Temperatures way below the freezing point cause stress for plants, and may lead to considerable damage. This past winter, 660.000 hectares of cereals and rape were affected in Germany alone. For two weeks in February, temperatures fell as low as –28°C. On fields without a protective layer of snow, farmers had no choice but to plough the land and to begin anew by sowing spring crops.
But damage from frost threatens crops in many regions of the world. Large parts of Europe and of North America, but also higher-altitude Asian and South American regions are affected.
Frost affects many functions of the plant and consequently there are several different specific types of damage:
Intracellular Ice Crystal Formation: Sugar and Potassium Serve as Antifreeze Agents
Plants may be directly affected by the formation of ice crystals which rupture and kill cells. Plants may better protect themselves by storing potassium and sugar in their cells. Both substances lower the cell sap’s freezing point, thereby actually serving as antifreeze agents. If water is drawn from the intercellular spaces by ice formation, this raises the potassium and sugar concentrations in the cell’s interior, and improves frost resistance.
This mechanism is developed during the acclimation phase, the plant hardening period. This requires alternating cool temperatures above 0°C and light frosts. In case of sudden severe freezing prior to the onset of the dormant period plants are not yet sufficiently adapted to low temperatures. Selection of the best-suited varieties also affects frost resistance.
Winter Desiccation: Potassium and Magnesium Protect
In addition to intracellular ice crystal formation, winter drought is the next most important cause of winter kill. Deeply frozen soil prevents plants from absorbing sufficient water. As a consequence, they “die of thirst”. Plants are most at risk when evaporation rates are high from sunny and windy weather occurring above ground with deeply frozen underground conditions preventing the replacement of water lost through the leaves.
For a limited period of time, a balanced supply of nutrients will help to stave off winter desiccation. The osmotic function of sugar and potassium, as well as the function ofpotassium in regulating the guard cells will help to improve the plant’s water balance, preventing uncontrolled loss of water via the foliage. Magnesium promotes root growth, allowing plants to use this better developed root system to draw water from even deeper layers of superficially frozen soils.
Frost Heaves: A true tension test for roots
Alternating frosts and thaws – such as warm days and strong frosts at night – will cause soil movements that may cause roots to rip. Uptake of water is severely restricted. At the beginning of each new vegetation period new roots need to be developed, before the plant can begin to grow. Heaving may be prevented by reconsolidating the seedbed. But promoting root growth by providing a sufficient supply of magnesium will also help crops to recover.
There is no 100%-protection against frost damage but a balanced supply of both potassium and magnesium will put your crops in much better shape to survive frost and drought stress. This will considerably decrease your yield losses.