Hardiness of Plants

the ability of plants to withstand unfavorable winter conditions without injury. During severe frosts plants may die owing to the formation of ice in the cells or interstitial spaces. The icy crust that appears on plantings during thaws interferes with the aeration of plant cells and weakens the plants’ resistance to frost. Winter crops that lie for a long time under deep snow at a temperature of about 0°C suffer from starvation and infection by fungi (damping off). The formation of a layer of ice in the soil breaks the roots and causes heaving. Many of these unfavorable factors often act simultaneously.

Hardiness and, especially, resistance to frost develop by the beginning of winter through the process of hardening. Plants can tolerate frosts: winter rye up to −30°C, winter wheat up to −25°C, and apple trees up to −40°C. Resistance to damping off may be acquired by the accumulation of a large quantity of sugars and other reserves in plants by the beginning of winter, by the economical expenditure (at about 0°C) of their reserves on respiration and growth, and by protecting plants against fungus diseases. Plant resistance to heaving is caused by the strength and elasticity of their roots. Since heaving generally occurs on compact, humic, and wet soils after repeated freezing and thawing, it is very important that the sowing area be correctly chosen. The stagnation of water in the fields during the fall is also harmful because it prevents plant hardening, causing plants to become more vulnerable to injury by frosts. Water stagnation in spring is even more destructive. Plants that were weakened or injured during the winter die if aeration is insufficient. For this reason, the physical properties of the arable layer of soil must be improved.

Agricultural techniques for collecting and storing moisture in the soil and irrigation must be used to increase the hardiness of fruit trees. Hardiness is also decreased by summer droughts. If water is in short supply, the trees cannot complete their developmental cycle and become dormant; therefore, windbreaks play an important role. The hardiness of fruit trees often decreases in highly productive years because they are unable to adapt themselves in time for winter. It is essential, therefore, to choose varieties that produce evenly from year to year. Pests and diseases must also be controlled if the plants are to become hardy. The correct acclimatization of existing varieties and the breeding of new, hardy varieties are very important. The hardiest varieties of winter rye and wheat, alfalfa, clover, and apple trees are found in the USSR. The hardiest winter rye varieties are grown in Siberia and in the southeastern part of the country, and the hardiest winter wheat is cultivated in the eastern regions.