By: Laura Lindsey, Alexander Lindsey, and Aaron Wilson
Overnight temperatures on April 15 and April 16, 2020 dropped into the low to mid 20s across a large portion of Ohio (Figure 1), and unofficial reports show a few locations briefly dropped into the upper teens! These temperatures were generally 12-20°F below average (1981-2010). A closer investigation at a few of the colder sites reveal temperatures remained below 32°F for 9-11 hours, below 28°F (hard freeze) for 7-9 hours, and below 22°F for 3-5 hours.
Figure 1. Daily overnight lows based on station observations for April 15-16, 2020. Figures generated at the Midwest Regional Climate Center
Injury to winter wheat depends primarily on three factors: 1) growth stage, 2) how cold, and 3) duration of cold temperature. Differences in freeze injury among cultivars can usually be attributed to slight differences in growth stage.
Although temperatures were low and there may be some yellowing/browning of leaves, the impact on wheat grain yield is likely to be minimal. In our research, at Feekes 6 growth stage, reductions in wheat grain yield began when temperatures fell to less than 20°F for a 15-minute duration. A 50% reduction in grain yield occurred at 12°F for a 15-minute duration.
Prior to Feekes 6 growth stage, the growing point of wheat is below the soil surface, protected from cold temperatures. However, at Feekes 6 growth stage, the first node appears and pushes the growing point (developing spike) up through the plant stem, and this developing spike can be damaged by low temperatures.
Damaged spikes can be observed by carefully cutting the wheat stem lengthwise to expose the developing spike at the first node. Damaged spikes will appear discolored and shriveled, which occurred at the 3°F temperature treatment (Figure 2).
Figure 2. At Feekes 6 growth stage, cold temperatures cause damage to the forming wheat spike within the stem. Wheat spikes pictured (left to right) were exposed to 39, 28, 21, 14, and 3°F temperature treatments.
At Feekes 6 growth stage, damage from low temperatures will cause yellow or browning (necrosis) of the leaf tissue, most likely leaf tips or edges exhibiting symptoms first (Figure 3). Death of leaf tissues and stems may result in the formation of tertiary (regenerative) tillers from surviving plant crowns (Figure 4). These tertiary tillers may produce seed, but often time do not fully mature, resulting in small, lightweight kernels. Overall, grain yield is reduced in these situations as primary and secondary tillers account for the majority of grain yield.
Figure 3. At Feekes 6 growth stage, low temperatures cause yellowing or browning of the leaf and stem tissue. Wheat plants pictured (left to right) were exposed to 3, 14, 21, 28, and 39°F temperature treatments, corresponding to death of 100, 80, 50, 25, and 0% of the aboveground biomass, respectively.
Figure 4. Low temperature damage to wheat leaves and stems may cause uneven tillering. Here, the wheat plant has primary (1st), secondary (2nd), and tertiary (3rd) tiller development as a result of regrowth after a freeze event.