2015 row-crop planting in South Dakota began relatively early this spring and has progressed at a rapid pace. As of May 17, 46 percent of corn had emerged.

"Which is well ahead of the five-year average of 21 percent," said Jonathan Kleinjan SDSU Extension Crop Production Associate.

Soybean emergence was recorded at 4 percent, only slightly higher than the five-year average of 3 percent. 

Kleinjan added that while early planting may increase yield potential, it also puts crops at risk for damage caused by early-season low temperatures. This was the case in the early morning hours of May 19, when temperatures of 32 degrees Fahrenheit and lower were observed in the central and northeast portions of the state. 

"Producers in these areas of the state may want to evaluate individual fields for crop damage, as some locations, especially low-lying areas, may have actually seen lower temperatures than those recorded by the weather stations," Kleinjan said.

What is the difference between a frost and a freeze? 

Kleinjan explained that a frost event occurs when the temperature of the soil surface is 32 degrees Fahrenheit or lower. While a freeze occurs when the air temperature is 32 degrees Fahrenheit or lower - at five feet above ground level. "It is important to remember that, if atmospheric conditions are right, it is possible for a frost to occur, even when air temperatures are slightly higher than freezing," Kleinjan said.

He added that while a mild frost should cause minimal damage to young crops, a hard freeze of 28 degrees Fahrenheit or colder for two hours, will most likely be lethal.

"The growing point for corn plants remain below the soil surface until the V6 growth stage, at which point the plant is approximately 1-foot in height, and is generally considered safe from freezing air temperatures," Kleinjan said.

However, he said if air temperatures remain below 28 degrees Fahrenheit for more than a few hours, the growing point may be injured or killed regardless.

Soybeans are at greater risk

Kleinjan added that soybeans are more likely to be damaged by low temperatures due to the fact that the growing point is above ground as soon as the crop emerges. "The soybean plant develops axillary buds at each leaf node, including the cotyledons," he said. "If any of these buds survive a low temperature event, recovery is possible. However, if the plant is damaged to a point below the cotyledons, seedling death will follow."

Wait three to five days

No matter what your fields look like the day after a frost or freeze, Kleinjan said this is not the time to make decisions about replanting. "Immediately following a frost or freeze, leaves on young corn and soybean plants may initially blacken and then become necrotic (wither and turn brown)," he said. "The actual extent of damage should be assessed three to five days after the event or when a few days of favorable growing conditions have returned."

When evaluating the field, Kleinjan said corn plants that will survive should either have new leaf tissue emerging from the whorl or a growing point that is white/light yellow and firm. 

The growing point can be observed by splitting the seedling lengthwise and examining the area one half to three-fourths-inch below the soil's surface. "A decayed and discolored growing point indicates a dead plant," he said.

Soybean plants should have new leaf tissue emerging from one or more of the undamaged nodes. "Plants that still look dead at this point will probably not recover," Kleinjan said.

A careful assessment of plant stands should be made prior to making replanting decisions. 

"Ninety percent of maximum yield potential is often still possible when stands are reduced by up to about 20 percent for both corn and soybeans," Kleinjan said. "However, stands should be somewhat uniform."

He encouraged growers to remember that when they are scouting for crop damage in fields with varying topography, they should keep in mind that frost damage is typically more severe in low-lying areas, as cold air tends to flow into depressions. "In other words, some areas of the field may warrant replanting while others do not," Kleinjan said.