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frosty_leaves.jpgPlants Can’t Come In From The Cold

Imagine for a moment that you had to stand outside your house or apartment, without moving, all winter long…and that you were naked.

How long do you think you could last?

Not long, especially if the temperature went below freezing. And certainly not if the temperature went below 0o F (-18o C).

In temperate zones on Earth that’s what perennial plants must be able to do to survive.

And even annual plants may have to withstand an early or late frost in order to complete their life cycles.

But, you may reply, plants are not warm-blooded organisms like mammals. What difference is it to them whether it’s cold or not?

How Cold Kills Plants

What happens if water freezes inside at plant?

At least a couple of things can happen…both bad for the plant.

The first, and likely lethal for the plant, is ice crystal formation inside cells. This has just about the same effect on a plant cell as the little alien chestburster did on the actor John Hurt in the movie Alien. (Sorry, but it’s an effective analogy.) It’s lethal.

If, however, the water freezes outside the cells, in the intercellular spaces, this may lead to the extreme desiccation of the plant. That is, it’s sort of the same as if the plant was drying out.

Other cold-temperature effects on plants include (1) decrease in enzyme activity and (2) changes in the fluidity of cellular membranes, both of which could severely harm plant cells, and, thus, the plant as a whole.

How Do Plants Cope With the Cold?frosty_tree.jpg

To answer this question we have to consider plants at the cellular level.

How do the cells of cold-tolerant plants survive sub-freezing temperatures, i.e., withstand dehydration and, more importantly at very low temperatures (below 0o F or -18o C) , avoid the formation of ice crystals in the cell?

1. Accumulation of solutes (sucrose, mainly, but also other organic compounds such as proline) by the cells to depress the freezing point of water (think salting ice on the sidewalk) and to stabilize membranes. (But this can only be effective at temperatures from 32o F to 20o F.)

2. So-called “antifreeze” proteins help prevent ice crystals from forming in the extracellular spaces (outside cell); plant cells that make these proteins typically secrete them into cell wall region (intercellular spaces).

3. The plant cells may synthesize proteins called “dehydrins”, which are inside the cell (cytoplasm), may bind water molecules and alter the collective structure of water in the cell to stabilize membranes.

4. Plant cells can alter lipid composition of cellular membranes in order to adjust the fluidity (functionality) to colder temperatures.

red_leaf.jpgCan Plants Generate Their Own Heat?

A silly question? I think not.

Some unusual plants, by partially uncoupling their cellular mitochondria, can generate small amounts of heat. (Please see ref #1 below for more information) But this is likely not very significant with regard to cold tolerance, however. (More on this interesting topic here.)

Bottom Line: Plant cells survive sub-freezing temperatures by adjusting their solutes, proteins, and membrane lipids in order to withstand desiccation and to avoid ice crystal formation.

Reference

1. Seymour, Roger S. (1997) “Plants That Warm Themselves.” Scientific American, March 1997, pp. 104-109. (Summary)

HowPlantsWork © 2008-2011 All Rights Reserved.

m4s0n501

11 Responses to “How Plants Survive The Cold (Or Not)”

  1. Brian says:

    Good article.

    This year I have a greenhouse and I am wondering about implication of not just frost but also of frost free periods.

    If a pot has frozen to say -2degrees centigrade overnight is it a good idea to let the greenhouse heat up to defrost the pot during the day (eg leaving all windows closed during sunny day) or is it better to leave the pot in its frozen state? Or in other words is it better for a pot to be frozen for 2 months or frozen and thawed and refroze over and over for 2 months? I could almost see getting re-frozen over and over as more damaging than one protracted freeze.

    • howplantswork says:

      It would be a shame to open the greenhouse windows during the winter. But I appreciate your concern regarding repeated freeze/thaw, although the cells of cold-hardy plants wouldn’t likely be frozen by -2 C. Of course, a simple experiment might be to move some pots outside and compare them to those left inside the closed greenhouse.
      I’ve used passive heat sources to warm small greenhouse overnight by painting several 5-gallon water jugs black, then filling them with water and placing them in a central location in the greenhouse that gets sunlight. They warm up during the day, then slowly radiate the heat overnight. Not a great heat source but may help on nights that dip slightly below freezing.

      Cheers!

  2. Brian says:

    Thanks for the reply.

    Afraid I don’t have spare plants to experiment with to find the answer to this.

    And the greenhouse is pretty full so I can’t accomodate large bodies of water to explot the specific heat capacity and latent heat of freezing of water. And at this time of year generally the bottom of the greenhouse (where I’d have the water containers) gets no direct sunlight at all.

    I have a heater but its not large enough to keep a 10×8 greenhouse from very cold temps (NW UK, Europe) and I don’t think I could afford to. The best I could do would be to be compartmentalise the most valuable pots into containers with reused 4 pint milk bottles , a covering of multipurpose compost and a bit of fleece.

