The onset of the era of global warming worries almost everyone. Let's debunk some myths about this phenomenon.
Plants can die due to severe frost. Directly frost, that is, low temperature, does not cause suffering to plants. But frost creates ice crystals in plant tissues, which damages living cells. The defrost mode is also very important. For a hardened plant, deep freezing is not terrible if the cooling takes place under conditions of slow ice formation, and thawing will also be slow. In laboratory conditions, in the middle of the 20th century, Russian scientists were able to freeze blackcurrant shoots to a temperature of -253 degrees, later the experimental plants developed as if nothing had happened, bloomed successfully. It should be borne in mind that the experimental varieties - Laxton and Leah fertile are not at all the most winter-hardy.
Winter hardiness determines the temperature in degrees that a plant can withstand without loss. Many believe that if there was a frost at -30 degrees, and the fruit trees were not affected, then their winter hardiness is -30 degrees. However, there is a special term for the ability of plants to withstand low temperatures - frost resistance. The term winter hardiness is quite wide, it includes the ability of a plant to endure the entire spectrum of winter hardships - these are the notorious crackling frosts, and sharp thaws, significant changes from heat to cold, and so on.
All plants are monolithic in terms of frost resistance - all parts of them withstand or succumb to the same temperature. This is not true, it has been noticed that different parts of the same plant can react differently to frost. In fruit trees, the roots are the most unprotected part, usually a temperature of 9-10 degrees of frost becomes critical for them, but this indicator depends on the culture and stock. In fruit plants, the buds are always more tender than in growth ones, their wood freezes more often than the bark.
The winter hardiness of a plant depends only on itself. Undoubtedly, the genes of a shrub or tree already have their ability to endure winter with accompanying problems, but the current state of the plant, its health, and living conditions in a particular year are no less important. Maximum winter hardiness accompanies a well-groomed and strong plant, but a plant weakened from diseases, abundant harvest or poor-quality nutrition or other problems may have a reduced winter hardiness.
Throughout the winter, the frost resistance of a shrub or tree does not change. Throughout the fall, the plant goes through a kind of hardening, during which frost resistance increases. In a state of deep dormancy, winter hardiness continues to increase. By about the end of December, for most plants of the middle zone, deep dormancy ends, at this time the winter hardiness is maximum. In the future, frost resistance begins to fall, and this does not even depend on the weather, each thaw only intensifies this phenomenon. The duration of the thaw also has an effect, the longer and warmer it is, the faster the frost resistance falls. However, one should not think that this process is irreversible, in part, resistance may increase, especially in cases where frosts come gradually, and not abruptly. Therefore, do not be surprised if a tree will endure a frost of 35 degrees in early December, and suffer from a thirty-degree cold in early March. And an unexpected frost of 5-6 degrees in March can cause blackening of the blossoming leaves.
In fruit trees, the lower parts of the trunks are the most frozen. It is in these places, after the endured winter, that areas of dead bark are found. In such injuries, the main cause of damage is not frost at all, but temperature drops in early spring. During the day, the sun heats up areas of the crust, which locally already lose their winter hardening, starting the "spring" mode of operation. And at night the temperature drops significantly and those parts of the bark that rushed to enter the spring can be damaged. That is why it is recommended to whitewash the trunks at the end of winter, and not in the spring, which is not clear to many.
In the period from the end of leaf fall to the beginning of budding, a deep sleep of garden plants occurs. It is believed that during this period no changes occur, but if you transfer the branches to heat, they will immediately begin to bloom. The reality is much more complicated. After the end of the leaf fall, the plants fall into a state of deep sleep, rest. Such a period is vital for plants, no heat can wake them up and interrupt this process. Gradually, deep calm gives way to forced. This means that such a pause is not vital for plants, but this remains the only way to survive the coming frosts. It is at this stage that the buds are able to begin to bloom with the onset of heat. For an outside observer, both states look the same - the plant looks lifeless, but at the same time its development occurs, so future inflorescences begin to form inside the buds.
Winter frosts are much more dangerous for buds than for ovaries, which tolerate cold better. Frost resistance of plants begins to decrease in the middle of winter, continuing in the course of spring. That is why the ovaries are always more tender than flowers, which can be damaged by freezing more than the buds. It is for this reason that unfolded leaves can freeze more than buds that have recently burst.
The climatic zones for cold resistance of plants indicated in foreign sources have nothing to do with and benefit to Russian gardeners. The very division of plants according to their suitability for different climatic zones was actually proposed in the USA, in the Department of Agriculture. But in Russia it is quite possible to be guided by such an approach, because in our territory there are several climatic zones. The zones are located using readings of the lowest temperatures recorded in these territories. The first zone is considered the coldest, on its territory there can be frosts below 45 degrees, while the tenth zone is considered the warmest, according to the classification on its territory the temperature does not fall below 1-4 degrees above zero. On the basis of this classification, for example, for central Russia, plants are recommended that are intended for the fifth zone or below.
By the belonging of a plant to the climatic zone, one can uniquely determine its winter hardiness. The correct number alone cannot guarantee the plant a happy life in this area. The fact is that one should not assume that winter difficulties are just frosts. Each site has its own nuances that affect the state of the plants - this is the microclimate, soil, moisture supply, the length of daylight hours. These parameters, which are very important for plants, are not reflected in any way by the zone number, since they are individual for the area. And an extreme temperature drop is not excluded, which have a record or anti-record character. However, the opposite example is also possible, for example, some delicate plants (grapes or roses) can successfully winter in rather cold zones with the help of special shelters. Therefore, the zonal method gives only general concepts about the winter hardiness of plants.