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A plant's ability to remain unfrozen at temperatures below 0°C (32°F) is known as 'supercooling', and the hardiest of plants, such as the black spruce from Alaska, can survive an incredible -40°C (-40°F) without suffering any ill effects.
Plants have several ways of withstanding freezing temperatures. When the cooler weather arrives in autumn, hardy plants increase the unsaturated far content in their cells. This has a lower freezing point than water and keeps the cells functioning at 0°C (32°F). As temperatures drop to just below freezing, ice crystals begin to form in less hardy plants between the |
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cells and in the water-conducting tubes in the stems and leaves. As temperatures fall at night and rise during the day, the continual expansion and contraction of ice between the cells can rupture them, causing sudden wilting and discoloration.
In addition, as ice crystals grow they draw water from inside the cells, dehydrating them and creating a concentration of sugars and salts. As a result, the sugars and salts begin to crystallise and prevent the cells from functioning properly.
Ice formation can be rapid in tender plants, and experiments on peach trees show that once ice starts to form it spreads to the whole plant within 20 minutes.
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Dehydration of the cells during a freeze followed by a rapid thaw can be fatal for a plant, as water suddenly re-enters the cells, causing them to burst. This is the reason why camellias should never be planted facing east towards the early morning sun: after a frosty night its heat can damage their flowers and buds.
Plants have other defences against more severe winter temperatures. At -2 to -4°C (28 to 24°F), sugars and salts in their cells act as antifreeze - just as salt on roads prevents ice forming - lowering the freezing point of sap by a few degrees to keep the plant functioning. |
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