Do I have to harden off plants?

Once, when I was looking for some information about plants, I came across one very interesting fact. It turns out the common cabbage dies when freezing 28,4°F (-2°C), but if we’ll harden off cabbages, they can withstand temperature 23°F (-5°C). And I tried out it.

Hardening is a process that increases the ability of plants to tolerate adverse environmental factors. It carries out before planting seedlings grown in closed soil. It is achieved by reducing irrigation and lowering the temperature of the air: frost and cold resistances of plants. However, hardening off plants should not be admired, as it entails sustained irreversible adverse changes in plants.

Let’s consider the pros and cons of this process.


The most important processes in hardening off plants are a cold resistance and a frost resistance.

Cold resistance

The cold resistance of plants is expressed in their ability to tolerate low positive temperatures (38,8°F to 50°F (1°C-10°C)). This property is especially important for heat-loving plants because they suffer from these temperatures. External symptoms of plant suffering are faded leaves and the appearance of stains.

We include cold-resistant plants, for example, barley, oats, flax. The degree of cold resistance is not the same in different plants. It is also a measure of various organs of the same plant. For example, the stalk is the least cold-resistant in corn and buckwheat, and roots – in peanuts.

The introduction of potassium fertilizers, growing plants at low temperatures, high humidity, with good illumination contribute to the cold resistance. The temperature is adjusted in accordance with the degree of it (from 32°F to 41°F or from 0°C to 5°C) and influence for short periods (12 hours) in order not to damage the germinating seeds. Then we put seeds in favorable conditions (59°F-68°F or 15°C-20° C). This change of cold and heat produced during the month or longer. We can also apply pre-sowing treatment with some solutions.

This method allows us even to transport our plants to other areas, having prepared them from appropriate conditions.

Frost resistance

Frost resistance of plants is the ability to survive in the period of short-term frosts or prolonged frost (low negative temperatures (below 32°F or 0°C)). In wintering plants, frost resistance develops annually as a result of a long and complex preparation of them for the winter. During the warm period of the year, when plants grow, their frost resistance is insignificant and maximum resistance during the winter frost. It drops sharply in thaw and rises again then when an increase in frosts proceeds slowly. Sharp temperature fluctuations are dangerous because plants do not have time to undergo repeated hardening.

The frost resistance is primary determined by the hereditary features of plants. Some types of plants die with weak frost (such as lemon trees dies at temperatures from 23°F to 10,4°F or from -5°C to -12° C). The other is able to survive in the harshest of winter (for example, some apple trees can withstand freezing temperatures up to -40°F (-40° C)); larch, birch, and other trees in the Eastern Siberia could survive at -94°F (-70° C).  The degree of frost resistance is not the same in different varieties of the same species of plants. For example, some varieties of winter wheat die at temperatures below 5°F  (-15° C), others only at 9,4°F (-23° C).

Thus, one of the most effective methods of improving frost resistance is the withdrawal of frost-resistant varieties and finding the correct area. Soil and climatic conditions and agrotechnical methods, that provide plants optimal conditions of a power supply, also influence it.

The important role in the frost resistance is played by the creation of heat by the plant itself. When the water freezes, it is allocated 331.6 J of energy per gram of water. That’s why we need abundant water spraying of vegetable and decorative crops on the night before the onset of frost.

This process gives opportunity plants to survive hard winter.


First of all, not all plants have the ability to harden off. It depends on the type of plants, its origin. Plants of southern origin, in general, are not able to harden. In plants in the northern latitudes, which experienced a significant decrease in temperature, the process of tempering is timed only to certain stages of development. Some plants need to finish the process of growing. If not, it can cause mass destruction of plants in winter.

Often the winter death is caused by summer drought. Drought suspends growth of plants in the summer, does not allow woody cultures to complete growth processes until autumn. As a result, plants are not able to harden processes and die even in small frosts.

Plants are grown with inappropriate photoperiods, do not have time to complete the annual growth and are not able to harden off. Studies have shown that spring wheat, in comparison with winter; grow at lower-than-optimal temperatures. Due to this, spring wheat almost does not reduce growth rates in the autumn. That’s why it is not capable of hardening. The ability to harden off is lost in the spring due to the lack of growth processes.

Also, hardening is possible only on an organism with the mandatory presence of the root system. It is important because the cells of the root produce substances that increase the body’s resistance to frost. If our plant has any violation of the root system, it can’t harden off correctly. And it will perish after some time.

Thus, the resistance of plants to frost, the ability to undergo hardening processes are closely linked to a sharp decline in growth rates, with the transition of plants to the resting state.


Hardening off plants should not be admired, because it entails persistent irreversible changes in plants. For example, if we want to harden off the seedlings, it is enough to begin it 3-5 days before planting them. Our actions have to ensure that the hardening does not damage our plants.

On the other hand, they are like people: birth, life, and death. And they need also strong health for normal existence. Hardening off them provides it and helps plants to produce immunity to cold and frost. And it’s the biggest plus when we can have strong and healthy plants.

So, if someone asks me to harden off plants or not, my answer is only yes. We must do it!

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