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How plants prepare themselves for winter

Bob Dluzen
Special to The Detroit News

The vibrant fall colors have peaked in Michigan, we’re now left with mostly shades of  browns punctuated with a few brighter colors here and there.

The color change of deciduous trees is the most obvious sign that trees are preparing for the long cold winter. Other perennial plants and evergreen trees are also going through a similar process, they’re just not as showy.

During winter all water movement from the roots to the upper parts of plants stops. If water was allowed to continue to move up into the plants like it did during the growing season, freezing would occur inside the plant cells causing them to burst rupturing the cell membrane. Freezing damage is irreversible and results in the plant’s death.

To counteract the havoc freezing causes, overwintering plants go through a process called cold hardening.

This white pine looks the same as it did in the summer but has been undergoing changes on the cellular level as it prepares for winter.

Cold hardening is not a single event, rather it is a complex conglomeration of processes that result in the increased concentration of sugars and certain proteins inside the cell membrane that acts as a kind of natural antifreeze.

The cell membranes themselves also undergo a change that in some species allows water to ooze out of the protoplasm into the space between the cells. Water then can freeze in that space without causing damage.

Two things are involved in stimulating cold hardening to start, shorter daylight hours and cooler temperatures. That much is obvious to most of us.

The cold hardening process begins just as the daylight hours fall below 12 hours, right after the vernal equinox. As each day passes, the plant is a tiny bit more cold hardy, just a fraction of a degree accumulates per day. After a couple of weeks, the process provides more and more cold temperature resistance per day.

The first light frost dramatically increases the daily cold resistance. Each subsequent frost pushes along the internal plant chemistry changes. The speed of the cold hardening process doesn’t change, just the amount of cold resistance changes.

Extreme cold hardening requires more and more frosts until the plants reach their low temperature limit. Oak trees resist freezing down to -40 degrees while some spruce tree species can handle temperatures down to -94 degrees fahrenheit. That’s why you only see evergreens and not deciduous trees in far northern regions.

Cold hardening will not occur in actively growing plant tissue. That is the main reason why fertilizer application is not recommended for many woody plants after mid-summer. Fertilization will stimulate new growth that will not cold harden properly. That non-hardened growth will freeze and die back during winter.