Temperatures on Easter into Monday (Figure 1) and last night into this morning (Figure 2) dipped low enough throughout a large portion of the state to potentially cause some level of injury to the wheat crop. There were a number of areas that spent a significant amount of time with temperatures in the mid to lower 20s over the course of these two cold snaps. Areas in the northwest and into the Panhandle even got as cold as the mid to upper teens. On top of all that, we have cold temperatures forecasted again for later this week.
Figure 1. Minimum air temperatures (top graph) during Easter into Monday (April 1-2) and hours spent below freezing (32°F) over the past 48 hours during that same time frame (bottom graph).
Figure 2. Minimum air temperatures (top graph) during last night into this morning (April 3-4) and hours spent below freezing (32°F) over the past 48 hours (bottom graph).
Keep in mind that the temperature recorded by the nearest weather station or at your house may not quite reflect the actual temperature that the wheat canopy experienced, especially as you increase the distance from where the temperature was recorded and the field itself. Factors such as elevation and topography can influence the temperature, as well as things like large amounts of residue in a no-till situation, for example.
What are the temperatures that can damage the wheat plants?
This will depend on the growth stage of the plants. Anecdotal evidence suggests there are varietal differences in resistance to spring freeze injury, but this is likely due to differences in plant growth stages when the freeze event occurred. Earlier maturing varieties are more likely to be injured from these recent freeze events than later maturing varieties because they are likely more advanced. The susceptibility of the wheat plants to freeze injury does steadily increase as we progress throughout the spring from jointing to heading and flowering. Figure 3 listed below is a general guide to the minimum temperature threshold and its impact on yield. These numbers are not exact but provide a decent rule of thumb. It is difficult to have exact numbers because each freeze event is unique. While a field at the jointing growth could spend two hours at 24 F, it is possible that the same amount of injury could occur with at a 28 F temperature that was sustained for a longer period of time.
Figure 3. Temperatures that can cause injury to winter wheat at different growth stages. Source: Kansas State University publication C646: Spring Freeze Injury to Kansas Wheat.
How long should I wait to assess injury?
Another important thing to keep in mind is that we need to be patient before going out an assessing freeze injury. The extent of a significant freeze event may not be apparent 1 or 2 days after. If warm temperatures return quickly, you should wait about 5-7 days before determining the injury. If temperatures remain cool after the freeze event, it may take 10-14 days before the extent of the injury can be fully assessed. Since we still have cooler temperatures in the forecast, we will likely need to wait closer to the 10-14 days.
What are some freeze injury symptoms to look for?
A common freeze injury symptom is leaf tips turning yellow and necrotic (Figure 4). This is very often just cosmetic and will not hurt yield in the end. More severe damage can result in the entire leaf turning yellow to white and the plants become flaccid (Figure 5). You may even notice a “silage” smell after several days.
Figure 4. Leaf tips which have turned necrotic due to freezing temperatures. Photo taken in March 2017 courtesy of Josh Bushong, OSU northwest area Extension agronomist.
Figure 5. More severe freeze damage causing the leaves to turn yellow-white with plants losing their overall turgidity. Source: Kansas State University publication C646: Spring Freeze Injury to Kansas Wheat.
The most important plant part to check is the growing point. This will be important for areas of the state that have fields with plants which are at jointing or past jointing. Sometimes we can see what look like healthy plants overall, but the growing point has been damaged or killed. To get a look at the growing point, you can slice the stem open lengthways. A healthy growing point will have a crisp, whitish-green appearance and be turgid (Figure 6). Often, you can lightly flick the head, and if it bounces back and does not break, it is still healthy. If it is mushy, limp, and breaks or parts of it break off when you lightly flick it, it has been compromised. It may also have a brown color (Figure 7). Another indication that the growing point has been compromised is the next emerging leaf is necrotic.
Figure 6. Close up of a healthy wheat head (growing point). Source: Brenda Kennedy and Dr. Carrie Knott, University of Kentucky
Figure 7. Plants that appear healthy could have damaged heads. Photo taken several years ago courtesy of Dr. Jeff Edwards.
Figure 8. A close up view of the damaged wheat head from Figure 7. Photo taken several years ago.
Also, the percent of damaged heads may not translate into percent yield loss. There is still opportunity for wheat at the jointing stage (GS 6) to produce additional tillers and/or retain secondary tillers. Whether or not these tillers are able to compensate for larger tillers that were lost due to freeze will depend on the subsequent weather. If conditions are favorable, there is a chance for late emerging tillers to have a shot at producing grain. If the wheat is more advanced, it will be tougher to make this type of recovery.
Do drought conditions exacerbate freeze injury?
When it comes to this question, it is really a mixed bag of results. A lot of anecdotal evidence suggests drought conditions can make freeze injury worse, and that could very well be the case in some scenarios. Water in the soil is a good buffer to resist temperature swings and can prevent the soil from cooling as quickly as the air around it. Therefore, the temperature at the soil surface of a conventionally tilled field with good soil moisture may not get as cold as a similar field with dry soil conditions for example.
In theory, the plants themselves under drought conditions should actually be able to withstand cooler temperatures than non-stressed plants as less water content in the plant cells increases the solute concentration (i.e., it takes longer for those cells to freeze). Using the conventionally tilled field example above, we cannot automatically say that a field with dry soil conditions will have worse freeze injury than a field with adequate moisture. Also, if the weather conditions during the day(s) prior to the freeze event were warm and sunny, a significant amount of heat may still be radiated from a field with dry soil conditions and provide some buffer against freeze injury.
Remember that each freeze event is unique. The amount of injury observed will depend on the growth stage of the plants, how low the temperature got, and how long it stayed at those cold temperatures. Other factors such as elevation, residue cover, and moisture can influence the observed temperature within the canopy as well. Because of the number of influential factors, it is important to check each field. It is possible to have variability in injury symptoms among fields and even within fields.