Given the current conditions and the forecast for cool and wet weather at the end of this week (the week of April 11 through 17), the best place for corn seed this week is probably in the bag. That caution is from Iowa State University Extension corn agronomist Roger Elmore, who issued the warning on April 12 in response to phone calls and emails he's been getting from farmers.

There are a few fields in Iowa that have already been planted. They were planted last weekend and during the first week in April. But remember the old saying: "The first guy to plant is often the first to replant."

Four-inch soil temperatures passed the 50 degrees Fahrenheit mark in the past few days, air temperatures broke 80 degrees Fahrenheit over the weekend, soil moisture conditions in many areas are good, and the sun is shining. "Because of the good planting conditions, I'm receiving many questions on whether we should start planting corn," says Elmore. "However, rain is expected by mid-week across much of the state and air temperatures may drop below freezing by this coming weekend. This calls for a caution flag."

You could run into corn germination and emergence problems

He adds, "While our current planting date data identifies the best time frames to start planting begin this week for the three Iowa regions and end at varying times, there is more to consider." 

Corn seed absorbs 30% to 35% of its weight in water during germination, Elmore explains. Soil temperatures have little effect on this process. However, the time required for the radicle to emerge is tied to temperature; it increases linearly if soil temperatures are between 46 and 90 degrees F. Little, if any, mesocotyl or coleoptile growth occurs in soils cooler than 60 degrees F. A constant soil temperature of 86 degrees F optimizes germination and seedling emergence.

Once the seed begins to germinate, a significant change in soil temperature can cause problems, says Elmore. Research shows that a swing of soil temperatures of 27 F (soil high temperature to soil low temperature) will affect mesocotyl growth. "It is possible that corn planted this week will experience this range of temperatures," he points out.

Seed put in ground, yet not emerged, can be hurt by cold snap

"Seed placed into the ground, yet not emerged, can be injured from a cold period. The drop in soil temperature can cause erratic and uneven corn stands as it did across the state in 2006, resulting in crop development that varied up to two developmental stages between plants in the same row."

"Seedlings can also have stunted and distorted leaves or may not emerge from the soil," he adds. "Given the current conditions and the forecast for cool-wet weather, the best place for seed this week is probably in the bag."

Remember the old saying, "The first farmer to plant in spring is often the first to replant." For more information, see Early season cold stress, an online article by Roger Elmore and colleague Lori Abendroth on their ISU Extension corn website.