No. 2 Article 11/April 6, 2007

Cold Temperatures and Wheat

The warm temperatures of recent weeks have helped the wheat crop to green up and grow nicely in many areas, especially southern Illinois. The crop continues to lag in parts of northern Illinois due to late planting last fall and wet soils in recent weeks. Overall, the state's wheat crop is rated as rather average at this time, but its condition ranges widely across Illinois.

There is a lot of concern about how the wheat crop will be affected by the cold temperatures that moved into Illinois on April 3 and 4. Nighttime lows are expected to be in the mid-20s in the central part of southern Illinois (Mt. Vernon) over the next few days and in the upper teens in the northwestern part of the state.

While it's optimistic to expect there to be no damage from low temperatures when the crop is in its present stage, history provides some reason to hope. Nighttime temperatures in the 20s have occurred at least once during April at Mt. Vernon in five of the last 10 years, including lows of 20 and 23 degrees on April 9 and 10, 1997. Low temps were 24 and 26 on April 4 and 5, respectively, in 2002. While it is possible that some badly damaged wheat might have been planted to a different crop in such cases, statewide yields did not suffer, and the damage was not reported to be widespread.

How well the wheat crop can tolerate temperatures dropping from highs in the 70s to lows in the 20s depends on crop stage and on how fast the temperature drops. Young, growing leaf tissue is vulnerable to freeze damage or death, but if the temperatures drop over two days or so, the leaf tissue can physiologically adjust to some extent, perhaps enough to escape serious injury. Wheat does not do this as well as bluegrass in a lawn, but it has much more ability to tolerate low temperatures than does a crop like corn.

One factor in the crop's favor is that the "growing point"--the small, developing head--is still down in the canopy, surrounded by a lot of leaf tissue. Leaf orientation also tends to be vertical as the stem grows upright, and this helps protect it from heat loss by radiation to the sky at night. Temperatures down in the canopy on a cold, still night, aided by heat radiating from the warmer soil, will be several degrees higher than the air temperature above the canopy. This is one reason that freeze injury to the head normally happens only after the head is "in boot," a week or so before head emergence. Exposed near the top of the plant, heads don't benefit much from warm soil or from surrounding plant tissue.

So what can we expect the wheat crop to look like after this cold period? The crop in southern Illinois has "jointed" by now, and most is about 12 to 18 inches tall, with the head located roughly a third of the way up the plant. I would expect temperatures in the mid-20s to cause some loss of leaf area, with damage increasing as plant height increases. Wheat planted early last fall and that with "extra" N applied early this spring thus will be more prone to injury. Such plants may not die, but they might lose so much leaf area that they can no longer produce full yields. Wet soils might provide a little more temperature protection to plants, but if wet soils cause stress, leaves may not be able to protect themselves as well physiologically.

In fields planted at the normal time or a little late, with leaves still upright and plants about a foot tall, there should be less loss of leaf area and yield potential. Leaf tips and margins might freeze, after which they will dry out and give the crop a "frosted" appearance. Once temperatures warm and new leaf growth emerges, the appearance will improve quickly. More winter-hardy varieties, which usually green up and start growing later in the spring, might show less freeze damage.

While temperatures in northern Illinois are predicted to be even lower than those in the south, the crop is considerably behind in its development there, and so should generally be safer from injury. Exceptions might be fields that were planted well before the optimal time, where plants are already growing upright. But most of the wheat there is not yet jointed and should not suffer much damage. Of course, highs in the 30s don't do much to encourage growth, so there will be some "lost" days there until temperatures return to normal.

Predicting damage from an upcoming weather event is often not very accurate. In the case of freeze injury in wheat, there can be large differences in damage based on things like cloud cover, how fast temperatures drop, how long they stay low, and even how much the sun shines on the days following cold nights. Coupled with differences in plant stage and activity, this makes such predictions more guesswork than we'd like.

If the temperatures get lower than predicted or if previous experience proves not to be a very good guide in this case, damage to larger plants or to plants in lower-lying parts of fields could be much more severe than I have outlined. Such damage will start to show up within a day of the low temperature; it will include limp plants with discolored leaves and death of the upper part of the stem, including the head. There is no chance that such wheat will revive. It might be possible to salvage such a crop (after it dries some) as forage. If not removed, heavy residue might make planting difficult, and such fields will dry slowly.--Emerson Nafziger

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