LIND, Wash. — Eastern Washington’s winter wheat crop experienced high winds and prolonged subzero temperatures during the third week of December.
“This is a big concern for farmers who did not have snow cover on their fields,” according to William Schillinger, a research agronomist at Washington State University’s Dryland Research Station here. “If the farmers had snow cover, they’re alright, but a lot of ground was not covered.”
At Lind, sustained northeast winds of 25 mph with gusts to 46 mph occurred for two consecutive days, blowing what little snow that was present into drifts and leaving more than 80 percent of the ground bare. Temperatures plummeted to below zero on several days following the windstorm. On Dec. 20, temperatures dropped to 10 degrees below zero and stayed below zero for 12 hours.
Some of the wheat plants Schillinger and WSU research technician Timothy Smith brought in for evaluation at the station did not recover from the cold.
“Plants collected from fully exposed hilltops with no snow cover had minimum to no recovery,” Schillinger said. “Other plants collected from less exposed areas were damaged but not dead. Plants under snow cover had no damage at all. I suspect we’ve had some winterkill in pockets, and we’ve certainly had some more widespread plant damage. How much remains to be seen.”
If a grower wants to estimate cold damage, Schillinger recommends digging up a representative sample of plants, making sure the crowns remain intact. Clip roots off within one-quarter of an inch of the crowns. Wrap plants in wet paper towels and place them in zip-lock bags to prevent dehydration. Put plants in a refrigerator for 24 hours, then remove from the refrigerator and leave at room temperature. If the wheat plant is alive, regrowth should begin within four days.
The critical process for the survival of winter wheat during extreme cold is hardening, Schillinger explained. “We had relatively warm temperatures at Lind during the second week of December, so the wheat did not have much time to harden off prior to the onset of the wind storm and extreme cold. Cold hardiness is not a static condition, but changes with time, temperature, soil moisture status and plant age.”
The major soft white winter wheat varieties grown in the Pacific Northwest vary considerably in their winter hardiness. Eltan is the hardiest of the soft white varieties; Stephens the least hardy.
“Generally, under Pacific Northwest conditions, the deeper a variety sets its crown, the more winter hardy the plants,” Schillinger said. “This is because the soil acts as an insulator, providing protection against rapid temperature changes. Soil temperature at the crown depth, not air temperature, is the critical factor affecting plant survival. Although extreme cold may kill above ground portions of the wheat plant, recovery is still possible as long as the crowns are alive. The most winter hardy varieties have their crowns up to one inch below the soil surface.”
Schillinger said that early planted winter wheat with several tillers, which are new shoots, hardens less and is more vulnerable to winterkill from cold temperatures than wheat planted at a later date with fewer tillers. However, larger well-tillered plants protect the crown better from the wind.
“Wheat is very susceptible to dehydration when exposed to cold, dry winds,” Schillinger said. “Even tillers on the same plant do not possess the same degree of winter survival. Younger tillers are more cold hardy and survive the winter better than older tillers. Plants that are in the three or four leaf stage and have a good root system are best suited for both winter survival and rapid growth in the spring. Plants in moist soil survive low temperatures better than in dry soil.”
He said that cold injury increases with the length of exposure to low temperatures and fully cold-hardened winter wheat crowns can tolerate minus 4 degrees for 24 hours and minus 9 for 12 hours.
“Crowns of hardy varieties can be held at 11 degrees for a period of 15 days or longer with little or no damage,” Schillinger said. “As little as 1 to 2 inches of snow cover can provide the insulation necessary to trap residual soil heat and buffer against air temperature extremes that prevent killing temperatures in the crown zone, even during periods when air temperatures go down to minus 30.
He said that more research is needed on how the duration of low temperature combined with wind and soil moisture factors affect winter survival before we can accurately predict the degree of winterkill and reduction in plant population.
Although a variety may achieve considerable hardiness by early winter, warm temperatures will cause wheat plants to lose some of their winter hardiness, Schillinger said. “Winter wheat begins to grow as the temperature rises, taking up water from the soil. Water dilutes the cell solute concentration or antifreeze, rendering the plant more susceptible to cold injury. For this reason, rapid onset of arctic temperatures is more damaging to non-hardened plants than hardened plants.”
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