Defense Against Freezing
Not all plants react to freezing temperatures the same way. Native plants 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.
What Happens Inside
Most plants contain more than 85% water. As water freezes, it expands. When this happens, cell walls and other parts of the plant can rupture. Frozen water between cells can also draw water out of the cells, causing them to shrink and rupture. Hard ice crystals that form between or within the cells cause the most damage. The crystals can be like miniature needles, piercing cell walls inside in the plant.
Water and Plants
There are many layers of cells inside the leaf and stem of a plant. Winter-hardy plants have some protection against freezing. Water inside the cells isn't pure water. The water in the living parts of cells contains dissolved salts, sugars, and other substances that act like anti-freeze. These substances lower the temperature at which the water freezes. This "anti-freeze water" can still freeze if temperatures drop suddenly or extremely.
If a plant is to survive freezing temperatures, it needs to protect cells from ice crystals, or prevent crystals from growing too big. Here are some cool plant adaptations:
- Seeds: Plants that only have an annual cycle get through the winter by making dry seeds which can survive freezing.
- Toughening/"Anti-freeze": Some plants toughen up or become hardy in the fall in response to slowly decreasing 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: 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 move 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. At planting depth, temperatures rarely fall much below 30 degrees Fahrenheit. The sugars in a hardy bulb, just like salt on a winter sidewalk, lower the temperature at which water freezes.
If you have a spring snow or freeze, what will happen to your tulips? The plants should be fine at 20 or 25 °F. Below those temperatures, any frozen part of the plant will be damaged and not able to make food for next year's bulb. If a tender flower bud freezes, it probably won't bloom.
The process of thawing out can be more dangerous than freezing! Water can flow back to a damaged cell and cause it to burst. You can see evidence of this process when you look at leaves after taking them out of the freezer and watch what happens as they thaw.
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 or 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