The short answer is: The optimum planting window for corn is mid-April to about May 5 in north-central and northeast Iowa, and mid-April to about May 10 in the rest of the state.

However, with today’s hybrids, yields of 95% to 100% of potential are common up to May 10 in north-central and northeast Iowa and May 15-18 in the rest of Iowa.

Keep in mind there is a lowering of yield potential when planting before these date ranges as well as after. Yields are usually no higher for crops planted in early April compared to those planted in late April or early May, so there’s little reward for taking the risk of very early planting.

When the optimum window is for any given year varies. It depends on weather, so one year May 10-15 may be better than April 15-18; another year April 25-30 may be the best time.

But wait, there is more to the answer than calendar date. Soil condition is at least as important, and many times more important, than calendar date. Some things to consider are soil moisture content, soil temperature and weather forecast. Here is a closer look:

• Freeze-thaw cycles. The numerous freeze-thaw cycles of this past January and February have helped break up compaction in the top 4 to 5 inches of soil. Now the stage is set for good planting conditions. Very little spring tillage should be needed. In many cases, especially following cover crops, no spring tillage will be needed. That takes some of the pressure off.

• Soil’s readiness. Be sure to let the soil get to good condition before working it or planting. Good soil moisture (lack of wetness) is far more important than some farmers realize.

After a rain, water that infiltrated is pulled down through the soil profile by gravity, until the excess is drained. At that point, the soil is at field capacity.  It’s best to do field work about two days after the soil gets down to field capacity. Before beginning a tillage operation, dig down to the depth of the tillage implement and make sure the soil is below field capacity. If you can make a ball of the soil at that depth, you are better off waiting. Same goes for planting.

• Soil temps and weather. Corn germinates at 50 degrees F, so aim for having the soil at or near 50 with a warm forecast. The corn seed will imbibe (take up) water during the first 12 to 48 hours. If that water is cold, it can interfere with the seed germination process. So watching the weather forecast is important. You don’t want the water being taken by the seed to be below 41 degrees.

• Safety. Spring fieldwork means long hours and at least some time driving on roads with equipment. Be safe, take your time and be careful. Take breaks to refresh yourself. Even just a couple of laps around the tractor and implement helps. Stand up, walk and drink plenty of water throughout the day.

On a personal note
I am retiring in May, while I am still healthy enough to enjoy it.  I’ve greatly enjoyed working with farmers and agribusiness people over many years, in large groups, small groups and one-on-one. I have had a very rewarding career thanks to the great people in Iowa agriculture.

Johnson is the ISU Extension field agronomist in central Iowa. Contact him at [email protected].