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Spring frost

Early corn emergence dates increase the risk of spring frost damage, especially in low-lying areas in Minnesota.

View dates of the last spring frost for numerous Minnesota locations (compiled by the Minnesota Climatology Working Group).

Areas susceptible to frost damage

Frost damage can occur when air temperatures are in the mid-30s on calm nights, as the lack of wind allows heat to transfer from air near the ground to the air above. This results in colder temperatures near the soil surface.

Settings where frost damage is generally worse include:

  • Low areas where cold, dense air settles.

  • Near field edges, as vegetation makes it harder for heat to transfer from the soil to the air above.

  • Fields with high levels of surface residue coverage, as this limits heat transfer from the soil.

  • Recently row-cultivated fields. These fields are more susceptible to frost injury because tillage dries the surface soil, reducing the amount of heat and moisture that can be transferred from the soil to the air.

  • Peat or sandy soils, as these lose heat more quickly.

Assessing frost injury

Carefully evaluate fields to determine if the frost-damaged corn will survive.

What to look for

Symptoms of frost-damage to corn are initially discolored and water-soaked leaves, which later dry and turn brown. Because the corn plant’s growing point remains below the soil until the fifth to sixth leaf-collar stage, frost before this stage does not typically kill the plant unless the upper part of the soil – where the growing point is located – freezes.  

Corn plants with growing points not damaged by the frost generally show new leaf growth after a few days. So, delay assessing frost-damaged fields until three to five days after the frost.

Larger frost-damaged corn plants can resemble buggy whips, as new vegetative growth tries to break free of dead tissue. Buggy-whipped plants generally recover. This happens at a faster recovery rate for smaller plants and when warm and windy conditions follow the frost.

Whether plants will survive

To determine whether frost-damaged corn will survive, dig up plants and split stems to examine the growing point and the tissue directly above the growing point. Healthy growing points will be firm and white to yellow.

If the growing point or tissue within 0.5 inches above the growing point is damaged, it will be water-soaked and orange to brown. In this case, recovery is unlikely.

Corn recovery is greatest when frost occurs before the three leaf-collar stage (V3) or, if after the three leaf-collar stage, when it damages only a limited amount of leaf area. This is because the amount of energy reserves in the seed and leaf area for growth partially influences recovery.

It’s best to let frost-damaged plants recover on their own. Removing the dead tissue by mechanical means or spraying health-inducing compounds will not improve the crop’s survivability and could make things worse.

Injured corn tissue, which will fall off.
Seedling showing frost damage on leaves, but healthy internal tissue.

Yield reductions due to frost

Whether or not frost will impact yield depends on stand loss, the severity of plant damage and recovery-slowing issues such as pathogens.

Minnesota growers can expect yield losses of 5, 12 and 24 percent when the final plant population reduces to 28,000, 22,000 and 16,000 plants per acre, respectively.

Research from the University of Wisconsin found that cutting off all corn plants at the soil surface during the second leaf-collar stage reduced yield by 8 percent, but that cutting off only half the plants resulted in minimal yield reduction.

In addition to yield losses, frost-damaged plants may reach maturity a few days later than normal.

Replanting considerations

Before replanting, consider the existing crop’s yield potential, replanting costs and the replanted crop’s yield potential. Replant costs including time, fuel, seed costs and penalties related to hybrid selection if the best genetics are no longer available.

In addition, consider the length of the remaining growing season and select hybrids of appropriate maturity.

Growing degree days available for various Minnesota planting dates

Weigh yield potential from reduced stands against potential yield reductions from later planting dates. For example, if the date is May 20, it would pay off to replant only if the populations were less than 26,000 - 28,000 plants per acre (Tables 1 and 2).

As shown in Table 1, planting date influences the yield potential of a replanted crop. In Minnesota, corn planted from May 20 to 25 yields about 87 to 95 percent of the corn planted in late April to early May.

Table 1: How planting date affects corn yield, as a percent of maximum

Planting date Percent of maximum grain yield Percent of maximum grain yield
Study 1: Lamberton (1988-2003)* Study 2: Lamberton, Morris and Waseca (2009-2011)
20-Apr 99% 98%
25-Apr 100% 99%
30-Apr 100% 100%
5-May 99% 100%
10-May 98% 99%
15-May 95% 98%
20-May 92% 95%
25-May 87% 92%
30-May 82% 89%
4-Jun 76% 84%
9-Jun 69% 79%

Table 2: How corn plant population affects yield potential

Population (plants per acre) Grain yield potential
36,000 100%
34,000 99%
32,000 99%
30,000 97%
28,000 95%
26,000 93%
24,000 91%
22,000 88%
20,000 84%
18,000 80%
16,000 76%

If you replant

When replanting very thin to non-existent stands, seeding directly into the existing seedbed is a better option than working up mud.

Remove existing plants by the appropriate herbicide or tillage when replanting low, but variable, stands. You may also need tillage for fields hardened by heavy rain or with previous tillage problems.

Finally, remember to adjust the relative maturity (RM) of your hybrid (Table 3).

Table 3: Corn maturity guidelines for late planting in Minnesota

Planting date Relative maturity (RM) units earlier than full-season for the region*
Prior to May 25 Plant normal seed choices.
May 25 to 31 Plant hybrids that are 5 to 7 relative maturity units earlier than full season.
June 1 to 10 Plant hybrids that are 8 to 15 relative maturity units earlier than full season.
June 11 to 15 Plant hybrids that are 15 or more relative maturity unites earlier than full season.

*For areas where 105 RM hybrids are full-season, plant hybrids with maturity ratings of 100 or less for grain production. Source: University of Minnesota research on plant populations.

Jeff Coulter, Extension agronomist

Reviewed in 2018

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