can see frost toward the end of August on any given year.
- Frost is funny – because air is always moving, ambient temperature is never completely homogeneous. One area of the garden could catch a patch of freezing air where another area is unscathed. One plant could get a nip of frost bite when the next plant is untouched.
- Frost damage looks like darker patches on leaves, limp and wilty looking leaves, and sometimes leaves have light or white patches on them.
- The amount of frost damage depends on the severity and length of time the sub-zero temperature lasts — a heavy frost (below -3C) for 1 hour could cause less damage than a light frost (down to -2C) for 6 hours.
What does frost do to plant structure?
- When temperatures dip below freezing, ice forms on top of the leaves and within the leaves. Ice crystals that form inside the leaves are sharp and they damage cell walls. Once the cell wall has been breached by an ice shard, the contents leak out. The cell dies, the leaf loses it turgor and it hangs limp.
- Cell injury like this is related to how quickly the cooling happened. If it was very fast, you will see more damage than if there is a slow winding down of temperatures over a longer period of time,
Why do some plants tolerate frost and others not?
- Genetics play a large role here – tropical plants are meant for tropical climates and they are engineered for that life, just as plants that live in mid-North America versus the Arctic.
- The plants that tolerate frost have a structures and abilities in place that make frost tolerance possible. As temperatures begin to cool over a period of time when autumn approaches, they begin to shift fluid out of their cell structures into extracellular tissues. This increases cellular solute concentration which in turn lowers the temperature at which the cells will freeze.
- Cell wall polysaccharides are at a higher concentration and antifreeze proteins contribute to a elevate tolerance to low temperatures.
- Bulky organs (aka thick leaves) are another way plants are protected from the cold. Plants like sedums and tulips have thick leaves. The thicker the leaf, the more insulated they are from frost.
- Plants that tolerate dry conditions tend to have less fluid in their cellular structure and they handle sub zero dips better.
So that’s the abbreviated version of the frosting story and we no longer have to stay up at night wondering why our garden plants behave the way they do when autumn marches in.