I know we live in Colorado, but it feels more like the arctic outside! As I write this, my thermometer is hovering around 2°F—and it’s been there all day! I’m glad I have a nice warm house to bundle up in, but my plants aren’t so privileged. Aside from the potted herbs that I hastily dragged indoors, my shrubs and flowers are stuck where they grow. I have a hunch they’re not all going to make it.
To make matters worse, this fall has been mild, at least until now. With highs in the 60s and even 70s and lows barely below freezing, many of my perennials still had green foliage. It takes gradually cooling temperatures for plants to properly harden for winter. These poor victims never saw it coming!
With 20:20 hindsight, I could wish that I had offered some sort of protection, although I’m not sure how much good it would do to cover a plant when the temperature plummets 40 degrees in an hour or two. Covers usually only add about 4° to the temperature surrounding the plant, and that doesn’t help when the low is -7°. It’s all just part of living in Colorado.
So, now I have a garden full of “plantcicles.” What kind of damage can I expect, and what, if anything, can I do about it?
There are two ways that plant tissue freezes. In “extracellular freezing,” the liquid between the cells freezes. The resulting crystals tear fragile cell walls, and damage occurs. Additionally, the ice outside the cells draws water from inside the cells, causing wilting and eventually death. It makes sense then that xeric plants are less damaged by extracellular freezing—they’re already used to having low levels of water in their cells. Plants suffering from extracellular freezing are more likely to recover, especially if they thaw before the fluids inside the cells freezes too.
“Intracellular freezing” occurs when ice forms inside the cell membrane. This occurs most often in cases of rapid cooling, e.g., the weather we’ve had this week. The ice crystals destroy the structure inside the cell, and it dies. The extent of the damage depends mainly on how fast the temperature drops. This is not good news. I highly suspect this is why my plants now look so pitiful.
While my plants look pretty awful, my biggest concern is for their roots. In some parts of the country, snowfall forms a natural insulating blanket, keeping the ground from dipping much below 32°. Here, we have the cold but little-to-no snow. Mulch definitely helps, but the lack of snow cover is why plants that are hardy in Minnesota often don’t survive Colorado winters. Most roots don’t acclimate to soil temperatures much below freezing, and they experience severe damage or death below 15°F—no matter how hardy the plant is above ground. Shallow-rooted plants are at the most risk.
A freeze-thaw cycle makes things even worse, heaving plants out of the ground. Now that our ground is (abruptly) frozen, I should add another layer of mulch to keep it that way until spring. Unfortunately, the mulch pile is just as frozen as everything else, so I’ll have to wait a bit until temperatures return to normal.
Woody plants can suffer tissue damage when the sun warms their trunks and stems, bursting cell walls. Wrapping the trunks of young trees helps avert this problem. Right now, the sun is hidden behind snow clouds, so this would be a perfect time to run outside with some paper tree wrap. White paint also does the job, but I think it would freeze onto my brush.
For the most part, I won’t know what survived and what died until spring. Pruning off any cold-damaged foliage and branches should wait until then, as even dead wood adds a degree of protection. The cold may kill leaf and flower buds, but if the plant is still alive, it can eventually grow new ones. Sure, I’ll miss the flowers, but there’s always next year.
Once spring starts in earnest, I can decide who the survivors are. Dead twigs turn brittle and snap off easily. Living limbs stay flexible. Before I give up, there’s one more place to check. Carefully scratch a small hole in the thin bark on some twigs and small branches. If there’s a green, growing layer of tissue underneath, your plant still has potential.
I’m perfectly content to wait until spring. I really didn’t want to go outside right now anyway.