A historical look at lethal soil temperatures

Each year as the first few warm spring days arrive, farmers begin gearing up to plant: checking over tractors and planters, arranging last-minute seed or input purchases, checking field conditions and checking the daily, short- and long-term weather forecasts for the region.

According to University of Illinois Agricultural Economist Dr. Gary Schnitkey, on average seed is the second most-expensive non-land cost of corn production. In checking field conditions and weather forecasts, farmers are looking for the earliest window in which they can safely sow this precious seed. One of the dangers that they are trying to avoid is exposing seeds and seedlings to low soil and air temperatures.

Lethal temperatures. Plant cells contain dissolved sugars and salts. These sugars and salts are the reason why plants can often experience short-periods of below freezing temperatures and still survive. The lethal air temperature for corn is typically considered to be 28 °F.

As a general rule, a hard frost doesn't kill a plant unless it kills the growing points. In corn plants that have fewer than five leaves, the growing point is protected under the ground. However, when air temperatures dip to 28 degrees or lower for more than a few hours, even below-ground growing points run the risk of severe injury or death.

Average date of last lethal frost: 1893 through 2013. In Monmouth, over the past 121 years, the average date that a lethal frost occurred was April 10. However, individual years varied widely (Figure). Historically, the last lethal frost has occurred during the month of March in only 21 of the past 121 years, approximately 17 percent of the time. The last lethal frost occurred in May even less frequently, in only 4 of the last 121 years: on May 1, May 3, May 10 and May 11 in 1903, 2005, 1966, and 1907, respectively. The last lethal frost occurred most frequently during the second week of April (Figure).

Average date of last lethal frost: 30-year average. According to Illinois State Climatologist Dr. Jim Angel, the 30-year average was established many decades ago as the international standard among climatologists and meteorologists for comparing weather measurements among locations. Looking at a 30-year period, gives these scientists a stable estimate of the recently occurring average. The most recent 30-year average is calculated based on data collected from 1981 through 2010.

According to the 30-year average, in Monmouth the average date of the last lethal frost is April 6, 4 days earlier than the entire 121-year data set. However, as with any average, there are many more final lethal frosts that occurred both prior to and after April 6 than on April 6.

Earliest planting date for crop insurance. The USDA – Risk Management Agency (RMA) operates and manages the federal crop insurance program. On a county-specific basis, the RMA sets the earliest planting date. If some acres need to be replanted, any acres planted before the earliest planting date are ineligible for replant payments through crop insurance. Earliest planting dates are determined based on several criteria: previous planting dates, the earliest and latest lethal frosts and input from commodity groups. After record-early corn planting in Illinois during the 2012 growing season, RMA revised its early planting dates for most counties for the 2013 growing season. In Warren County and other central Illinois Counties the earliest planting date for corn is now April 5.

April temperature and precipitation outlook. Illinois State Climatologist Dr. Jim Angel recently summarized the National Climate Center's temperature and precipitation outlooks for April. It appears that the below average temperatures are predicted to continue throughout our region at least through April.