Damage to plants during the winter is something we gardeners worry about, and after a few questions about it I thought I should write something about how it happens (and how it doesn’t).
There are three main causes of winter damage: freezing, breakage due to snow and ice loads, and dehydration.
Freezing (or “cold damage”) seems like an obvious problem, but it’s almost never the cause of winter damage on our garden plants. Freezing causes damage when ice crystals form within the plant tissue, which bursts the cell walls. This is what causes tropical plants such as tomatoes, basil, and zinnias to die when the first frost comes.
Plants that are perennial here have a defense against freezing: they have anti-freeze!
The sap of a plant isn’t just water, but rather is a solution containing salts, sugars, soluble fats, and other phytochemicals that lower the freezing point considerably (just like the solution of brine that freezes at 0ºF doesn’t freeze at 32ºF). Plants that can overwinter to the south of us (“tender plants”) have a sap solution with a higher freezing point than we typically experience during our winters.
Snow and ice breakage
We often get heavy snows or ice storms in our area, which puts a lot of weight on branches, and if it’s too much weight, they break.
This is something we can prevent, through proper pruning. Shorter and thicker branches are more stable than longer skinnier branches. Wide branch crotches are stronger than narrow ones. Dead and damaged branches contain dehydrated wood, which isn’t flexible like healthy wood.
This is the worst culprit in winter damage, and is often given a pass because we assume that the damage was due to freezing. It’s also more accurately called early spring damage, because the worst of the harm is done during springs when the air is warm but the soil is frozen.
Plants become dehydrated during winter because their pores open during warm weather to pull water from the roots (“transpiration” is the technical word). They do this to enable the cell expansion and growth which we observe as flowering and leafing out. The problem occurs when the plants try to start growing when the water in the soil is bound up as ice.
A few plants (notably figs) are very prone to this, but can be helped by protecting them from the wind, which dries them out even faster.