Wallaces Farmer

Temperatures of 28 degrees F or lower for a couple hours can be lethal to emerged corn and soybean tissues.

Rod Swoboda

May 11, 2020

6 Min Read
Corn plant 4 days after frost
FROSTED CORN: If temperatures drop to 28 degrees F or lower for a few hours, it could be lethal for corn or soybean plant tissue aboveground. This photo was taken four days after a frost. Photos courtesy of ISU

Temperatures fell to below freezing or near that mark overnight on May 9-11 in parts of Iowa. How does the cold weather affect newly planted corn and soybeans? Will the young seedlings survive? What about alfalfa fields?

“Temperature is critical for both corn and soybean seedlings,” says Mark Licht, Iowa State University Extension cropping systems agronomist. “Temperatures between 28 degrees F and 32 degrees F will result in minimal frost injury to aboveground tissue. Temperatures between 32 and 50 degrees could result in poor vigor and growth.”

How long the corn and bean seedlings experience the cold temperatures makes a difference. “Also, keep in mind that dew point temperatures are several degrees lower than the temperature,” Licht says. “In low-lying areas or areas with no wind, the air temperature could be slightly lower than the reported air temperature for your general area.

“Another consideration is warmer soils help buffer emerged seedlings by radiant warming of air just above the soil surface. The soil is warmer and gives off radiant warmth. The point is, if you really want to know what temperature the emerged seedlings are exposed to, you should place a thermometer on the soil surface and check it just before sunrise.”

Automated weather stations usually take the air temperature 4 feet above the ground. Air temperatures taken by those stations are what you normally hear on news and weather reports.

Wait to assess stands

Regardless of the exact low temperatures, the next couple of days after cold temperatures occur will put planted corn and soybeans at greater risk for disease infection, Licht says. He offers the following guidelines for checking the health and viability of corn and soybean stands after a frost.

Be sure to get out into your fields toward the middle to end of this week to assess stands. For checking frost-injured crops, wait at least three days (maybe even five to 10 days if temperature is slow to rebound) following the last frost occurrence to fully assess living versus damaged or injured plant tissue.

Yellow corn will quickly respond to the sunlight and increased temperatures. Frost-injured, water-soaked corn likely won't grow, but if the growing point is still viable (cold temperature didn’t penetrate to the growing point), the plants will have to slough off the dead tissue. A dead growing point is unlikely unless temperatures get down to 28 degrees or lower for several hours.

Check growing point

Soybean cotyledons are resilient to cold temperatures. You can expect to see some frost damage to cotyledons and the hypocotyl around the margins, but survival is likely unless there were several hours of temperatures below 30 degrees. The apical meristem or auxiliary buds are not expected to be killed at temperatures down to 28 degrees.

Soybeans 4 days after frost

FROSTED BEANS: Emerged soybean plants are more easily impacted than corn, since growing points of beans are aboveground. This photo was taken four days after a frost.

Licht says regardless of emergence or not, patience will be the key. Remember, assess corn and soybean stands after three to five days to give them a chance to show some recovery from the frost. If aboveground vegetation is dead, look belowground to check for viable growing tissue.

“When corn’s growing point is below the soil surface, it takes a hard freeze to cause the plant to die,” says Brian Lang, ISU Extension field agronomist in northeast Iowa. Frost damage to exposed leaves has little impact on potential corn yields, especially when the frost occurs at very early dates of the plant’s growth. “Lethally cold” temperatures for emerged young corn plants are those that dip to 28 degrees or lower for one to two hours.

Bean seedlings more easily hurt

Emerged young soybeans are more easily impacted since their growing points are aboveground, although their thick high-moisture cotyledons are highly resistant to frost damage. “However, as development continues and the cotyledons open, they expose the developing unifoliate leaves,” Lang says. “This tissue is more fragile and exposes the main growing point. Fortunately, it is common for soybeans to produce new growing points from the axils of the cotyledons if the unifoliate is damaged.” Damage would first look “water soaked” and dark greenish in color, changing to a tan color as tissue dries out.

With corn, even though we assume corn’s growing point is safe belowground, when assessing stands, it would still be good for you to dig some plants and check for a viable growing point, he says.

“Healthy growing points will be firm and white to yellow in color. If the growing point or plant tissue within a half-inch above the growing point is damaged, it will be watery and a brownish color, and the corn plant will not likely recover,” he says. “For 29 degrees to 32 degrees , frost injury should be minimal. However, the continued cool weather is still contributing to reduced vigor and growth, and a greater risk for infection from pathogens.”

Evaluating alfalfa for frost injury

How are alfalfa stands affected by frost? “It depends on whether you have a new alfalfa stand that was seeded this spring, or an older stand, one that’s at least one year old or older,” Lang says. He provides the following tips for evaluating alfalfa fields.

Light frost injury to alfalfa  turns some of the leaves a tan color

ALFALFA: Light frost injury to alfalfa eventually turns some of the leaves a tan color; new shoots closer to the crown are quite cold-tolerant.

New alfalfa stands. New seedings this spring that have emerged are quite tolerant of frost up to about the second trifoliate stage. At that point, a few hours of around 26 degrees could kill these young plants. If a young seeding is frost-damaged, it will first appear to wilt, and then die over the next three to five days. If at least one set of leaves escapes damage, the plant should recover. Unless mortality is absolutely obvious, wait a week after the frost and count the number of living plants per square foot. If more than 20 plants per square foot remain, the stand will survive in good shape. If there are less than 15 plants per square foot, consider interseeding more alfalfa into the stand.

Taller alfalfa shoots hit by frost may wilt if tops of shoots are killed

INJURY: Taller alfalfa shoots hit by frost may wilt if tops of shoots are killed. New growth will start below the killed portion.

Older alfalfa stands. Generally, if stand height in the field is less than 10 inches, no matter how hard it is hit by frost, just leave it alone. Regrowth will come from below the frost-killed part of the shoots. For light frost damage, expect to see white leaf edges, and then leaves will later turn somewhat tan in color. New shoots near the crown are quite cold-tolerant. As the shoots lengthen, they are more susceptible to cold injury. Taller frosted shoots may wilt or gooseneck.

Alfalfa plants damaged and killed by frost with new growth emerging.

DAMAGE: Major frost injury killed much of the aboveground shoots of these alfalfa plants, but new growth is coming from the base of the plant.

You may see some significant frost damage, but with recovery of new shoot growth 10 days later. Again, the stands will recover, but those with more significant frost damage will set back the first crop alfalfa harvest date by a week or more. There will also be some variability in damage across fields due to slope position and valleys.




About the Author(s)

Rod Swoboda

Rod Swoboda is a former editor of Wallaces Farmer and is now retired.

Subscribe to receive top agriculture news
Be informed daily with these free e-newsletters

You May Also Like