Fall Frost and Corn Maturity

Physiological Maturity or Black Layer

Corn typically reaches physiological maturity when the plant has obtained enough growing degree units (GDUs) required for full maturity and the kernel becomes sealed off from the flow of nutrients and water. The sealing is a result of the formation of a black layer or a thin film of tissue on the tip of the kernel (Figure 1). Black layer formation can be influenced by environmental conditions. Drought can cause the black layer to form prematurely and cool conditions can delay its formation. The kernel begins its drying process after the black layer forms.

Light Versus Killing Frost

A light (or “simple”) frost can develop on leaves with temperatures above 32 °F and very clear and calm conditions.¹ Light frosts do not kill the entire corn plant but can damage or kill some leaf tissue. In the days following a light frost, undamaged remaining leaves can continue to photosynthesize and carbohydrates from stalk tissue are remobilized to immature grain. The stalk itself is a temporary storage organ and if not killed by frost, can contribute a 7 to 20 percent grain yield increase after light frosts even if leaves are killed.² Research from Minnesota suggests that yield losses due to the entire loss of functioning leaf area at growth stages R4 (soft dough), R5 (full dent) and 1/2-milkline were 35, 27 and 6%, respectively.¹ Similar results can also be found in the National Crop Insurance Services hail loss chart where yield losses at the same growth stages is considered to be 41, 23, and 8%, respectively.³

Killing frosts occur when temperatures remain at or below 32 °F for several hours. Plant metabolism is stopped, and no further dry matter accumulates in the grain. Kernel black layer develops after a killing frost and is triggered by the reduction or lack of photosynthates. Drydown of grain following a killing frost may be temporarily slowed or delayed immediately after a freeze event occurs in corn that had not reached physiological maturity.¹ The premature formation of the black layer due to frost may result in lower test weight and kernels becoming more susceptible to breakage.⁴

Table 1. Potential percent loss of grain yield after frost at different growth stages.

Late Planting and Timing of a Killing Frost

A late-planted corn crop could be at risk of being killed by frost. However, research indicated that corn products require about 6.8 fewer GDUs per day for delayed corn planting as compared to typical planting dates (after May 1 and through mid-June) to reach black layer.⁵ Below-normal temperatures during the growing season, followed by warm, sunny fall conditions keeps green plants photosynthesizing. These conditions may result in delayed black layer formation based on calendar date and expose the plant to frost damage. However, this situation may also provide an opportunity for late season grain fill and potentially be beneficial.

Black Layer Development

The calendar time from silking (R1 growth stage) to black layer formation, which occurs at physiological maturity, or the R6 growth stage, is similar across a wide geographical area in the Midwest under normal planting dates and growing conditions. Black layer formation (R6) for adapted corn products typically occurs about 65 days after silking in the central Corn Belt and about 55 to 60 days after silking in the northern corn growing region.⁶ Black layer may not be achieved before a late-September or early-October killing frost if silking does not occur until early August or later. This is especially true in the northern corn-growing area where black layer can be prematurely induced by frost.

Estimating Corn Plant Maturation Before Frost

There are two ways to estimate the potential for a corn plant maturing before a killing frost: adding calendar days or calculating GDUs:

• Calendar Day Method. Depending on geography, add the 55 to 65 calendar days required for black layer development after silking and check historical information for the average first killing frost date in the area. This rule of thumb can be influenced by temperature during grain fill period; warmer than normal temperatures can shorten the grain fill period and hasten maturity, while cooler than normal temperatures can lengthen grain fill and delay maturity.
• Growing Degree Unit. Estimate maturity date based on the anticipated GDU requirement (which could be in a range around 1500 GDUs) from silking to black layer. It typically takes early-maturing corn products fewer GDUs to reach black layer after silking. There is also potential for early season soil temperatures and growth-limiting stresses to influence timing of maturation. Models have been developed by several Land Grant Universities to help predict physiological black layer and may be helpful in forecasting the date that it is reached. Please see https://hprcc.unl.edu/agroclimate/cligrow/ for more information.

Summary

Yield potential and grain quality can be affected if a killing frost occurs before physiological maturity (black layer) occurs. The effect on corn yield depends on the stage of corn development, the low temperatures reached, the duration of the low temperature, and other factors. The closer the plant is to physiological black layer, the less effect there is on grain yield (Table 1). Even if a frost damages most of the leaf tissue on the plant, the translocation of sugars from stalks to ears can still increase kernel dry weight unless the freeze is severe enough to kill the husks, stalks, and kernels.

Sources:

¹ Nielsen, R.L. 2019. Frost or freeze damage to immature corn. Corny News Network. Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/FrostFreezeImmatureCorn.html.

² Lauer, J. 1997. Killing frost in corn. Wisconsin Crop Manager. Corn Agronomy. University of Wisconsin. http://corn.agronomy.wisc.edu/WCM/W048.aspx.

³ Klein, R. and Shapiro, C. 2011. Evaluating hail damage to corn. University of Nebraska-Lincoln Extension. Publication EC126. http://extensionpublications.unl.edu/assets/pdf/ec126.pdf.

⁴ Osler, O., Wilson, A. and Lindsey, A. 2022. Watch for potential frost damage to non-mature corn. Agronomic Crops Network. Ohio State University Extension. https://agcrops.osu.edu/newsletter/corn-newsletter/2022-36/watch-potential-frost-damage-non-mature-corn.

⁵ Nielsen, R.L. 2019. Hybrid maturity decisions for delayed planting. Corny News Network. Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/hybridmaturitydelayedplant.html.

⁶ Nielsen, R.L. 2019. Predicting corn grain maturity dates for delayed plantings. Corny News Network Articles. Purdue University. https://www.agry.purdue.edu/ext/corn/news/timeless/RStagePrediction.html.

Web sources verified 4/25/2025. 1222_137013