# Using Growing Degree Days to Schedule Sweet Corn

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Sweet corn is one of the most important fresh market vegetable crops in the region. Direct market growers and wholesalers strive to have a continuous supply for their customers during the growing season.

Some growers will make a planting and then wait until that corn has emerged to make the next planting while others use their experience to create a calendar or determine when to plant.

With sweet corn, maturity will be based largely on two factors: the variety grown, and the amount of heat accumulated. There can be other factors that delay maturity such as crop injury, flooding, or hail; however, the genetics and temperature will be the major drivers.

To ensure a steady supply of sweet corn, the most accurate way to space plantings in the spring is by the use of growing degree days.

Growing Degree Days (GDD) are calculated as follows GDD = ((Maximum Temperature + Minimum Temperature)/2) -50°F. So, if the daytime temperature is 68 and the nighttime temperature is 48, you would add 68+48=116 then divide 116/2 = 58 and then subtract 58-50= 8 GDD. Negative numbers are not counted. You can find growing degree days already calculated for nearby weather stations at this DEOS site: http://www.deos.udel.edu/data/agirrigation_retrieval.php

To schedule sweet corn by growing degree days you will need the following information:

1.) How much corn you plant to sell per day and the number of acres or row feet to plant to supply that amount,

2.) How many days you expect to harvest from that planting (1-4 days usually),

3.) The GDDs required to harvest for the varieties of sweet corn that you grow (an alternative is your records of the first harvest for the varieties you use),

4.) The average GDDs during the expected harvest period,

5.) GDDs during your planting season (calculate daily). Having your own maximum and minimum thermometer is the best way do this. Information from the nearest weather station is an acceptable alternative.

So, for example, you have determined that you need 200 dozen ears per day. This requires 2400 ears or about 3000 seeds accounting for germination losses and unmarketable ears. At 24000 seeds per acre this is 0.125 or ⅛ acre and if you plan to harvest over three days this would be 0.375 or ⅜ acres to plant.

Historically, your records indicate that the specific variety you plant April 10 matures July 1. Or you can use historical GDD information and GDDs for that variety from your seed supplier to calculate first harvest (a 1300 GDD corn will mature on average between July 1 and July 4 when planted in early April in southern Delaware).

As our weather becomes more variable, the use of the GDD scheduling model will be more useful as historical temperature data changes. You can create your own historical GDD charts from the information provided at the DEOS site (or other historical weather records for your area). Use a 10-year rolling average. A GDD tracking tool with 15 and 30 year averages and extremes is available online at: http://climatesmartfarming.org/tools/csf-growing-degree-day-calculator/

Average growing degree days in July for southern Delaware are 25 per day (from weather records). To have the corn you require every 3 days, you would multiply 3 x 25 = 75 growing degree days. Therefore, you would space your plantings in the spring 75 growing degree days apart. As you move into August, the growing degree days are similar but for September corn the growing degree days drop to 20 per day and plantings should be 60 GDD apart. This means that the first 20 plantings should be spaced 75 GDD’s apart (April through early June) and after that you would space plantings 60 GDD apart (mid-June onward).

In summary:
• Make the first planting as you normally do for you first intended harvest date.

• Estimate the time of harvest and calculate average GDD per day in the harvest period.
• Determine the number of days you plan to harvest the planting (three in our example).
• Calculate the GDD that will accumulate during the harvest period (3 days x 25 GDD/days) = 75.
• Record maximum and minimum temperatures and calculate GDD = ((Maximum Temperature + Minimum Temperature)/2) -50° F
• Add daily GDD from planting until they equal the GDD in the intended harvest period (75).
• When GDD equal those in the harvest period, make the next planting.
• The process can be repeated for subsequent plantings and other varieties.
• To obtain specific variety GDD information, contact your seed supplier