Stubbornly cold temperatures and snowstorms affecting much of Iowa this spring shouldn’t cause undue alarm for farmers anxious to begin planting. At least not for another week or so. That’s how Iowa State University Extension cropping systems agronomist Mark Licht sizes up the current situation.

“I’m not overly worried that we haven’t got a lot of corn in the ground yet in Iowa,” Licht commented on April 19. “In another week or so if this situation persists, then I become a bit more worried.”

 Although farmers are chomping at the bit to get in the field, Licht recommends patience. “You can do more harm than good running over fields with heavy equipment and planting before soil conditions are favorable. Soil that’s too wet and cold can increase degrees of compaction, seed rot and germination problems,” he cautions.

Still time for a good planting season
With today’s superwide planters and GPS technology, a number of farmers can plant around the clock, day and night. Licht notes that Iowa farmers can scale up their planting in a hurry, planting between 1 million and 1.5 million acres a day across the state when operating at full capacity.

Thus, there’s still time this spring for most farmers to get virtually all their corn and soybeans planted before the optimal window closes for each crop, even with this month’s temperatures freezing farmers out of their fields. 

Farmers in Iowa usually want to have their corn completely planted between May 5 and May 10. Soybean planting can stretch a little later, to around May 20 in an average year. Waiting too long after those dates shortens the growing season for crops and risks diminishing yields. 

So, the clock hasn’t run out yet, but additional delays most likely will take some farmers down to the buzzer for optimal planting. “We need 15 to 20 good days to get all the planting done across Iowa,” he says. “You realize quickly when looking at the calendar, we’re going to be under some pressure if we keep getting delays.”

Good reason to wait for soil to warm up
Licht recommends farmers wait until soils reach 50 degrees F before planting. He also recommends paying attention to the forecast to make sure soil temperatures won’t slip below that threshold in the first few days after planting. Seeds won’t germinate in cooler soil, which gives insects and pathogens an opportunity to strike.

Farmers are most concerned with the top 2 inches of soil, which can warm quickly once the weather finally turns around for good. For instance, most of the state’s soils were below 30 degrees at the beginning of last week. By week’s end, some soil in southern Iowa had reached 50 degrees. This week with colder weather, soil temperatures dropped back down again.

“A few days of 60s and 70s during the daytime, and with nighttime temperatures that remain above 40 degrees, can make a huge difference,” notes Licht. “With some sustained warm daytime temperatures, we can see soil temperature respond and increase quickly.”

To check daily soil temperatures for Iowa on the ISU Mesonet website (temperature at planting depth for corn), visit ISU Soil Moisture Network and click on the tab for recent "4-inch soil temperatures."

Optimum planting window varies
The optimum planting window for corn (98% to 100% yield potential) varies depending on your location in Iowa. ISU research shows northeast Iowa has the narrowest window, April 12-30. For northwest and central Iowa, it’s April 15 to May 9. For the southern three tiers of counties, it’s April 17 to May 8.

For soybeans, ISU research shows little to no yield loss in Iowa if beans are planted by May 15. After that date, yields on average decline by one-fourth to one bushel per acre per day that the seed isn’t in the ground.

Farmer Bill Shipley of Nodaway in southwest Iowa likes to start planting corn by April 15 to 20. That’s not happening this year. On April 16, soil temperatures ranged from 32 degrees in northern Iowa to 41 degrees in the far southwest corner of the state.

“It’s going to be awhile before we plant,” Shipley says. “We want the soil at 50 to 55 degrees and trending higher, not going lower. Cold soil isn’t good for putting seed in the ground.”

Patience a virtue at planting
Getting into a field with a corn planter on cold, wet soils — even a half-day too soon — can cause sidewall compaction, hampering root growth. That leads to restricted uptake of moisture and nutrients, and causes crop lodging and ultimately yield loss, says Licht. “I’ve been saying the word ‘patience’ a lot lately,” he adds.

It’s always a good idea to have a contingency plan. Licht recommends farmers consider the following options if corn planting is pushed back well into May or later:

• Delay herbicide application so you can plant first, spray later. But there is risk of weed escapes if timely spraying can’t be done.

• Postpone pre-plant nitrogen application so you can plant first, apply the N later.

• Switch to earlier maturing corn hybrids if you are planting in June.

For soybeans, Licht doesn’t recommend switching to an earlier maturity group; plant the bean variety with a maturity rating that’s adapted for your area. “You usually give up yield potential by going earlier,” he says. “The likelihood of soybeans getting killed by frost before they are mature in the fall is fairly low.”

Despite cold and snowy conditions this spring, there’s still enough time for most Iowa farmers to plant corn and soybeans without having to worry about reduced yields due to a shortened growing season. But further delays to planting could mean some farmers will be cutting it close.

“We need 15 to 20 good days to get all the planting done. You realize quickly looking at the calendar, we’re going to be under some pressure if we keep getting delays," says Mark Licht, ISU Extension agronomist.