Arctic cold is no trouble thanks to snow

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Snow is a great insulator. The air may be -20F but the soil around roots can remain above freezing, if fluffy snow is blanketing the ground. 

Hi Janet and Steven

Could you comment on how the recent extreme temperatures will affect our gardens?

I am confused about the zoning temperature numbers.  If we were a zone 5b before (-15 to -10F)  but are a Zone 6a now, (-10 to -5 F) how does the extreme temperature and duration of the cold impact our plants? We had -16 F at our house with a wind chill of -28 F. (I got a cool new Davis weather station for Christmas.) We don't know how cold the ground is because we can't get the soil probe in.

I know that soil moisture and mulch are factors in survivability of plants but what is your educated guess?

Hope you are staying warm.  - P.N. -


Last year's Zone 5 plants didn't read the new map

Dear P.N.,

If your landscape is populated with plants chosen because they had proved hardy to zone 5 temperatures (-10F to -20F average minimum temperature), it doesn't matter that your neighborhood is now included in a warmer zone 6 on the redrawn USDA maps. Your plants can still handle that cold.

With respect to the USDA weather experts, we remain skeptical of the new zoning and will keep selecting plants as we did before. We will keep choosing -20F plants because we have seen -20F in our garden in our lifetime.

Our skepticism rests fairly simply on the difference between the hardiness zone map's revision interval and plant longevity. The hardiness zone map was revised based on something more than 20 years of data. Sounds like a lot except the trees and shrubs we plant can live a century or more and although we won't see all of that we certainly love to imagine them living on into venerable old age and the future gardeners who may enjoy them.

Cold? Snow worry!

As for this recent cold, we don't think it caused much trouble to our garden. We had wonderful, early, deep snow cover with an extra blanket laid on just before the deepest cold descended. The soil was not even frozen below the snow -- check it now, you'll see. So the crowns of perennials and the most tender parts of woody plants -- the bases of trunks and the roots -- were cozy at 32F or above and well protected from wind. (This, by the way, may be a mixed blessing. There are many people rooting for the cold to kill invasive species and that wish wouldn't be fulfilled in cozy warm soil.)

We're not surprised that a soil probe won't penetrate. Even a very thin layer of ice can stop a blade, and there may be an inch or more of frozen soil capping the ground, especially in places where the sun doesn't reach. However, that soil is not super cold though, not cold enough to hurt plants. It can hold at 32°F, not becoming super chilled unless it's directly exposed to even colde air -- not going to happen with a snow buffer between!

Rabbits, mice another matter

The snow may buffer something we don't want, too. We may see extra trouble from rodents and rabbits. It may be happening now but we won't see it until spring unless we go digging now. That is, all that snow provides protection (from hawks and owls) for trunk-gnawing rodents, and elevation for hungry rabbits. Poor plants, poor gardeners, we slip one punch only to meet another -- it's always something!

And then, the "garage plants" - a 3rd different story... RosemaryCarry5621.jpg

Argh! We should have thought about this when we first heard the weather forecast: The marginally hardy plants we store in our unheated garage over winter. They survive out-of-zone because they have a few degrees of extra protection. This year the zone 6 rosemary (Rosmarinus officinalis) was there, rather than in the house. Sad, when something we've had for 20+ years may be a goner. Too late now!

Weather and work

Aren't weather stations fun? Now you can become a weather reporting station, too. I love to check the weather (we like weather underground's format) and see exactly where the reported temperatures occurred.

We are indeed staying warm - thanks for your concern. We did stay indoors when the mercury dropped to -17F and we didn't stay out long when it was -10F and windy. However, with thermal underwear, warm pants and shirt, sweatshirt, vest and coat plus boots and insulated gloves, we actually got sweaty while working. Things we had to do: Shovel snow, lift up overburdened branches so snow could slide off, clear deep snow from the bird feeding areas, and keep an air vent in our pond.