MSU Extension Vegetables

Many southern Michigan sweet corn growers seed their first sweet corn plantings by the end of April and finish later plantings by early July. These dates change by two to three weeks as production moves north through the state. No matter where you are located, once harvest begins, the goal is to have a continual supply through Labor Day or beyond. Timing of successive plantings is important in maintaining this supply, and if planting time is off or the weather does not cooperate, producers could experience supply issues of too much or too little. Neither of which they like.

Timing of early plantings when the soil and air are colder can be difficult to judge. Often times the second or third planting, even though planted two or more weeks later, easily catches up to the first planting causing excessive sweet corn harvest followed by a lull. How can growers better time their plantings to prevent over and under supplies? Sweet corn producers can follow three types of scheduling.

1. Days between plantings

With this method, growers seed subsequent plantings a certain number of days after the previous one. However, they know to leave more days between early plantings and less between June plantings. The more experience the grower has, the better they are at determining the number of days between plantings. Of the three methods, this is by far the easiest but the least accurate and the most prone to supply chain problems. Just like all crops, corn is subject to temperature for growth, and day-to-day temperatures can vary substantially, especially early in the season. While this method is common, it is not the most reliable.

2. Plant stage

With this method, planting time of the next seeding is based on the amount of growth of the previous planting. For this method, the grower has to have a certain amount of knowledge on the terminology of corn growth stages. The plant growth stage of the previous planting signals growers when the next planting should occur. The grower does not have to do any math—the plant does it all. However, depending on how tight the desired scheduling will be, the grower may have to determine plant stage by digging up a few non-emerged, germinating seedlings.  The University of Vermont has a fact sheet that describes this method in depth and contains helpful pictures.

3. Growing degree days (GDD)

This method is based on how many GDDs are needed between plantings to maintain a steady supply. The GDD model for corn growth uses 50 F as the baseline temperature. The University of Connecticut describes this method and it is the most reliable in maintaining a constant supply.

To calculate degree-days on your own, you will need a maximum – minimum thermometer and paper and pencil to perform some basic math. To make the calculations, add the daily maximum and minimum temperatures, divide by 2 to get the daily average, and then subtract 50 degrees from that number to get that day’s GDD units. Daily averages for each day are added together to obtain a cumulative number (see example). Daily averages below 50 F are counted as “0.” There is no subtraction of units since once plants grow, they are not capable of growing backwards.

Growing degree example calculation

Day 1:
Low: 45 F
High: 65 F
Average: 50 + 64 = 114/2 = 57
GDD: 57 – 50 = 7

Day 2:
Low: 48 F
High: 70 F
Average: 48 + 70 = 118/2 = 59
GDD: 59 – 50 = 9

Cumulative GDD (CGDD): CGDD: 7 + 9 = 16

Your local Michigan State University Enviroweather station can both track temperatures and do the math for you. To do this:

• Visit MSU Enviroweather and click the weather station by clicking the yellow dot closest to your location.
• Scroll down to the “Degree-day tools” header, then click “Degree days: region (alfalfa and corn)”
• In the new window, you can select a start date that aligns with your first planting date, then hit “Get Report”
• Corn has a degree day base of 50ºF, so look at the last two columns. The first column gives you the number of corn growing degree days since the date you entered, the second gives you the predicted degree day accumulation over the next week.

The last two seasons have presented challenging planting conditions and this one seems to be shaping up to be equally difficult. To help things go right and avoid potential mid-season supply problems, plant sweet corn using the best method and information available.