Some farmers who were feeling so smug a week ago about having planted so much corn so early are now feeling less certain of the wisdom of their actions in light of the recent cold temperatures and frost this week.
Should they be concerned about the health of their newly planted and, in a few cases, newly emerged crops? Well, we'll know for certain come harvest time. But in the mean time, we can talk about possibilities.
Newly Planted Corn
One of the risks that newly planted corn faces is that of imbibitional chilling injury due to cold soil temperatures during the initial 24 to 36 hours after seeding when the kernels imbibe water and begin the germination process. In response to the imbibition of water, kernels naturally swell or expand.
If the cell tissues of the kernel are too cold, they become less elastic and may rupture during the swelling process. Symptoms of imbibitional chilling injury include swollen kernels that fail to germinate or arrested growth of the radicle root and/or coleoptile following the start of germination.
Instances of chilling injury following germination during the emergence process can also occur, often causing stunting or death of the seminal root system, deformed elongation of the mesocotyl (the so-called "corkscrew" symptom) and either delayed emergence or complete failure of emergence (i.e., leafing out underground).
It is not clear how low soil temperatures need to be for imbibitional chilling or subsequent chilling injury to occur. Some sources simply implicate temperatures less than 50 F (10C).
Others suggest the threshold soil temperature is 41 F (5C). Daily minimum soil temperatures at the 4-inch depth (typical depth for National Weather Service measurements) have certainly dropped into the mid- to high-40's F in recent days, with some growers reporting temperatures as low as 40 F at seed depth.
Newly Emerged Corn
Damage from exposure of above-ground plant tissue to frost can range from minor leaf injury to complete death of all exposed leaf tissue. That's the bad news.
The good news is that the all-important growing point region of a young corn plant remains below the soil surface, safe from exposure to frost, until the V4 to V6 stages of development. That means that the above-ground plant tissue you see in fields younger than about V4 is composed primarily of leaves and rolled up leaf tissue in the whorl, but does not include stalk tissue or the growing point.
As long as temperatures are not lethally cold, "simple" frost injury usually does not literally kill such young corn plants. Damaged plants will begin to show recovery from the whorl within 5 to 7 days, depending on temperatures following the frost event.
Disclaimer: Repeated frost events that re-inflict damage to recovering corn plants can cause permanent stunting or death.
When folks worry about the effects of cold weather on corn, they often fail to distinguish between simple frost events and lethal cold temperatures. Frost can occur at temperatures easily up to the high 30's F, but lethal cold temperatures for corn are generally thought to be 28 F (-2C) or colder. Air temperatures in recent days have certainly dropped to 28 F or lower in areas of the state. Whether such cold temperatures "penetrated" the upper inch of soil near the growing point region of corn seedlings is not clear, but may be possible in fields where soils are excessively dry and free from surface residue.
Only time will tell whether this week's cold temperatures and frost will cause permanent damage or death of early-planted corn around the state. Recovery of damaged plants will usually be evident within 5 to 7 days following such events. Come October, we will know for certain whether this year's early planting risk takers will have "won the game" or not.
Nielsen, RL (Bob). 2001. Symptoms of Low Temperature Injury to Corn and Soybean. Corny News Network, Purdue Univ. online at http://www.kingcorn.org/news/articles.01/Frost_Corn_Soy-0418_Gallery.html [URL accessed Apr 2012].
Nielsen, RL (Bob). 2007. Growing Points of Interest. Corny News Network, Purdue Univ. online at http://www.kingcorn.org/news/timeless/GrowingPoints.html [URL accessed Apr 2012].
Nielsen, RL (Bob). 2009. Corkscrewed Corn Seedlings.Corny News Network, Purdue Univ. online at http://www.kingcorn.org/news/articles/timeless/Corkscrews.html [URL accessed Apr 2012].
Nielsen, RL (Bob). 2010. Determining Corn Leaf Stages. Corny News Network, Purdue Univ. online at http://www.kingcorn.org/news/timeless/VStageMethods.html [URL accessed Apr 2012].
Nielsen, RL (Bob). 2010. Germination Events in Corn. Corny News Network, Purdue Univ. online at http://www.kingcorn.org/news/timeless/GerminationEvents.html [URL accessed Apr 2012].
Nielsen, RL (Bob). 2010. Requirements for Uniform Germination & Emergence of Corn. Corny News Network, Purdue Univ. online at http://www.kingcorn.org/news/timeless/GermEmergReq.html [URL accessed Apr 2012].
Nielsen, RL (Bob). 2010. The Emergence Process in Corn. Corny News Network, Purdue Univ. online at http://www.kingcorn.org/news/timeless/Emergence.html [URL accessed Apr 2012].