Dr. Ajay Nair
Department of Horticulture
Iowa State University
In few weeks growers will be planting sweet corn in Iowa. Sweet corn is an important commercial crop in Iowa. Based on the 2017 Agriculture Census, sweet corn is planted across 329 farms in Iowa with a collective acreage of 2,739 acres. Of the total acres, 1,743 is harvested for fresh market and the remaining for processing. For the fresh markets, growers practice succession planting of sweet corn to have a continuous supply for their customers during the growing season.
Every year growers try to push the boundaries of early planting to have sweet corn ready by July 4th. It is a risky proposition as there is risk of frost late April and early May. It is important to note that sweet corn seeds do not germinate well when soil temperatures are below 55°F. Seeds could rot in cold, wet soils therefore it is important to seed when soil conditions are optimum. Standard sweet corn cultivars (Butter and Queen, Peaches and Cream, Silver Queen) may be planted in late April in central Iowa. Sugary enhanced cultivars (Ambrosia, Cuppa Joe, Temptation) prefer slightly warmer soil temperatures; therefore, they should be planted one week later than standard sweet corn. Shrunken-2 (Northern Xtra sweet, Honey and Pearl,) and augmented supersweet cultivars (Anthem XR, Kickoff XR, Solstice, Rosie) should be planted later in the season because their seeds germinate poorly when soil temperatures are below 60°F. For a continuous supply of sweet corn, plant early-, main-, and late-season cultivars or plant every 2-3 weeks.
Many growers decide their succession planting schedule by planting their first cycle and then wait until that corn has emerged to make the next planting. A vast number of growers will use their experience from previous years to create a calendar or determine when to plant. During any growing year, 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 affect maturity such as inclement weather, soil fertility, flooding; however, the genetics and temperature will be the major factors. To ensure a steady supply of sweet corn and follow a more scientific approach, the accurate way to space plantings in the spring is using 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 70 and the nighttime temperature is 50, you would add 70+50=120 then divide 120/2 = 60 and then subtract 60-50= 10 GDD. Negative numbers are not counted. Growers can find growing degree days already calculated for nearby weather stations by using the mesonet data available at https://mesonet.agron.iastate.edu/. Typically scheduling sweet corn by GDD would require the following information:
- Estimate of how much corn will be sold every day and the number of acres or row feet to plant to supply that amount
- Expected harvest days from that planting (1-4 days usually)
- The GDDs required to harvest for the specific cultivar
- The average GDDs during the expected harvest period
- GDDs during the planting season (calculate daily) from personal or nearest weather station.
One can use the historical records for example: it has been found that a particular cultivar if planted April 15 matures July 1. Or use the historical GDD information for that variety from your seed vendor to calculate first harvest. Average growing degree days in July for central Iowa is 25 per day (from mesonet records). To have the corn every 3 days, you would multiply 3 x 25 = 75 growing degree days. Therefore, one would space the plantings in the spring 75 growing degree days apart. Assuming similar GDD’s for August harvests but September GDD drops 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 one would space plantings 60 GDD apart (mid-June onward).
Isolation Requirements: All types of corn (field corn, ornamental corn, popcorn, and sweet corn) can cross-pollinate with one another. This cross-pollination can markedly alter the sugar levels, texture, color, and other characteristics of sweet corn. To prevent cross-pollination and contamination, all sweet corn types should be isolated from field corn, popcorn, and ornamental corn. Within the sweet corn types, shrunken-2 (sh2) cultivars should also be isolated from standard (su), sugary enhanced (se), and synergistic (syn) sweet corn types. Augmented supersweets (shA) should be isolated from standard (su), sugary enhanced (se), and synergistic (syn) cultivars. For best results, isolate sugary enhanced (se) cultivars from standard (su) and synergistic cultivars (syn). To maintain color purity, isolate white sweet corn from yellow or bicolor sweet corn. Pollen from yellow or bicolor corn will produce yellow kernels in white cultivars. Pollen from yellow corn produces more yellow kernels in bicolor cultivars. Pollen from white sweet corn has no effect on yellow or bicolor cultivars. Isolation can be achieved by planting different types at least 250 feet from one another. Another option is to stagger planting dates or select cultivars that mature at different times. A minimum of 14 days should separate the tasseling time of different corn types.