How Freezing Temperatures Affect Plants

— Teacher Background—

Standards Covered
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When Plants Freeze . . .
Not all plants react the same way to freezing temperatures. Native plants and others that originated in cold climates have adaptations to survive typical winter conditions. Plants that originated in more tropical locations have fewer defenses against freezing temperatures.

Water, Ice, and Plants, 101

Plant leaves contain many layers of cells. Water inside the leaves, but outside the cell walls, is relatively pure, so it freezes at 32 degrees F. But water inside the living cells of many "hardy" plants has dissolved salts, sugars, and other substances that act like anti-freeze. (They lower the temperature at which the water freezes.) So water in the living parts of cells in winter-hardy plant leaves has some protection against freezing. But it will freeze if temperatures drop suddenly or extremely.

As water freezes, it expands; this can rupture cell walls and other parts. Frozen water between cells can also draw water out of the cells, causing them to shrink and rupture. The ice crystals that form between or within the cells exact the most damage; these can act like miniature needles, piercing cell walls and the living parts inside.

The process of thawing out can be more dangerous than freezing! Water can flow back to a damaged cell and cause it to burst. Students may see evidence of this when they observe leaves after taking them out of the freezer and again once they thaw.

Plant Strategies
Frozen Tulips?
If you have a spring snow or freeze, what will happen to your tulips? The plants should be fine at 20 or 25 degrees. But below that, any frozen part will turn white and not be able to make food to form a flower or to store for next year's bulb. If a tender flower bud freezes, it will not likely bloom.
If a plant is to survive freezing winter weather, it needs to protect cells from ice crystals, or prevent crystals from growing big. Here are a few cool plant adaptations for getting by:
  • Seeds: Plants that have only an annual cycle get through the winter by making dry seeds; can survive freezing.
  • Toughening/"Anti-freeze": Some plants toughen up or "harden" in the fall in response to slowly increasing temperatures. Most produce sugars and salts that prevent ice crystals from forming or growing large. They also gradually lose water; the drier tissues are less likely to freeze.
  • Leaf Loss/Adaptations: Some plants simply lose their leaves altogether and store energy in their roots. Others, such as spruce trees, have needle-like leaves with waxy coatings; these prevent water loss and cold damage.
  • Controlling Water Location: Some plants keep water in cell walls, but keep it out of the living part of the cell where ice could kill the plant.
  • Underground Storage: Some plants, like tulip bulbs, store energy in underground structures. Temperatures at that depth rarely fall much below 30 degrees F. Sugars in a hardy bulb, like salt on a winter sidewalk, lower the temperature at which water freezes.

When the Strategies Might Fail
Even a hardy plant can suffer or die under certain winter conditions. These include the following:

  • When temperatures fall below a plant's maximum low-temperature limit even after it has acclimated (toughened up!)
  • When early freezes occur before the plant has acclimated in the fall
  • When unusually late freezes occur in the spring after the plant has emerged