The root systems of perennial plants are much less cold tolerant that the above ground portions of the plants. For the most species we can grow in Iowa, root injury can begin occur when the soil temperature drops below 18 to 15 degrees F. Fortunately, the soil has a tremendous buffering capacity and the temperature in the root zone seldom drops this low, but it can when there is a shortage of soil moisture going into the winter.
Soil moisture is important for the transfer of heat energy in the soil. During the growing season, the soil temperature near the surface is warmer than at the lower depths, and the net movement of energy is from the surface to the lower depths. A moist soil will conduct greater energy downward than a dry soil because much of the pore space between particles is occupied by air and air is a very poor conductor of energy. Also, because of the lack of moisture holding capacity, sandy soils conduct less energy than loamy soils.
During the winter, the soil temperature at the lower depths is warmer that near the surface and the net movement of energy is from the lower depths of the soil to the surface where it escapes to the atmosphere. Again, a moist soil will conduct greater energy upward than a dry soil and temperature of the moist soil will be warmer near the surface than the dry soil. If there is snow cover or mulch on the soil surface, it will trap the energy, and the temperature of the soil near the surface will be warmer than an exposed soil.
Therefore, if we continue to go into the winter with a shortage of soil moisture, the potential for winter injury to the roots will remain high. However, for injury to occur, the dry soil conditions would have to be coupled with the lack of snow cover and an extended period of sub-freezing temperatures. So besides hoping for snow cover and a mild winter, measures that can be taken to reduce the risk of winter root injury would include:
- Irrigating before the ground freezes.
- Applying a mulch to minimize heat loss from the soil. When applying mulch under trees, some space should be left between the trunk and mulch to reduce the risk of vole damage.
- Taking measures to aid in trapping snow - allowing the grass to remain tall, installing snow fencing at intervals in the field.