Why Don't Plants Freeze In Winter?
Excerpted from Beautify Botany.com
Freezing rain has been falling since the early hours of the morning. The mercury has plunged to a record
low for this date and the radio warns us that exposed flesh can freeze in mere minutes. A Wisconsin
blizzard is on its way. Brrrrrr, we think, as we bundle ourselves up just to walk the short distance to the
But spare a thought for the poor plants in our gardens. What about that yew under the front window with
the icicles hanging from its needled branches? The evergreen rhododendron with its leaves curled into
tight little rolls? The magnolia with its furry, brown buds? How do they protect themselves from the
extremes of winter weather?
Although plants are built of cells and viscous fluids just as animals are, they are at a great disability when
it comes to seasonal environmental stress. A man, a marmot or a moth can remove themselves from
harm’s way, but plants are stationary. The yew needles must submit to sheets of freezing rain. The
magnolia buds must hold tight against a blizzard. The rhododendron leaves must brave Arctic winds.
Yet nature has armed plants with remarkable defense mechanisms to help them withstand the stresses
of both heat and cold.
These adaptations help trees and shrubs withstand the worst extremes of winter weather. But they’re no
guarantee against that once-in-century ice storm that tears jagged branches from sturdy trunks; the
atypical January cold front that sets record lows while confounding a tree’s genetic definition of
“normal”; or that sudden spring freeze that pierces newly-awakened cellular protoplasm with lethal ice
They are merely the best that nature can do.
Most important is a plant’s inherited genetic adaptability to seasonal change. In woody plants that must survive
winter, this is called cold acclimation. Plants vary in their degree of adaptability, of course, depending on their
locality or ecoregion. A rhododendron native to the subarctic tundra (Zone 1) such as Lapland rosebay,
Rhododendron lapponicum, will survive far lower winter temperatures than the American rosebay rhododendron,
Rhododendron maximum native to the cool, moist Appalachian mountains (Zone 4). To that end, the Lapland rhododendron
will maintain a compact height (rarely over 30 cm or 1 foot), thus exposing a minimum of its above-ground growth
to the elements, whereas the southern species can afford to grow to an extravagant 7 meters (20 feet).
Woody plants may also employ two metabolic genetic strategies to help them deal with freezing temperatures.
The first strategy is avoidance, for instance, the prevention of the formation of ice crystals in cells
through the encoding of genes for super-cooling proteins that protect intracellular tissues.
The second strategy is freeze-tolerance. Plants that must survive even lower temperatures, like the
Lapland rhododendron, contain genes that encode cold-stress-responsive proteins. These proteins cause the plant to evacuate
water from cellular protoplasm into intracellular spaces where ice crystals can form without damaging the plant.
Provided these adaptations occur in a timely fashion after the plant has acclimated in fall, provided winter
temperatures do not exceed the plant’s maximum low temperatures, and provided the plant is not lured out of
dormancy by prematurely warm spring temperatures that plunge again later, there should be no tissue damage
to the plant.
These movements (nastic) are plant movements triggered by an external factor such as cold, heat, light
or humidity. Unlike the nerve-generated movements of animals, plants “move” through changes in the
internal cellular pressure of their parts. In extreme cold, the leaves of many evergreen rhododendrons
exhibit thermo-nastic movement, curling the upper surface of the leaf inwards and pointing it down toward
the ground, thus minimizing exposure to freezing temperatures and also reducing winter-burn caused by
winter sunshine. It has been shown that rhododendron species indigenous to mild climates do not
Reduction of leaf size is an important adaptation of conifers, many of which are indigenous to the
northern boreal forest zone where cold, dry winters are a fact of life. Trees must invest abundant energy
to produce their leaves, but the lean, nutrient-poor soil, the harsh atmospheric conditions and the short
growing season of the northern forest do not favor an annual leaf cycle, as with deciduous trees.
Therefore, depending on the species, conifers retain their oldest leaves for 2 years (white pine) to 45
years (Great Basin bristlecone pine) – that is, they stay “evergreen."
While a full leaf canopy allows a
tree to begin photosynthesizing early in spring and stay photosynthetically active later in autumn, it also
invites dessication and freeze damage in winter. Thus, conifers have adapted by producing small,
narrow, needle-like leaves (spruces, pines, firs, hemlocks, yews) or scaled leaves (cedar, cypress),
reducing the surface area to reduce transpiration of water and the risk of freezing. They also have fewer
stomata (pores) in the needles than deciduous leaves.
Evergreen conifers and broadleafed evergreens also protect their leaves from dessication in winter by
covering them with a new layer of cuticular wax each summer.
Those furry winter buds on the magnolia and the shiny, dark-brown ones on the horsechestnut are more
than mere cold season finery. Their overlapping scales form a watertight, protective covering for the
embryonic leaf and flower shoots that will emerge in spring. The buds of conifers are covered with
protective wax or pitch, giving them extra insulation against winter weather.