The optimal time to plant corn is when the soil reaches a temperature of at least 60°F and threat of frost has passed.
More Info: According to the Corn Planting Guide from the University of Iowa Cooperative Extension, you can plant your corn earlier as long as the soil has reached a temperature of 50°F but there may be little benefit to planting earlier as the colder the soil conditions the longer it takes the plant to emerge.
Plant Corn According to Soil Conditions
Rather than attempt to find an exact date for your area that will be best for corn performance, the soil conditions should dictate your decision to plant. General rule of thumb-plant as early as the conditions will allow.
Soil Temperature: You can plant corn early as soon as soil temperatures have reached at least 50-55°F. Planting when the soil is this cold may take the plant up to three weeks or more to emerge. If you wait until the soil temperature reaches 70°F the plant will generally emerge within a week.
Soil Moisture: It is not advisable to plant when the soil is too wet. Wait until soil conditions have sufficiently drained.
Staggering for Optimal Yields
For best yields stagger the planting of your corn. Depending on variety, corn requires 60-90 days from planting to maturity so adding new plants every 10-14 days will result in a continuous harvest throughout the season.
 “Sweet Corn in the Garden.” Utah State University Extension. Utah State University, n.d. Web. 9 May 2011. <extension.usu.edu/files/publications/publication/HG_Garden_2005-10.pdf>.
 ” Planting Sweet Corn in the Home Garden | Horticulture and Home Pest News .” Integrated Pest Management at Iowa State University. N.p., n.d. Web. 9 May 2011. <http://www.ipm.iastate.edu/ipm/hortnews/2001/4-20-2001/sweetcorn.html>.