The first half of April was dry and warm in Illinois, and 159 growing degree days accumulated, compared to the average of 99. The second half of April was also dry in most areas, but only 92 GDD accumulated, compared to the average of 132. Some producers who would normally plant as fast as possible during the second half of April decided to delay planting due to low temperatures. That’s very unusual when soils are in good working condition, but with little rain in the forecast and temperatures low enough to delay germination and emergence, there seemed to be (and was in fact) little penalty for waiting.
By April 16, 10 percent of the corn and 4 percent of the soybean crops had been planted, compared to the 5-year averages of 4 percent for corn and 1 percent for soybean. By April 30, 40 percent of corn and 39 percent of soybean had been planted, compared to 5-year averages of 29 percent for corn and 15 percent for soybean. The 5-year averages were lowered a lot for both crops by the very late planting in 2019. Over the last 33 years, the average dates to have 50% of corn and soybean crops planted in Illinois are May 4 and May 22, respectively.
With the upside-down temperatures in April, crops planted during the first half of April emerged faster than those planted the last half of the month. Most corn and soybean planted by the middle of April, except in the area north of I-80, should have had enough GDD to emerge by the end of April. The numbers for soybean back this up: 4 percent were planted by April 16 and 3 percent emerged by April 30. But 10 percent of the corn crop was planted by April 16, and only 4 percent emerged by April 30. It is not clear why corn required more GDD to emerge than soybean, but it may have been related to the unusual pattern of warm followed by cool soils.
As it turned out, early emergence ended up as more of a bust than a boon, especially for soybean. While we normally consider a soybean plant relatively safe from death by freezing once its hypocotyl hook straightens and the cotyledons unfold, low temperatures on the morning of April 24 were in the upper 20s throughout most of Illinois: the highest minimum temperature reported in the WARM network of the Illinois State Water Survey that morning was 32.5 degrees at Springfield, and the lowest was 25.7 at Bondville, just west of Champaign. With cold air sinking into low-lying areas, temperatures in parts of some fields probably dropped to less than 25 degrees, cold enough to freeze the plant.
Soils that are dark-colored on the surface and warmer than the air provide protection against freezing to small plants, although this may not be enough when air temperature drops to 25 degrees. This protection comes from the soil’s radiating to the plant tissue, The physics principle is that for any two bodies within “sight” (radiated heat travels as waves, like light) of one another, the warmer one radiates to the colder one. On a clear, cold night, small soybean plants—the leaves and cotyledons (horizontal parts)—“see” the cold sky, and radiate to it, lowering their temperature. Soil that’s warmer than the soybean plant parts radiates to the plant, warming it. The darker and warmer the soil, the more it radiates. Dry soils do not store heat as well as wet ones, and they’re also not as dark as wet soil, so in this case they weren’t nearly as good a radiator as they would normally be after crop emergence. Residue from the previous corn crop also radiates poorly, so offers little protection.
Figure 1 is a photo taken on April 27 of soybean plants that were planted near Urbana at the end of March, and that emerged several days before the April 24 freeze. More than 95 percent of the plants in this planting were dead with no green tissue, and many with green cotyledons had no living buds from which to regrow their stem. With no surviving plants to protect during replanting, fields like this can be replanted back in the same row or next to the same row.
There is a chance that the low temperature might also have killed emerged corn plants in low-lying areas of fields, although we have heard no reports of that. The meristematic tissue from which leaves develop (the “growing point”) is at the tip of the stem, an inch or so beneath the soil surface, for a few weeks after emergence. This provide protection, so that even when the aboveground leaf area is killed, new leaves can begin to appear as soon as warmer soils produce regrowth, and there may be no loss in yield potential. Cold air can infiltrate into dry soil better than into wet soil, though, so be sure to check to see if plants have begun to regrow. Having all of the above-ground leaf tissue killed, as it may have been in some emerged corn fields, may allow the new leaf tissue to push past the old with less interference than when the emerged leaves are damaged but not killed.
Although freeze damage to soybean was greater than to corn this time, this was due to the unusually low air temperatures more than to the decision to plant soybeans before corn. The forecast in early April did not indicate the return to below-normal temperatures by later in April, so there was no help there in making the decision of which crop to plant first. Nor did we have indications that April would turn out to be so dry. This reminder that small plants suffer in cold and thrive in warmth can help us make planting decisions in the future, although it won’t eliminate uncertainty. For this year, we can take comfort in the fact that crops planted, or replanted, the first or second weeks of May will have warmer conditions for early growth, and this should set them up to thrive.