Assessing Freeze Injury to Wheat in Michigan

By Dennis Pennington and Jeffrey Andresen

Figure 1. Minimum temperatures (degrees Fahrenheit) across Michigan, May 9, 2020. Data obtained from the MSU Enviroweather and the National Oceanic and Atmospheric Administration.

An arctic-origin air mass brought unseasonably cold weather and widespread hard freeze conditions to much of Michigan May 8-9, 2020 (Figure 1). In northern sections of the state, the event followed an almost daily series of freezing overnight temperatures back to May 4. The freezing conditions on May 8-9 were generally advective in nature, meaning there was a horizontal movement of a cold air mass versus more typical radiative freeze events with vertical release of heat to the atmosphere under relatively clear, calm conditions. Breezy, cloudy conditions continued at many sites throughout the evening.

Another distinguishing factor was the very dry, low humidity nature of the air mass. Dewpoint temperatures during the event generally remained in the teens, resulting in little or no visible frost on the soil or vegetation (black frost conditions). Coldest temperatures statewide were observed across interior western sections of the Upper Peninsula and across interior sections of the southwest half of Lower Michigan where low to mid-20’s were common. Minimum temperatures were somewhat milder across the eastern Upper Peninsula southward through northern and eastern sections of the Lower Peninsula (28-33 degrees Fahrenheit) due to the moderating influence of lake effect clouds and snow showers.

A representative vertical cross section of temperatures during the event is given in Figure 2 from the Michigan State University Enviroweather tower site near Sparta, Michigan. Five-minute mean air temperatures measured at 66 feet (brown), 33 feet (orange), 5 feet (red) and 4 inches (green) above the surface collectively dropped below freezing at approximately 9:30 p.m. and remained there until about 8 a.m. on May 9 (approximately 10.5 hours). With advective freeze conditions and a persistent 5-15 mph wind (blue vertical bar graph just above the temperatures) throughout the evening and early morning hours, temperatures at the different levels generally remained within 2 F of one another, with only weak inversion conditions (increasing temperatures with height) near the ground surface.

The duration of temperatures below 28 F, a common threshold for cold injury, varied significantly across the state, ranging from zero hours across eastern sections to as much as 9 hours across interior sections of west central and southwestern Lower Michigan and the western Upper Peninsula. Below the surface, 2-inch bare soil temperatures during the event generally ranged from the upper 40s south to the upper 30s in the north.

Winter wheat tolerance to subfreezing temperatures is highly dependent on growth stage. In young, tillering wheat, the growing point remains below the soil surface where it is protected from very cold temperatures. Feekes 6 marks the beginning of jointing or rapid stem elongation, where the developing head is moved up the stem and emerges. Feekes 6 is when you can feel/see one node above the soil surface. Feekes 7 and 8 are marked by two and three detectable nodes, respectively. The developing head is located just atop the uppermost node. It is this developing head that is of utmost concern. If conditions get cold enough, water in the extracellular space freeze, causing the cell walls to burst. The severity of this damage will determine how much yield loss to expect.

Wheat that is in the jointing stage (Feekes 6-9) can tolerate temperatures down to 24 F for only about 2 hours. As wheat progresses in development toward boot, cold tolerance is reduced. Damage during the jointing stage include leaf yellowing or burning, death of the growing point, splitting or bending of the lower stem and a “corn silage” odor. Yield reduction is moderate to severe and is dependent on how many florets are damaged. More information on growth stage, damage and yield effect can be found in Table 1.

To assess a plant for viability, slice open the stem longitudinally at the uppermost node with a sharp knife. Locate the developing head. A normal head should be white to yellow-green and turgid. A damaged head may be off-white or brown in appearance and have a silage odor. How cold and length of the cold period will have an impact on how many florets may be damaged. The level of damage on each head and the number of damaged heads in a field will be directly related to final crop yield. Also, check the health of lower stems. Often, freeze damage shows up as swollen nodes or lesions on the stem near the nodes. This could increase the risk of lodging.

Affected tillers may remain green but if the growing point dies, growth will stop. Where you see chlorotic leaves emerging from the whorl, the growing point in that tiller is likely damaged. It may take a week after a freeze event before the impacts are fully known. Scouting too early may result in inability to identify damage.