    But that doesn’t answer the question of what’s best for plant survival one deep freezing period or many cycles.

    • howplantswork says:

      Indeed, I didn’t answer your question.
      That’s because it depends…….on what plant species are in your pots.
      Plants vary greatly with regard to their ability to adapt to cold (or not). Some woody perennial plants enter a “resting phase” or endodormancy to over-winter. Other plant species may transition in and out of cold acclimation in a matter of days, depending on the temperatures. This post may help a bit: http://howplantswork.wordpress.com/2010/02/04/how-do-plants-chill-out/ .
      I wish that I could provide a simple answer, but it’s a complex subject. If I had to choose one or the other, one deep freeze or cycles, I’d be inclined to go with the deep freeze. This would be to try to avoid the chance that the plants lose their cold tolerance during a warm period and then be hit with a subsequent killing freeze.

  3. Brian says:

    Well I’m please to have read in your cited post that day length can drive some winter dormancy because here in the UK the seasons were going ‘as normal’ and then suddenly have plumeted from average. I thought the speed of onset of the freeze may have done for some of my plants that had not got ready for winter because of the experienced temperatures.

    I also try and dry down pots as I’ve always thought this too increases winter survival but also plays to your final line…that is to better manage when plants really get going during spring. Interesting that drying down and winter dormancy are similar for the plant too (Cited article).

    The whole thing is made more complicated though by having plants in pots. Eg I have bulb in a large pot and planted IRO 6inch deep. If that bulb was in the ground it would take heavy frosts to penetrate that far down into the earth. However in the pot 2 inches of frost will be knocking on the bulb’s door so-to-speak, accessed easily enough through the side of the pot. So a ‘dangerous minus 17 degrees’ becomes a ‘dangerous minus 6 degrees’ or whatever.

    Anyway after posting to you though I realised I could compartmentalise a lot more than a few pots in storage boxes. I made a large bench put my most precious pots on it, covered it with fleece and bubble wrap overhanging the edges of the bench by a good amount and put a low burning twin flue heater beneath. So I’m heating up a compartment where prettymuch all the heat is contained in a fleece/bubble wrap tent. I decided I could aim for a ‘slightly frozen’ minus 2 to 4 degrees minimum (ie frosty but not deeply penetrating).Tried it live last night and it shows the capacity to keep at plus 2 degrees in a minus 4 degrees night…so plenty of wiggle room for using numbers of burners, burner settings, varying insulating layers to keep the minimums around minus 4.
    The setup seems so fuel efficient I may even get away with using a very small heater even on a minus 8 night. LOL well all intersting to me:)

  4. Susanna says:

    I am have a small courtyard, what kind of tree, or shrub will survive outdoors, in a pot, during the winter in Denver??

    • howplantswork says:

      Susanna,

      I would not be so presumptuous as to provide you advice re. your excellent question.

      Please allow me to refer you to your local Colorado State University Ag Extension in Denver:

      http://www.denverext.colostate.edu/

      888 E. Iliff Avenue, Denver, CO 80210
      (720) 913-5270

      My experience has been that such offices offer a wealth of practical and reliable information.

      And, besides, your tax dollars are helping to pay for these services, so your are certainly entitled to access their expertise.

      Good luck!

  5. ck says:

    so im doing this science project which is which plant would die quicker in a freezer or a cabnet pls help me

  6. Sherwood says:

    I grow trees in central Alberta. Here our winters can get to -40 although usually at that point we have about a foot of snow on the ground.

    I have been growing my trees in pots. Everything I’ve read says that I should kill most of my trees every year. A #2 pot (roughly 8″ across x 9 tall) should allow the roots to freeze solid. While I’ve found that growth is not as swift as I would like, I run under 3% mortality per winter.

    That said: I’ve notice that mortality is concentrated on the NW corner of a block — the direction that our bitter winds come from. How much of adaptation to severe cold is not “how cold it gets” but “how fast it gets cold”

    • plantguy says:

      Great question.
      The development of tree cold hardiness is a complex process that certainly takes time, sometimes weeks to months. So, a brief answer to your question is: Yes, how FAST it gets cold is certainly an important factor.
      I’m no expert on tree cold hardiness, but I know that the induction of cellular cold hardiness in plants is typically triggered by both the photoperiod (think longer nights) and low temperatures. In perennial trees such as yours, in general, the living tree cells first change – membrane lipids, solute accumulation, “anti-freeze” proteins – to become more cold tolerant. Next they enter the next stage, namely, dormancy, which is more complex and more poorly understood. It’s this deep winter dormancy that likely what allows trees to survive extremely low temperatures.
      I lived in Bozeman, Montana for over 20 years. And during this time there were several Novembers in which temperatures rapidly (one day) plummeted 60 to 70 degrees F to wind up 15 to 25 below zero F. I wasn’t the only one who noticed that many mature trees around town (especially beech) were obviously dead the following spring. I presumed that this was due to the rapid onset of extreme low temps the previous fall.

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