Michigan is no stranger to wide fluctuations in weather patterns. In southern Lower Michigan, recent daytime temperatures reached 60 degrees Fahrenheit. While the warmer temperatures were welcomed by most people, some are asking how this will impact their wheat crop.
Winter wheat goes through a vernalization period where the plant hardens and adjusts to the colder winter temperatures. The hardening off period begins in the fall once temperatures at the crown (growing point, generally placed about 1-2 inches below ground surface level) drop below 48 F and continues as the temperature decreases.
The hardening process causes a reduction in moisture content in the cells of the crown, which slows growth processes and the accumulation of soluble carbohydrates, all of which help the plant to resist frost damage.
The hardening process takes place over four to eight weeks and the level of hardiness is directly related to the soil temperature at the crown depth. Daylength also impacts hardiness, as shortening days in the fall induce wheat winter hardiness. Conversely, longer days in spring bring wheat out of hardiness.
Cold tolerance is dynamic and can be lost if soil temperatures rise above the previous temperature that the plant hardened to. According to a study by D.B. Fowler from the University of Saskatchewan in 1982, if the crowns of the plants are exposed to warmer temperatures for as little as 50 hours, the cold hardiness can be decreased substantially. The loss of cold tolerance has two major implications:
- If soil temperature decreases below the current level of winter hardiness, injury will occur.
- Once a crop loses winter hardiness, it will never reach the original level of hardiness and it will de-harden more quickly each time the soil temperature rises above the minimum survival temperature.
The chart below shows the rolling 50-hour, 2-inch soil temperature data for three locations in Michigan from Jan. 1-25, 2017. Notice that Dundee had the largest spike up, but it never reached the 48 F critical temperature to break dormancy. What you want to see is a steady line, without large spikes in either direction. Whenever the temperature spikes up followed by a spike down below the original temperature, injury can occur.
So how can these recent warm weather conditions affect the winter wheat crop? The answer is “it depends.” Here are a few points to consider:
1. The warm weather causes snow to melt. Snow acts as an insulator and even as little as 1-2 inches of snow can help protect crowns against severe swings in ambient air temperature.
If less than 1-2 inches of snow remain on the soil surface, the next time temperature drops could result in winter injury, provided temperatures at the crown level fall below 9 F.
2. Snow melt causes ponding in the field. If the water is able to drain before freezing, there may be little or no injury. However, if the water freezes and persists, injury can occur to crowns by waterlogging them, creating anaerobic conditions as well as the crowns absorbing more water, increasing the risk of cell rupture next time the crown is frozen.
3. Pay attention to soil temperatures. Michigan State University has a network of weather stations online at Enviro-weather where you can access soil temperature data for the station nearest to you. If we get more snow cover before temperatures decrease again, we have low risk for injury.
On the other hand, if temperatures decrease, ponded water freezes and we don’t get any insulating snowfall, we could see significant injury.
4. Ice damage is another thing to consider. The warm temperatures melted much or all of the snow plus additional rainfall caused water ponding in the field. When temperatures cool below freezing, before the water has a chance to drain, you can get icing of the ponded spots.
For an established wheat crop to suffer damage from ice, previous research from Kansas State University has shown that a minimum 10 to 40 days of ice surrounding the leaves is necessary. Ice generally damages plants by sealing leaves, stems and buds from the surrounding air, creating an anaerobic environment.
When ice surrounds the crown of wheat for long periods of time, it allows toxic metabolites resulting from this anaerobic environment (ethanol and carbon dioxide) to build up, preventing the natural gas exchange that occurs during respiration. In other words, it “suffocates” the plants. Still, for suffocation to occur, a long period of ice-covered leaf surface is needed.
There have been some areas in Michigan where I would expect damage from icing this year. Scout your fields this spring to determine the level of damage.
What to do if you suspect winter injury
There is a “bag test” that can be done if you suspect you have winter injury or if you just want to confirm your plants did survive the winter. This test can be used to get an early indication of survival. If you have injury, there are some general guidelines to help you determine if you should reseed or tear up the seeding to plant another crop.
Optimum plant stands 20-28 plants per square foot (12-16 plants per foot of row if 7-inch rows). A general rule of thumb is that 50 percent or more of the optimum stand is adequate. In areas of the field with reduced stands, tillering will help fill in the gaps.
Bag test for determining winter kill
- Carefully dig wheat plants from different parts of the field with a spade–remove up to 3 inches of soil containing the plant crown and roots.
- Thaw the samples at room temperature.
- Wash with cold water to remove soil from the roots.
- Cut leaves about 1.5 inches from the crown and roots just below the crown.
- Rinse crown with clean, cool tap water.
- Place at least five crowns in a plastic freezer bag, inflate and tie shut.
- Place bag in a lighted room but not in direct sunlight.
- After five to seven days, a healthy crown should show one-half inch or more of new growth.
- Plants that are not growing after six days are assumed to be dead.
For more information, contact Dennis Pennington, wheat systems specialist at MSU Extension, at firstname.lastname@example.org or 269-833-0497.