The forecast for the next few weeks is cold – the first severe cold temperatures we have seen in western Oregon this winter. It may be a good time to review how plants are damaged by freezing weather.
During a “normal” autumn, along with declining day length comes decreasing temperatures. I’m sure this happens somewhere in an orderly fashion, but usually not in the Pacific Northwest. Our temperatures tend to fluctuate quite a bit, with cool days interrupted by relatively warm days and mild nights.
This pattern of down/up/down temperatures delays the hardening-off process by which plants survive cold temperatures. A steady decline of temperatures gives the plant time to prepare, by moving unbound water outside of living cells, into the inter-cellular spaces. This allows the living cells to have very little water within them that can freeze, for it is the formation of ice within the cells that kills them.
This process is delayed when the plant are teased by temperatures that wander all over the place. For example, in the Willamette Valley, September day temperatures varied from 60 to 84; in October, they ranged from 60 to 72, but in both months, the weather noodled back and forth between warm and cooler. By December, temperatures had settled down to a respectable level, although night temperatures were bouncing around, like children on a bed when the parents aren’t looking.
Whether the plants are sufficiently acclimated to tolerate the anticipated frigid temperatures is a head-scratcher. My guess is that some plants will be damaged by the expected sudden cold temperatures, but it depends on how far along the plants are in the hardening-off process, which in turn depends on the plant species, its exposure to the weather (e.g. protected or exposed to cold winds), how cold the temperatures will get, and for how long.
At this time of year, sapwood is least tolerant of cold. Sapwood is the relatively thin layer of living wood through which water and nutrients move in the tree (most of the wood in a woody plant is dead). If this is killed by formation of ice crystals in its cells, the plant will no longer be able to transport water up to its branches.
However, the damage won’t be noticeable until the spring, when the buds start to swell, or in early summer, when warm temperatures mean an increased demand for water. At that time the leaves or needles may suddenly wilt, and entire branches may die back for no apparent reason.
What can be done to prevent freeze damage to plants? Protect marginally hardy plants such as figs and olives by wrapping or covering them, if possible. A bed sheet, canvas, or Reemay, firmly secured so it won’t be battered by the wind, would work. Small plants can be covered by plastic garbage bins, weighted down so they won’t blow away. Just be sure to remove the bins when the weather moderates again – plants do need light! Large trees and bushes are on their own. Just don’t be surprised if they may need more pruning than usual, come summer.