By Romulo Lollato
Extension Wheat and Forages Specialist, KSU.edu
Temperatures dropped below freezing in many areas of Kansas during April 3-4, with as many as 10 hours or more of temperatures below 24 degrees F. While the majority of the Kansas crop is delayed in development this growing season due to late planting in the fall and a cooler-than-normal winter and spring, some fields in south central Kansas are approaching jointing and some injuries might be sustained. This article addresses some factors affecting the likelihood of freeze damage, potential consequences of the recent freeze to the wheat crop, and guidelines for producers to check for freeze damage.
Important factors determining freeze damage
A number of key factors determine the potential for freeze damage: the stage of development of the wheat, the density of the stand and condition of the plants, the amount of residue on the soil surface, the extent and duration of low temperatures, temperature gradients within the field (position on the landscape), soil moisture, and wind speed.
Stage of development
- Greenup. Wheat that hasn’t started to joint yet (Feekes 3 through 5) might suffer damage to the existing foliage, but the growing points will be protected by the soil and should escape injury. This wheat will have cosmetic damage to the leaves that will show up almost immediately as leaf tip burn (Figure 1).
- Jointing. Wheat that is jointed or past is more sensitive to freeze damage than wheat at greenup because the growing point is above ground. It can still tolerate temperatures in the mid 20’s with no significant injury, but if temperatures fall into the low 20’s or lower for several hours, the lower stems, leaves, or developing head can sustain injury.
Figure 1. Leaf burn from freeze damage. By itself, this is cosmetic damage only.
Density of the stand and condition of the plants
If the stand is thick, that will tend to reduce the extent of freeze damage as the warmth of the soil will radiate up into the canopy. On the other hand, well-fertilized, succulent wheat has often sustained more freeze injury than wheat that is not as well fertilized. Thin stands are at higher risk of injury because the air can penetrate the stand more easily. If the plants were wet before the freeze, this can result in a coat of ice on the plants that may protect the growing point to some extent. If temperatures get too low, however, the cold will go through the ice.
Amount of residue on the soil surface
No-till fields can often sustain more freeze damage because the residue acts as a blanket and doesn’t allow the heat from the soil to radiate up into the plant canopy.
Extent and duration of low temperatures
Significant injury becomes much more likely if the temperatures in the damaging range (below 24 degrees F for jointed wheat) last for two hours or longer, which is the case for parts of Kansas. For more detailed information on how cold parts of Kansas got and the duration of temperatures below the above threshold, please see accompanying eUpdate article “Cold snap on April 3-4: Can this damage the 2018 Kansas wheat crop?”
There is often less freeze injury at a given temperature when soils have more moisture present. Wetter soils tend to radiate a little more warmth than dry soils. On the other hand, drought-stressed plants tend to be more hardened against cold injury and their lower leaf water content tends to decrease the severity of the freeze injury.
Windy conditions during the nighttime hours when temperatures reach their lows will reduce the amount of warmth radiating from the soil and increase the chance of injury.
Temperature gradients within the field (position on the landscape)
Low spots in the field are almost always the first to have freeze injury. The coldest air tends to settle in the low areas, especially under calm wind conditions.
Although the sensitivity to cold temperatures at a given growth stage is very similar across all varieties, varieties can differ in their release from winter dormancy as much as three weeks. Because of differences in winter dormancy release, late-release varieties may escape a freeze injury for being delayed in their development. For instance, a late-release variety at Feekes 4 or 5 is less sensitive to freeze damage than an early-release variety planted at the same date which might have reached Feekes 6. For more detailed information of different varieties release from dormancy during the current growing season, please see the accompanying eUpdate on first hollow stem.
There are many possible scenarios after a freeze and producers should not take any immediate action following a freeze event. Several days of warm temperatures are needed to properly assess freeze damage to the wheat crop. Thus, producers should watch their fields closely over the next 7 to 10 days after the freeze event for the following:
- The color of newly emerging leaves - If they are nice and green, that probably indicates the tiller is alive. If newly emerging leaves are yellow, that probably indicates the tiller is dead. The color of existing leaves is not terribly important, except for the flag leaf, which should not have emerged at this point yet. 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.
- The color of the developing head or growing point in wheat that has jointed - As long as heads are light green, crisp, and turgid, the head in that tiller is fine. If it is whitish, flaccid, and mushy, it has died (Figure 2).
- Ice in the stems - If there was ice in the stems below the first node the morning of the freeze, those tillers may be damaged (although not always) and may not produce grain. You may see split stems from ice accumulation.
- 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.
Figure 2. A developing head still within the stem killed by freezing temperatures. The dead head is whitish and flaccid. If it were healthy, it would be light green and turgid.
The best thing producers can do for the first few days is simply walk the fields to observe lodging, crimped stems, and damaged leaves. Producers should not take any immediate actions as a result of the freeze, such as destroying the field for recropping. It will take several days of warm weather to accurately evaluate the extent of damage. After several days, producers should split open some stems and check the developing head.
Where stems and/or growing points were killed by the freeze, new tiller growth (coming from the crown area) will occur (Figures 3 and 4). 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 begin growing normally and fill out the stand, the wheat may look ragged because the main tillers are absent. Producers should scout for bird cherry oat aphids and other potential insect or disease problems on these late-developing tillers. 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.
Figure 3. A stem that was split open by having ice form within the stem. This stem has died and a new tiller has begun to grow at the base.
Figure 4. Some of the tillers on this plant had freeze damage to the lower stems. These stems are dying, but the symptoms may not be immediately evident. The growth of new tillers from the base of the plant is a sure sign that the main tillers are dead or dying. Note the brown lesion on the stem with the two new tillers.