Optimum Iowa corn planting dates range from mid-April to the end of April in North Central and Northeast Iowa and to the first or second week in May in other parts of Iowa (see Table). If we consider the differences between early-planting and late planting, yields are reduced less early in the planting season than late. That means that planting early during the optimum window is generally a better practice than planting a few days after the optimum window. Yields drop off dramatically in mid-May across Iowa. If possible, plant corn prior to May 15 to avoid this "slippery slope" of rapidly reducing yield potential.