Cold Temperatures - Temperature Fluctuations - Desiccation

Since plants are "cold blooded" life forms that depend on the heat of their environment for energy, their internal chemical processes are influenced by the temperature of their surroundings. For those of us in the temperate zones of the world, the key climate factor of concern is cold weather.

In other climate zones of the world, plant hardiness may be primarily a consideration of heat, drought tolerance, soil conditions, drainage, snow cover and other factors.

Although most people would associate subfreezing temperatures with the term "cold hardiness" however, plants are affected by several factors in dealing with low temperatures including:

  1. Cold Temperature - Most cold hardiness ratings for landscape plants are based on that plant's ability to live through some level of cold temperatures. In this case, the concern is for the freezing of the plant cells. Unlike animals, plants have solid cell walls, if the contents of the cell freezes and expands, it will burst and die.

    Plants have developed mechanisms to help them survive sub-freezing temperatures of the winter months. Those from the tropics do not experience freezes so they have no defense and may be damaged by temperatures in the 30's. Other plants of the arctic regions can survive temperatures of 40 below zero or more.

    In the United States, the Department of Agriculture (USDA) has developed a cold hardiness map that shows the absolute low temperatures experienced by the land area of North America. Horticulturists and gardeners have categorized plants by the minimum temperature at which they can survive and avoid serious cell damage.

    For example, a plant that can routinely survive exposure to -10 to -20 degrees F are said to be hardy to Zone 5 on the USDA map.


     

  2. Temperature Fluctuations - In addition to the absolute minimum cold temperature, plants are also affected by fluctuations in temperatures. As they adapt to the seasons in the temperate zones, plants gradually build their level of hardiness starting in the fall and hitting their peak hardiness in the coldest months of the winter. Then they gradually lose hardiness as spring progresses.

    Therefore, a drop to 25 degrees in October or March may damage buds and plant tissue more than a temperature of -25 degrees in February. Warm periods in the winter may cause plants to begin to lose their hardiness and them be damaged by a return to normal cold temperatures.


     

  3. Desiccation - Another impact of cold weather is that it can have a dehydrating effect on plant tissue. Cold air tends to hold less water i.e. humidity, than warm air so winter winds can pull water out of the leaves or needles of evergreens, buds and even stems.

    Broadleaf evergreens such as rhododendrons, boxwood, wintercreeper and others tend to be impacted by this factor more than deciduous plants which drop their leaves. Often when you hear that a plant should be placed in a "sheltered location", it means that it needs to be protected from winter winds and their drying effect.

    Also a factor in winter damage through desiccation is the position of the sun in relationship to the plant. In most temperate zones, the strongest winter sun comes from the south or southwest. Even on very cold days, if the sun is out, it may warm the surface layer of the leaves of evergreens and cause them to loose water.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.

 

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