The winter of 2013/14 was cold and abnormally dry across north-central and western Iowa. Dry soil conditions coupled with a lack of snow cover and an extended period of sub-freezing temperatures were ripe conditions for root and crown injury in fruit crops, including strawberry, raspberry, and blackberry. The root systems and crowns of perennial plants are less cold tolerant than the above ground portions of plants. For most species grown in Iowa, root injury can begin to 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. However, it can when there is a shortage of soil moisture going into winter and/or a lack of snow cover throughout winter.
Soil moisture is important for the transfer of heat energy in the soil. During the winter, the soil temperature at the lower depths is warmer than 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. A moist soil will conduct greater energy than a dry soil because much of the pore space between particles is occupied by air and air is a poor conductor of energy. Thus, during winter, a wet soil will stay warmer than a dry soil.
Snow also affects soil temperature. A heavy snow cover will trap energy in the soil keeping the surface up to 20 degrees F warmer than an exposed soil. Mulch is often substituted for snow or applied in the fall to ensure a heavy cover all winter long. However, without fall moisture or winter snow cover, mulch easily blows away leaving the ground exposed and allowing soil temperatures to plummet.
It can take several weeks or months after the growing season starts for injury to become noticeable. Often, plants will show no injury until summer when heat and water stress begin to take their toll on plants. They may show stress through continued wilting, stunting, or even dropping their leaves prematurely. Winter injury can often be very sporadic from plant to plant or across a field with no obvious pattern. Winter injury in crowns and roots can be identified by browning of the whitish/ivory colored tissue. Minor injury should not significantly impact overall plant health and yield.
Plants with minor winter injury will eventually repair themselves or compensate for the injury. Following good cultural practices, watering, and fertilizing as needed will help plants through the summer. Plants may require replacement if injury is severe.