Frost and highly variable soil temperatures are two factors that can place corn under significant stress after planting. The amount of stress experienced will vary based on whether the corn seed has imbibed (absorbed) water, started growth but not yet emerged, or if the seedling has emerged above the soil surface.
Variable soil temperatures have little effect on the first phase of corn germination, water imbibition. Seed will absorb about 30 percent of its weight in water. The time required for radical emergence is directly related to temperature; it increases linearly if soil temperatures are between 46 and 90° F. Little, if any, mesocotyl or coleoptile growth occurs in soils cooler than 60° F. See Photo 1. A constant soil temperature of 86° F optimizes seed germination and seedling emergence.
Photo 1. Corn seedling
Researchers determined that a swing of soil temperatures of 27° F (soil high temperature-soil low temperature = 27 °F) will particularly affect mesocotyl growth (Buckle and Grant). It is possible that corn planted in early April will experience this range of temperatures. Large temperature variation occurred in 2006 across the state. See Figure 1. Seed placed into the ground, yet not emerged, can be injured from a cold period. The drop in soil temperature in April 2006 caused erratic and uneven stands across the state, with crop development varying up to 2 growth stages often. (See the Image Gallery for an example of uneven stands.) Seedlings adversely affected by wide swings in soil temperatures will have stunted and distorted leaves and may or may not emerge from the soil (Photo 2).
Figure 1: Statewide soil temperature average in April 2006 versus the 9-year average for Ames, IA. Data is from Iowa Mesonet, Iowa State University.
Photo 2. Seedlings affected by fluctuating soil temperatures.
Recovery from a frost event
On the other hand, emerged corn seedlings are relatively resistant to cold weather. Air temperatures near 30° F may kill or damage exposed above-ground tissue, but the growing point of the seedlings remains below the soil surface until approximately the V6 stage of development (six collared leaves visible). Recovery from a moderate freeze is usually rapid and nearly complete when the growing point is below ground. Frost injury on very young corn plants surprisingly has very little effect on yield if the plants survive the frost (Photo 3).
Photo 3: Seedlings affected by frost showing tied-up and off-color leaves.
When poor growing weather follows an early season frost, corn seedlings may occasionally die. Seedlings that have been killed will begin to rot. Check for rot by splitting the seedling and looking for dark, water-soaked tissue. Yet, young corn plants that only had above-ground tissue damaged will resume growth, if the growing point was not killed. A hasty decision to replant would be ill advised. Wait a few days to see if growth resumes. If the growing point is not damaged, a new leaf should emerge in 3 to 4 days. If this does not occur, refer to the Iowa State University Extension publication, Corn Planting Guide, for replant decisions.
Appearance of reddish-purple leaves
Another symptom of cool air and soil temperatures, combined with wet conditions, is a purplish tint to corn seedlings. Purple leaf coloring is more pronounced in some hybrids' genetics than others. Most often though, the leaf purpling is related to stress experienced by the young seedling and/or restricted root development. Phosphorus unavailability is often mentioned as a culprit for the purple leaves. Phosphorus deficiency will result in reddish-purple leaves, yet it is not likely the primary cause. A reddish-purple tint on leaves can be due to anything that disrupts sugars within the plant. Cool and/or compacted soils, as well as shallow planting, can each create the opportunity for purpling to be expressed in corn leaves. If root development is restricted (due to temperature or seedbed problems), then the observed symptoms are simply an expression of this since the plant is not developing normally.
Any root restriction that may be causing this purpling is most likely temporary and seedlings should regain a healthy green color as weather conditions improve. As such, yield should not be affected. It is good practice to check a few plants after they 'green up' to ensure that root development resumes and is not hindered in any way.
Herbicide influence on development
Seedling stress caused by environmental factors should be identifiable from herbicide-induced influences. There are instances, though, when the two are interrelated. We know that corn seedlings weakened from cold weather will have a heightened sensitivity to herbicide injury. Postemergence herbicide applications should be evaluated carefully until corn begins to demonstrate "normal" growth and development. If weeds are small and growing slowly, there is likely sufficient time to allow delayed application timing. However, if weed pressure is high, and if weeds are large relative to the optimum size for control, growers should consider the risks of crop injury and the benefits of weed control before making a herbicide application. Often, the benefits of early weed control, thus protecting potential corn yields, outweigh the risks of herbicide injury to a previously stressed crop.
Growth regulator herbicides such as 2,4-D and dicamba are of particular concern. If these herbicides are applied in combination with other translocated herbicides, such as sulfonylureas that require surfactants or crop oils, the potential for injury increases significantly. Contact herbicides such as bromoxynil also may have a higher risk of significant corn injury if applied to stressed seedling plants.
If weeds are small, rotary hoeing or careful cultivation may be a better management option than a postemergence herbicide application. Each field must be evaluated to determine the best weed control strategy.
Buckle and Grant. 1974. Rhodesian Journal of Agric. Research. 12:149-161.
Portions of this text, written by Roger Elmore, Mike Owen, and Lori Abendroth, originally appeared in the Integrated Crop Management extension newsletter on pages 103-104 of the IC-496 (9) - May 1, 2006 issue, on page 115 of the IC-496(10) - May 8, 2006 issue, and on pages 146-147 of the IC-496(13) - May 30, 2006 issue.