When conditions have been unusually cold throughout Kansas during most of December and there has been little or no snow cover for most of that period, this can mean that soil temperatures have been colder than normal. Will this make some wheat fields susceptible to winter die-off?
The following are some of the factors to consider when evaluating the outlook for winter survival of wheat:
How well has the wheat cold hardened?
When temperatures through fall and early winter gradually get colder, that helps wheat plants develop good winterhardiness. When temperatures remain unusually warm late into the fall (which can lead to excessive vegetative growth) then suddenly drop into the low teens, plants are less likely to have had time to cold harden properly and will be more susceptible to winterkill. This fall, temperatures generally fell gradually. It did not go from unusually warm with strong plant growth to bitterly cold in a single day. As a result, the wheat should be adequately cold hardened in most cases.
How well developed is the root system?
Good top growth of wheat doesn't necessarily indicate good root development. Poor root development is a concern where conditions have been dry. Where wheat plants have a good crown root system and two or more tillers, they will tolerate the cold better. If plants are poorly developed going into winter, with very few secondary roots and no tillers, they will be more susceptible to winterkill or desiccation, especially when soils remain dry. Poor development of secondary roots may not be readily apparent unless the plants are pulled up and examined. If plants are poorly developed, it may be due to dry soils, poor seed-to-soil contact, very low pH, insect damage, or other causes.
How cold is the soil at the crown level?
This depends on snow cover and moisture levels in the soil. Winterkill is possible if soil temperatures at the crown level (about one-half to one inch deep if the wheat was planted at the correct depth) fall into the single digits. If there is at least one inch of snow on the ground, the wheat will be protected and soil temperatures will usually remain above the critical level. Also, if the soil has good moisture, it's possible that soil temperatures at the crown level may not reach the critical level even in the absence of snow cover. But if the soil is dry and there is no snow cover, there may be the potential for winterkill, especially on exposed slopes or terrace tops, depending on the condition of the plants.
Is the crown well protected by soil?
If wheat is planted at the correct depth, about 1.5 to 2 inches deep, and in good contact with the soil, the crown should be well protected by the soil from the effects of cold temperatures. If the wheat seed was planted too shallowly, then the crown will have developed too close to the soil surface and will be more susceptible to winterkill. Also, if the seed was planted into loose soil or into heavy surface residue, the crown could be more exposed and could be susceptible to cold temperatures and desiccation.
Is there any insect or disease damage to the plants?
Plants may die during the winter not from winterkill, but from the direct effects of a fall infestation of Hessian fly. Many people are familiar with the lodging that Hessian fly can cause to wheat in the spring, but fewer recognize the damage that can be caused by fall infestations of Hessian fly. Wheat infested in the fall often remains green until the winter when the infested tillers gradually die. Depending on the stage of wheat when the larvae begin their feeding, individual tillers or whole plants can die. If the infestation occurs before multiple tillers are well established then whole plants can die. If the plants have multiple tillers before the plants are infested then often only individual tillers that are infested by the fly larvae will die.
The key to being able to confirm that the Hessian fly is the cause of the dead tillers is to carefully inspect the dead plants or tillers for Hessian fly larvae or pupae. This can be done by carefully removing the plant from the soil and pulling back the leaf material to expose the base of the plant. By late winter all of the larvae should have pupated and thus the pupae should be easily detected by elongated brown structures pressed against the base of the plant. The pupae are fairly resilient and will remain at the base of the plant well into spring.
Damage from winter grain mites, brown wheat mites, aphids, and crown and root rot diseases can also weaken wheat plants and make them somewhat more susceptible to injury from cold weather stress or desiccation.
Fall armyworms and army cutworms may have fed on emerging wheat in the previous month (prior to the recent frigid weather, we even found live worms as recently as Dec. 3) leaving bare patches. If the worms were fall armyworms they have died by now. If the worms were army cutworms, they will overwinter right there in the soil and continue to feed on wheat plants anytime the temperature is 45 degrees or more from now through about April.
So if you have bare patches now, it is a good idea to keep an eye on them and if they slowly expand over the winter, get out and check in the soil around the base of the plants to see if there are small worms curled up about an inch or two below the surface, especially in loose soils. A spot application of a registered insecticide on a warm (above 55 degrees) winter afternoon will do a pretty good job of controlling the worms and allow the plants to come back in the spring as the worms only feed on the above ground leaf tissue, and not on the roots or crown.
Symptoms of winter survival problems
If plants are killed outright by cold temperatures, they won't green up next spring. But, if they are only damaged, it might take them a while to die. They will green up and then slowly go "backwards" and eventually die. There are enough nutrients in the crown to allow the plants to green up, but the winter injury causes vascular damage so that nutrients that are left cannot move, or root rot diseases move in and kill the plants. This slow death is probably the most common result of winter injury on wheat.
Direct cold injury is not the only source of winter injury. Under dry soil conditions, wheat plants may suffer from desiccation. This can kill or weaken plants, and is actually a more common problem than direct cold injury.