Small Grains Focus

What Determines Winterkill, How to Evaluate It

By Jochum Wiersma
U of M Small Grains Specialist
wiers002@umn.edu

It’s been a few winters since we’ve seen sub-zero temperatures like these in the Northern Plains – readings plunging past –30°F, even –50°F (that’s just air temperature folks, not factoring wind chill) in Fosston, Minn. It’s these kind of temperatures that, for those who grow winter wheat or are contemplating doing so, beg the question: what determines winterkill, and how do you evaluate the extent of winterkill damage this coming spring?

Winterkill is a complex problem. It is not just minimum temperature or the length of the exposure that will determine the final amount of damage.  To survive freezing temperature, the physiology of the young winter wheat seedling changes in late fall.  This cold hardiness is induced when temperatures dip to under 50°F for a number of days. 

The cold hardiness is not a static condition.  Canadian research has shown that the cold hardiness followed a bell shape curve in which hardiness increased during the fall, peaked between December and February, and decreased in early spring. The loss of hardiness was hastened by factors such as a mid-winter thaw, alternate freezing and thawing, frost heaving, desiccation, suffocation, and rain.

Winterkill itself can be caused by freezing of plant tissues, physiological drought, frost heaving, and/or ice encasement. In the Northern Plains, freezing of plant tissues appears to be the primary cause. A relatively mild frost of 14°F in early April may be just as damaging as -10°F in January, given the fact that cold hardiness is not a constant and the crown’s ability to withstand freezing temperatures changes during the course of the winter.

The crown (see figure below) is the part of the wheat plant located about 0.5 inches below the soil surface, where the nodes are tightly packed together, tiller buds develop, and coronal root buds develop. Fully hardened winter wheat crowns can tolerate soil temperatures of 5°F for up to 6 days. The minimum temperature fully hardened crowns can survive is approximately -10°F for about 12 hours. A continuous frost of 27°F will ultimately kill hardened plants after about 5 months.

now cover can greatly reduce winterkill. The snow cover insulates and allows warmer soil temperatures in the crown area. Three inches of snow cover already is sufficient for most North Dakota winters. Four to six inches will even further reduce crown injury and increase survival (Table 1). 

To evaluate winterkill in the spring requires some patience. As roots are less winter hardy than crowns, regrowth may be very slow, even if roots and shoots appear dead.  Don’t be too hasty to destroy winter wheat stands. 

Table 1. Predicted daily minimum temperatures at crown depth relative to two daily minimum air temperatures for snow depths.

1.2 – 2.5 0.9 -9.9 2.5 – 3.5 6.1 -3.6 3.5 – 4.7 11.1 2.5 4.7 – 20.5 18.1

1Source:  Winter Wheat Production in North Dakota. NDSU Extension Service.

Recent research at Kansas State University has shown that the percentage of winterkill is not equal to the amount of yield loss that can be expected. In a controlled experiment, stand reductions of 25, 50, and 75% resulted in yield decreases of 7, 14 and 21% in the first year and 15, 30, and 60% yield loss in the second year of the study. 

The ability of the winter wheat crop to compensate for the reduction in stand through formation of additional tillers and compensation between the yield components (number of tillers per unit area, number of kernels per heads, and kernel weight) allowed the reduction in yield to be less than the reduction in stand.  The authors of the K-State study attributed the difference between the two years on unfavorable conditions during grain fill in the second year.

Therefore, as long as stands are even, plant populations of 15 to 17 plants per square feet in the Northern Plains are probably adequate and should not be abandoned. This would equate to a stand reduction of about 35%. If large areas have been lost, an option would be to plant a spring crop in just those areas that failed.