K-State Research and Extension wheat production specialist Romulo Lollato said that wheat growers concerned about recent low temperatures will be able to assess any damage to their crop that has already jointed or is more advanced in the next few days. He described what to look for.
Leaf tissue
The extreme low temperatures measured around March 19-20 will give the crop a rough look for a few weeks. The first apparent sign of freeze injury will be leaf dieback and senescence, which should occur across most of the state regardless of damage to the actual growing point. Existing leaves will almost always turn bluish-black after a hard freeze, and give off a silage odor.
Those leaves are burned back and dead, but that in itself is not a problem as long as newly-emerging leaves are green. Provided that the growing point is not damaged, the wheat will recover from this damage with possibly little yield loss. If newly-emerging leaves are nice and green, that probably indicates the tiller is alive. If newly-emerging leaves are yellow, that probably indicates the tiller is dead.
Developing head
After a few warm days, the color of the developing head or growing point in wheat that has jointed will be the most important indicator of possible freeze damage. As long as heads are light green, crisp, and turgid, the head in that tiller is fine. If the head is whitish, flaccid, and mushy, it has died.
Stem integrity
If the wheat lodged immediately after the freeze, that indicates stem damage. Later tillers may eventually cover the damaged tillers. Even if there is no immediate lodging, look for lesions or crimps anywhere on the stems. If these symptoms are present, it usually means the wheat will lodge at some point during the season. If the stems look undamaged, that’s a good sign.
New tillers
Where stems and/or growing points were killed by the freeze, new tiller growth (coming from the crown area) will occur. In many cases, new tiller growth can be observed even when the stems do not show any symptoms of freeze damage for some time. In those cases, the first sign that the tillers are dead is the sudden growth of new tillers at the base of the plant. If secondary tillers may begin growing normally and fill out the stand, the wheat may look ragged because the main tillers are absent. Enough tillers may survive to produce good yields if spring growing conditions are favorable. If both the main and secondary tillers are injured, the field may eventually have large areas that have a yellowish cast and reduced yield potential.
Minimum air temperatures across Kansas dipped well below freezing March 19 and 20, which could pose a problem for some of the state’s wheat crop, said Mary Knapp, assistant climatologist with the Weather Data Library at Kansas State University.
“Most of the state was exposed to minimum temperatures below freezing, with the exception of small isolated pockets. The western half of the state had minimum temperatures below 24 degrees Fahrenheit, the threshold below which there can be damage to the wheat’s growing point when at the jointing stage of development. Even more concerning, the far western fifth of the state had minimum temperatures in the single digits,” Knapp said.
The risk of damage to wheat is a function of the minimum temperature and duration of time spent at potentially damaging temperatures, said Romulo Lollato, K-State wheat production specialist.
In this case, counties along the western border, neighboring Colorado, were exposed to as many as 27 hours below 24 degrees F over the past four days, Knapp said. In general, the western half of the state had more than 11 hours of temperatures below 24 degrees. On the other hand, many counties in the eastern one-fifth of the state did not have a single hour below 24 degrees.
The coldest night in the period was on March 19-20, she said.
“Temperatures were below 24 (degrees F) in that night for as much as 12.3 consecutive hours. The western third of the state, where about 40 percent of the wheat is grown, experienced colder temperatures for longer durations than other areas of the state,” she said.
While temperatures this cold are not uncommon for this time of the year, the wheat crop is well advanced throughout the state this year due to a relatively warm winter, and producers who have jointed wheat might be concerned with possible damage to their crop, she said.
Different stages of wheat development vary in their sensitivity to cold temperatures, Lollato said.
“Where the developing head is already above ground, in the jointing or later stages, cold temperatures can damage the developing wheat head. The threshold below which economic damage can occur when wheat is jointed is approximately 24 degrees. Additionally, temperatures need to be sustained at levels below 24 degrees for a minimum of two to three hours to be potentially damaging to the developing head,” he said.
The risk of freeze injury is probably greatest in south central Kansas, particularly in Harper, Barber, and Sumner counties, and possibly surrounding regions, said Lollato and Erick DeWolf, K-State extension plant pathologist.
“These counties are far enough west to be exposed to temperatures below 24 degrees for a minimum of four hours while having a more advanced stage of wheat, beyond jointing in the majority of the region. Counties in southwest Kansas bordering Oklahoma might also see damage in the more advanced fields that have the growing point above ground due to the long exposure to temperatures below 24 degrees,” DeWolf said.
Although other scattered cases of freeze injury might be observed in more advanced fields throughout Kansas, the risk of severe freeze injury in other areas of the state appears to be low because either the crop is not as advanced in development or it did not get cold enough to sustain damage, Lollato said.
The extent of a possible freeze damage to the developing wheat crop will depend on several variables, including canopy density, soil moisture, crop residue, and wind speed, Lollato added.
As a result of so many interacting variables, evaluating only air temperatures may not completely reflect the conditions experienced by the wheat crop, he said. In this situation, soil temperatures can help in determining the extent of the cold stress at crown and lower canopy levels, especially for crops in which the growing point is still below ground or just starting to elongate, he said.
While air temperatures reached critical levels for damage to the developing wheat head, soil temperatures at a 2-inch depth were above 32 degrees F all across western Kansas, and in most cases above 40 degrees in other regions of the state, Knapp said.
“Higher soil temperatures may have helped buffer the cold air temperatures experienced, minimizing possible injury to the wheat crop especially for crops still in developmental stages where the developing head is below ground and therefore insulated,” she said.
For more information on freeze damage to wheat, please see the accompanying article or publication “Spring Freeze Injury to Kansas Wheat,” K-State Research and Extension publication C646, available at county and district extension offices and online at: http://www.ksre.ksu.edu/bookstore/pubs/C646.pdf
