Justin Glisan, the Iowa state climatologist, says the trend is showing more time between the last freeze in the spring and the first freeze in the fall.

“What we’ve seen is an expansion of the growing season across the Midwest and Iowa,” he says.

States have weather data going back to the 1800s, but Glisan says looking at the last 30 years can help illustrate the recent trends. He says Iowa is experiencing 1 to 3 days more per decade during its growing season. Overall, Glisan says Iowa is seeing about 8 to 12 days more in its growing season than the historical number. He says the average growing season ranges from 170 to 210 days in Iowa.

Pat Guinan, the Missouri state climatologist, says his state’s data shows a similar trend.

“Missouri air temperatures are trending warmer over the last couple of decades, which is extending the growing season,” he says.

Guinan says statewide averages for spring minimum air temperatures over the last 127 years show a warming of 1.4 degrees per century. Missouri’s autumn air temperatures have trended 0.8 degrees warmer.

When comparing the last 20 years to the 127 years of weather data, Guinan says the most recent decades show the last spring freeze occurring 3 to 6 days earlier than the historical average, and the first autumn freeze is occurring almost one week later than the long-term average.

Illinois state climatologist Trent Ford sees the trend in his state as well.

“In Illinois we’ve seen an increase of the growing season of 10 to 15 days over the last 50 to 60 years,” he says.

Ford says the climate projections indicate this trend will continue.

Despite the trends, growing seasons can see a lot of variation from year to year in the Midwest, so it is difficult for growers to simply plant earlier due to longer growing seasons.

“Missouri weather is still variable, and seemingly random freeze events can and will occur late in spring or earlier in the fall,” Guinan says.

As an example, Ford says central Illinois saw a freeze as late as May 7 in 2020, bucking the recent trend.

“If we go year to year, the variability is really high,” he says.

In general, Ford says very late spring freezes will become more rare.

Warmer winters

Glisan says there are other factors beyond air temperature at play.

“What we also have to think about is the soil temperature,” he says.

He says farmers do think about changes in typical frost and freeze dates, but more so on the end of the growing season.

“Farmers are more keen to look at those during the fall,” Glisan says.

The long weather history shows many extremes, and Glisan mentions the extreme heat in the 1930s as one example.

He says Iowa’s June, July and August temperatures are the same as in 1895, the year the state’s widespread weather records begin. But he says overall the state is 1.3 degrees warmer than 1895, warming about a tenth of a degree per decade on average, with the warmer winters offsetting the fact the summer months have stayed about the same.

“Winter is the fastest warming,” he says.

Ford says Illinois has similarly seen a slight overall temperature increase over the last 125 years, with it being most pronounced in the winter.

Glisan says an increase in temperature increases the amount of water vapor that can be held in the atmosphere, meaning more intense, more infrequent precipitation events. He says an increase of 1 degree in average air temperature means 4% more water vapor can be stored.

Glisan says July is Iowa’s hottest month, but is on average the state’s third-wettest month. A dip in July rainfall in recent years has contributed to a three-year drought for parts of Iowa, triggered in part by years of La Niña conditions in the Pacific.

New strategies

Still, despite drought and challenges from the frequency and duration of rains, Iowa set records for corn and soybean yields in 2021, and came within a few bushels of doing so again last year. Crop technology continues to advance.

“We’ve had timely rainfalls that have helped the crop on,” Glisan says. “With hybridization, crops are more drought tolerant.”

Also, farmers themselves have adapted, using new strategies such as cover crops and no-till to maintain organic matter on soils and reduce the effects of erosion.

Farmers are also looking to irrigation and drainage options to manage weather challenges. University of Missouri research agronomist Kelly Nelson has been studying drainage in crop fields for decades. Based at MU’s Greenley Research Center at Novelty, Nelson works on the MU Drainage and Sub-Irrigation (MUDS) project. This project involves installing drainage structures to remove excess water, but also collect it for sub-irrigation later in the year when hotter, drier conditions generally settle in. He says it helps manage for weather extremes.

“We were raising the water table up in that soil profile with subsurface irrigation,” Nelson says.

Nelson says in nearly two decades of data, installing drainage-only systems has shown a 20% increase in corn and soybean yields.

“That’s just getting the water off in a timely manner since we sometimes run into a wet spring and fall and sometimes even summer,” he says.

Combining the drainage with sub-irrigation, Nelson says the numbers show a 28% increase for soybeans and a 40% increase for corn.

The longer growing season does give farmers more time for growing their crops.

“Farmers have been able to take advantage of that extension of summer into fall, and make up for spring planting delays,” Ford says.

Spring planting seasons have generally seen more precipitation, he says.

“The wetness in the spring has been a challenge,” Ford says. “Even though the growing season has expanded, the number of workable days in the spring has stayed about the same.”

He says a longer growing season can help make up for a later start, which happened in 2019 when corn planting was delayed but a longer growing season helped the crop catch up and thrive. He says as the growing season lengthens farmers may be able to look at longer-season hybrids and varieties of crops.

On the downside, weeds can be more of a challenge in a longer growing season.

“It’s not just the plants we want that take advantage of the longer growing season,” Ford says. “The weeds enjoy it quite a bit as well.”

The state climatologists study how the climate trends affect agriculture. Glisan says one of his favorite parts of his job is getting out and scouting fields. Also, he says state climatologists help tell the story of their states.

“We basically see ourselves as the weather historians of our state,” he says.