roots in winter

roxanna(z5b MA)October 30, 2010

here in zone 5, the ground freezes hard in winter. i don't know the depth that it does so, but obviously perennial roots manage to survive in the ground in spite of the freeze. why, then, do potted plants that would be perennial in-ground not make it thru the cold months outside in their pots??


Perpetually Perplexed

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mad_gallica(zone 5 - eastern New York)

Be more specific please. Not every potted plant automatically dies over the winter.

    Bookmark   October 30, 2010 at 4:02PM
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Good question.

Not every plant will die over the winter, but there is a greater chance that this may happen. Even though the ground is frozen, it is often warmer than the air and it is much more stable than the air temperature. When temps dip down into the teens (and probably single digits in your region), the plant roots are still somewhat protected in the ground. On milder days, a pot and the soil within can warm significantly, especially a black or dark green nursery pot exposed to the sun.

In addition to being exposed to more extreme cold, the plants, being exposed to repeated freeze-thaw cycles, may be induced to come out of (and go back into) dormancy repeatedly, which is not really good for most. That being said, I've had hostas and daylilies come through our winters with crowns and roots completely exposed and not only survive and put out leaves, but even bloom. Naturally, these were ones that I wished would turn into compost.

Burying the pots in the ground or surrounding and topping them with a good layer of mulch should keep most perennials in pots safe through winter. (I usually sink the inhabitants of my "pot ghetto" into a part of the raised-bed vegetable garden over the winter, since the soil is nice and soft, is not in use through winter, and, since the bed will be needed for spring planting, there's no chance I'll just leave them there. Haven't lost a plant yet.)

I've seen more damage during "iffy" on-again-off-again winters when the temperatures swung wildly than during bitter cold winters.

    Bookmark   October 30, 2010 at 4:26PM
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ken_adrian Adrian MI cold Z5

winter in ground freeze areas is all about suspended animation ....

BUT .. and its a big but ... its all about what happens inside the pot....

generally drainage is the issue.... if the roots freeze into an ice cube.. because the pot was too wet.. then the plant will die .... roots need air as much as water .. and a frozen plant does not need water ...

or about media temps... if the pot repeatedly thaws and freezes.. the roots die...

if you have an adult beverage.. and start thinking about other conditions which would cause root death.. you will start to understand why pots in winter in z5 .. are a nightmare...


    Bookmark   October 31, 2010 at 8:09AM
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There is a big difference between the temperatures of frozen soil and ambient air temperatures. And that's where containerized plants in winter become vulnerable. They are lacking the insulation properties of a large soil mass around their roots that their inground cousins enjoy. Inground soils decline in temperature slowly and when fully frozen, maintain a temp right around the freezing point of water, 32F., if properly hydrated. That temperature maintains the integrity of the roots of hardy plants without freeze damage, which typically occurs somewhere around 25-26F for many plants.

Containerized plants don't have that luxury. The soils of container plants are much more closely aligned with air temps and can and will freeze at lower temps. The roots do not have significant soil mass surrounding them to provide insulation and for all intents and purposes are at the whim of ambient air temps, not to mention any wind chill they may be exposed to. Container plants are also far more likely to dry out easily (one tends not think a lot about watering in winter) and that contributes to the cold damage risk as well - like snow, well-hydrated soils do provide insulating capacities. And there is the freeze-thaw issue mentioned above, which can be far more pronounced than what occurs in the ground. Repeated freeze-thaw cycles that containerized plants may experience can be just as stressful and damaging to the plants as having their roots frozen completely.

The larger the container, the less likely the plant(s) it contains will be exposed to significantly colder temps than the same plant(s) inground, but the risk does exist unless steps are taken to protect or insulate the container by wrapping or by sinking into a spare planting bed or mulch.

    Bookmark   October 31, 2010 at 11:32AM
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